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| author | Thomas Voss <mail@thomasvoss.com> | 2024-06-21 23:36:36 +0200 |
|---|---|---|
| committer | Thomas Voss <mail@thomasvoss.com> | 2024-06-21 23:42:26 +0200 |
| commit | a89a14ef5da44684a16b204e7a70460cc8c4922a (patch) | |
| tree | b23b4c6b155977909ef508fdae2f48d33d802813 /vendor/gmp-6.3.0/doc/tasks.html | |
| parent | 1db63fcedab0b288820d66e100b1877b1a5a8851 (diff) | |
Basic constant folding implementation
Diffstat (limited to 'vendor/gmp-6.3.0/doc/tasks.html')
| -rw-r--r-- | vendor/gmp-6.3.0/doc/tasks.html | 896 |
1 files changed, 896 insertions, 0 deletions
diff --git a/vendor/gmp-6.3.0/doc/tasks.html b/vendor/gmp-6.3.0/doc/tasks.html new file mode 100644 index 0000000..9a25bef --- /dev/null +++ b/vendor/gmp-6.3.0/doc/tasks.html @@ -0,0 +1,896 @@ +<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> +<html> +<head> + <title>GMP Itemized Development Tasks</title> + <link rel="shortcut icon" href="favicon.ico"> + <link rel="stylesheet" href="gmp.css"> + <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> +</head> + +<center> + <h1> + GMP Itemized Development Tasks + </h1> +</center> + +<font size=-1> +<pre> +Copyright 2000-2004, 2006, 2008, 2009 Free Software Foundation, Inc. + +This file is part of the GNU MP Library. + +The GNU MP Library is free software; you can redistribute it and/or modify +it under the terms of either: + + * the GNU Lesser General Public License as published by the Free + Software Foundation; either version 3 of the License, or (at your + option) any later version. + +or + + * the GNU General Public License as published by the Free Software + Foundation; either version 2 of the License, or (at your option) any + later version. + +or both in parallel, as here. + +The GNU MP Library is distributed in the hope that it will be useful, but +WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY +or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License +for more details. + +You should have received copies of the GNU General Public License and the +GNU Lesser General Public License along with the GNU MP Library. If not, +see https://www.gnu.org/licenses/. +</pre> +</font> + +<hr> +<!-- NB. timestamp updated automatically by emacs --> + This file current as of 29 Jan 2014. An up-to-date version is available at + <a href="https://gmplib.org/tasks.html">https://gmplib.org/tasks.html</a>. + Please send comments about this page to gmp-devel<font>@</font>gmplib.org. + +<p> These are itemized GMP development tasks. Not all the tasks + listed here are suitable for volunteers, but many of them are. + Please see the <a href="projects.html">projects file</a> for more + sizeable projects. + +<p> CAUTION: This file needs updating. Many of the tasks here have +either already been taken care of, or have become irrelevant. + +<h4>Correctness and Completeness</h4> +<ul> +<li> <code>_LONG_LONG_LIMB</code> in gmp.h is not namespace clean. Reported + by Patrick Pelissier. + <br> + We sort of mentioned <code>_LONG_LONG_LIMB</code> in past releases, so + need to be careful about changing it. It used to be a define + applications had to set for long long limb systems, but that in + particular is no longer relevant now that it's established automatically. +<li> The various reuse.c tests need to force reallocation by calling + <code>_mpz_realloc</code> with a small (1 limb) size. +<li> One reuse case is missing from mpX/tests/reuse.c: + <code>mpz_XXX(a,a,a)</code>. +<li> Make the string reading functions allow the `0x' prefix when the base is + explicitly 16. They currently only allow that prefix when the base is + unspecified (zero). +<li> <code>mpf_eq</code> is not always correct, when one operand is + 1000000000... and the other operand is 0111111111..., i.e., extremely + close. There is a special case in <code>mpf_sub</code> for this + situation; put similar code in <code>mpf_eq</code>. [In progress.] +<li> <code>mpf_eq</code> doesn't implement what gmp.texi specifies. It should + not use just whole limbs, but partial limbs. [In progress.] +<li> <code>mpf_set_str</code> doesn't validate it's exponent, for instance + garbage 123.456eX789X is accepted (and an exponent 0 used), and overflow + of a <code>long</code> is not detected. +<li> <code>mpf_add</code> doesn't check for a carry from truncated portions of + the inputs, and in that respect doesn't implement the "infinite precision + followed by truncate" specified in the manual. +<li> Windows DLLs: tests/mpz/reuse.c and tests/mpf/reuse.c initialize global + variables with pointers to <code>mpz_add</code> etc, which doesn't work + when those routines are coming from a DLL (because they're effectively + function pointer global variables themselves). Need to rearrange perhaps + to a set of calls to a test function rather than iterating over an array. +<li> <code>mpz_pow_ui</code>: Detect when the result would be more memory than + a <code>size_t</code> can represent and raise some suitable exception, + probably an alloc call asking for <code>SIZE_T_MAX</code>, and if that + somehow succeeds then an <code>abort</code>. Various size overflows of + this kind are not handled gracefully, probably resulting in segvs. + <br> + In <code>mpz_n_pow_ui</code>, detect when the count of low zero bits + exceeds an <code>unsigned long</code>. There's a (small) chance of this + happening but still having enough memory to represent the value. + Reported by Winfried Dreckmann in for instance <code>mpz_ui_pow_ui (x, + 4UL, 1431655766UL)</code>. +<li> <code>mpf</code>: Detect exponent overflow and raise some exception. + It'd be nice to allow the full <code>mp_exp_t</code> range since that's + how it's been in the past, but maybe dropping one bit would make it + easier to test if e1+e2 goes out of bounds. +</ul> + + + +<h4>Machine Independent Optimization</h4> +<ul> +<li> <code>mpf_cmp</code>: For better cache locality, don't test for low zero + limbs until the high limbs fail to give an ordering. Reduce code size by + turning the three <code>mpn_cmp</code>'s into a single loop stopping when + the end of one operand is reached (and then looking for a non-zero in the + rest of the other). +<li> <code>mpf_mul_2exp</code>, <code>mpf_div_2exp</code>: The use of + <code>mpn_lshift</code> for any size<=prec means repeated + <code>mul_2exp</code> and <code>div_2exp</code> calls accumulate low zero + limbs until size==prec+1 is reached. Those zeros will slow down + subsequent operations, especially if the value is otherwise only small. + If low bits of the low limb are zero, use <code>mpn_rshift</code> so as + to not increase the size. +<li> <code>mpn_dc_sqrtrem</code>, <code>mpn_sqrtrem2</code>: Don't use + <code>mpn_add_1</code> and <code>mpn_sub_1</code> for 1 limb operations, + instead <code>ADDC_LIMB</code> and <code>SUBC_LIMB</code>. +<li> <code>mpn_sqrtrem2</code>: Use plain variables for <code>sp[0]</code> and + <code>rp[0]</code> calculations, so the compiler needn't worry about + aliasing between <code>sp</code> and <code>rp</code>. +<li> <code>mpn_sqrtrem</code>: Some work can be saved in the last step when + the remainder is not required, as noted in Paul's paper. +<li> <code>mpq_add</code>, <code>mpq_sub</code>: The gcd fits a single limb + with high probability and in this case <code>binvert_limb</code> could + be used to calculate the inverse just once for the two exact divisions + "op1.den / gcd" and "op2.den / gcd", rather than letting + <code>mpn_bdiv_q_1</code> do it each time. This would require calling + <code>mpn_pi1_bdiv_q_1</code>. +<li> <code>mpn_gcdext</code>: Don't test <code>count_leading_zeros</code> for + zero, instead check the high bit of the operand and avoid invoking + <code>count_leading_zeros</code>. This is an optimization on all + machines, and significant on machines with slow + <code>count_leading_zeros</code>, though it's possible an already + normalized operand might not be encountered very often. +<li> Rewrite <code>umul_ppmm</code> to use floating-point for generating the + most significant limb (if <code>GMP_LIMB_BITS</code> <= 52 bits). + (Peter Montgomery has some ideas on this subject.) +<li> Improve the default <code>umul_ppmm</code> code in longlong.h: Add partial + products with fewer operations. +<li> Consider inlining <code>mpz_set_ui</code>. This would be both small and + fast, especially for compile-time constants, but would make application + binaries depend on having 1 limb allocated to an <code>mpz_t</code>, + preventing the "lazy" allocation scheme below. +<li> Consider inlining <code>mpz_[cft]div_ui</code> and maybe + <code>mpz_[cft]div_r_ui</code>. A <code>__gmp_divide_by_zero</code> + would be needed for the divide by zero test, unless that could be left to + <code>mpn_mod_1</code> (not sure currently whether all the risc chips + provoke the right exception there if using mul-by-inverse). +<li> Consider inlining: <code>mpz_fits_s*_p</code>. The setups for + <code>LONG_MAX</code> etc would need to go into gmp.h, and on Cray it + might, unfortunately, be necessary to forcibly include <limits.h> + since there's no apparent way to get <code>SHRT_MAX</code> with an + expression (since <code>short</code> and <code>unsigned short</code> can + be different sizes). +<li> <code>mpz_powm</code> and <code>mpz_powm_ui</code> aren't very fast on one + or two limb moduli, due to a lot of function call overheads. These could + perhaps be handled as special cases. +<li> Make sure <code>mpz_powm_ui</code> is never slower than the corresponding + computation using <code>mpz_powm</code>. +<li> <code>mpz_powm</code> REDC should do multiplications by <code>g[]</code> + using the division method when they're small, since the REDC form of a + small multiplier is normally a full size product. Probably would need a + new tuned parameter to say what size multiplier is "small", as a function + of the size of the modulus. +<li> <code>mpn_gcd</code> might be able to be sped up on small to moderate + sizes by improving <code>find_a</code>, possibly just by providing an + alternate implementation for CPUs with slowish + <code>count_leading_zeros</code>. +<li> <code>mpf_set_str</code> produces low zero limbs when a string has a + fraction but is exactly representable, eg. 0.5 in decimal. These could be + stripped to save work in later operations. +<li> <code>mpz_and</code>, <code>mpz_ior</code> and <code>mpz_xor</code> should + use <code>mpn_and_n</code> etc for the benefit of the small number of + targets with native versions of those routines. Need to be careful not to + pass size==0. Is some code sharing possible between the <code>mpz</code> + routines? +<li> <code>mpf_add</code>: Don't do a copy to avoid overlapping operands + unless it's really necessary (currently only sizes are tested, not + whether r really is u or v). +<li> <code>mpf_add</code>: Under the check for v having no effect on the + result, perhaps test for r==u and do nothing in that case, rather than + currently it looks like an <code>MPN_COPY_INCR</code> will be done to + reduce prec+1 limbs to prec. +<li> <code>mpf_div_ui</code>: Instead of padding with low zeros, call + <code>mpn_divrem_1</code> asking for fractional quotient limbs. +<li> <code>mpf_div_ui</code>: Eliminate <code>TMP_ALLOC</code>. When r!=u + there's no overlap and the division can be called on those operands. + When r==u and is prec+1 limbs, then it's an in-place division. If r==u + and not prec+1 limbs, then move the available limbs up to prec+1 and do + an in-place there. +<li> <code>mpf_div_ui</code>: Whether the high quotient limb is zero can be + determined by testing the dividend for high<divisor. When non-zero, the + division can be done on prec dividend limbs instead of prec+1. The result + size is also known before the division, so that can be a tail call (once + the <code>TMP_ALLOC</code> is eliminated). +<li> <code>mpn_divrem_2</code> could usefully accept unnormalized divisors and + shift the dividend on-the-fly, since this should cost nothing on + superscalar processors and avoid the need for temporary copying in + <code>mpn_tdiv_qr</code>. +<li> <code>mpf_sqrt</code>: If r!=u, and if u doesn't need to be padded with + zeros, then there's no need for the tp temporary. +<li> <code>mpq_cmp_ui</code> could form the <code>num1*den2</code> and + <code>num2*den1</code> products limb-by-limb from high to low and look at + each step for values differing by more than the possible carry bit from + the uncalculated portion. +<li> <code>mpq_cmp</code> could do the same high-to-low progressive multiply + and compare. The benefits of karatsuba and higher multiplication + algorithms are lost, but if it's assumed only a few high limbs will be + needed to determine an order then that's fine. +<li> <code>mpn_add_1</code>, <code>mpn_sub_1</code>, <code>mpn_add</code>, + <code>mpn_sub</code>: Internally use <code>__GMPN_ADD_1</code> etc + instead of the functions, so they get inlined on all compilers, not just + gcc and others with <code>inline</code> recognised in gmp.h. + <code>__GMPN_ADD_1</code> etc are meant mostly to support application + inline <code>mpn_add_1</code> etc and if they don't come out good for + internal uses then special forms can be introduced, for instance many + internal uses are in-place. Sometimes a block of code is executed based + on the carry-out, rather than using it arithmetically, and those places + might want to do their own loops entirely. +<li> <code>__gmp_extract_double</code> on 64-bit systems could use just one + bitfield for the mantissa extraction, not two, when endianness permits. + Might depend on the compiler allowing <code>long long</code> bit fields + when that's the only actual 64-bit type. +<li> tal-notreent.c could keep a block of memory permanently allocated. + Currently the last nested <code>TMP_FREE</code> releases all memory, so + there's an allocate and free every time a top-level function using + <code>TMP</code> is called. Would need + <code>mp_set_memory_functions</code> to tell tal-notreent.c to release + any cached memory when changing allocation functions though. +<li> <code>__gmp_tmp_alloc</code> from tal-notreent.c could be partially + inlined. If the current chunk has enough room then a couple of pointers + can be updated. Only if more space is required then a call to some sort + of <code>__gmp_tmp_increase</code> would be needed. The requirement that + <code>TMP_ALLOC</code> is an expression might make the implementation a + bit ugly and/or a bit sub-optimal. +<pre> +#define TMP_ALLOC(n) + ((ROUND_UP(n) > current->end - current->point ? + __gmp_tmp_increase (ROUND_UP (n)) : 0), + current->point += ROUND_UP (n), + current->point - ROUND_UP (n)) +</pre> +<li> <code>__mp_bases</code> has a lot of data for bases which are pretty much + never used. Perhaps the table should just go up to base 16, and have + code to generate data above that, if and when required. Naturally this + assumes the code would be smaller than the data saved. +<li> <code>__mp_bases</code> field <code>big_base_inverted</code> is only used + if <code>USE_PREINV_DIVREM_1</code> is true, and could be omitted + otherwise, to save space. +<li> <code>mpz_get_str</code>, <code>mtox</code>: For power-of-2 bases, which + are of course fast, it seems a little silly to make a second pass over + the <code>mpn_get_str</code> output to convert to ASCII. Perhaps combine + that with the bit extractions. +<li> <code>mpz_gcdext</code>: If the caller requests only the S cofactor (of + A), and A<B, then the code ends up generating the cofactor T (of B) and + deriving S from that. Perhaps it'd be possible to arrange to get S in + the first place by calling <code>mpn_gcdext</code> with A+B,B. This + might only be an advantage if A and B are about the same size. +<li> <code>mpz_n_pow_ui</code> does a good job with small bases and stripping + powers of 2, but it's perhaps a bit too complicated for what it gains. + The simpler <code>mpn_pow_1</code> is a little faster on small exponents. + (Note some of the ugliness in <code>mpz_n_pow_ui</code> is due to + supporting <code>mpn_mul_2</code>.) + <br> + Perhaps the stripping of 2s in <code>mpz_n_pow_ui</code> should be + confined to single limb operands for simplicity and since that's where + the greatest gain would be. + <br> + Ideally <code>mpn_pow_1</code> and <code>mpz_n_pow_ui</code> would be + merged. The reason <code>mpz_n_pow_ui</code> writes to an + <code>mpz_t</code> is that its callers leave it to make a good estimate + of the result size. Callers of <code>mpn_pow_1</code> already know the + size by separate means (<code>mp_bases</code>). +<li> <code>mpz_invert</code> should call <code>mpn_gcdext</code> directly. +</ul> + + +<h4>Machine Dependent Optimization</h4> +<ul> +<li> <code>invert_limb</code> on various processors might benefit from the + little Newton iteration done for alpha and ia64. +<li> Alpha 21264: <code>mpn_addlsh1_n</code> could be implemented with + <code>mpn_addmul_1</code>, since that code at 3.5 is a touch faster than + a separate <code>lshift</code> and <code>add_n</code> at + 1.75+2.125=3.875. Or very likely some specific <code>addlsh1_n</code> + code could beat both. +<li> Alpha 21264: Improve feed-in code for <code>mpn_mul_1</code>, + <code>mpn_addmul_1</code>, and <code>mpn_submul_1</code>. +<li> Alpha 21164: Rewrite <code>mpn_mul_1</code>, <code>mpn_addmul_1</code>, + and <code>mpn_submul_1</code> for the 21164. This should use both integer + multiplies and floating-point multiplies. For the floating-point + operations, the single-limb multiplier should be split into three 21-bit + chunks, or perhaps even better in four 16-bit chunks. Probably possible + to reach 9 cycles/limb. +<li> Alpha: GCC 3.4 will introduce <code>__builtin_ctzl</code>, + <code>__builtin_clzl</code> and <code>__builtin_popcountl</code> using + the corresponding CIX <code>ct</code> instructions, and + <code>__builtin_alpha_cmpbge</code>. These should give GCC more + information about scheduling etc than the <code>asm</code> blocks + currently used in longlong.h and gmp-impl.h. +<li> Alpha Unicos: Apparently there's no <code>alloca</code> on this system, + making <code>configure</code> choose the slower + <code>malloc-reentrant</code> allocation method. Is there a better way? + Maybe variable-length arrays per notes below. +<li> Alpha Unicos 21164, 21264: <code>.align</code> is not used since it pads + with garbage. Does the code get the intended slotting required for the + claimed speeds? <code>.align</code> at the start of a function would + presumably be safe no matter how it pads. +<li> ARM V5: <code>count_leading_zeros</code> can use the <code>clz</code> + instruction. For GCC 3.4 and up, do this via <code>__builtin_clzl</code> + since then gcc knows it's "predicable". +<li> Itanium: GCC 3.4 introduces <code>__builtin_popcount</code> which can be + used instead of an <code>asm</code> block. The builtin should give gcc + more opportunities for scheduling, bundling and predication. + <code>__builtin_ctz</code> similarly (it just uses popcount as per + current longlong.h). +<li> UltraSPARC/64: Optimize <code>mpn_mul_1</code>, <code>mpn_addmul_1</code>, + for s2 < 2^32 (or perhaps for any zero 16-bit s2 chunk). Not sure how + much this can improve the speed, though, since the symmetry that we rely + on is lost. Perhaps we can just gain cycles when s2 < 2^16, or more + accurately, when two 16-bit s2 chunks which are 16 bits apart are zero. +<li> UltraSPARC/64: Write native <code>mpn_submul_1</code>, analogous to + <code>mpn_addmul_1</code>. +<li> UltraSPARC/64: Write <code>umul_ppmm</code>. Using four + "<code>mulx</code>"s either with an asm block or via the generic C code is + about 90 cycles. Try using fp operations, and also try using karatsuba + for just three "<code>mulx</code>"s. +<li> UltraSPARC/32: Rewrite <code>mpn_lshift</code>, <code>mpn_rshift</code>. + Will give 2 cycles/limb. Trivial modifications of mpn/sparc64 should do. +<li> UltraSPARC/32: Write special mpn_Xmul_1 loops for s2 < 2^16. +<li> UltraSPARC/32: Use <code>mulx</code> for <code>umul_ppmm</code> if + possible (see commented out code in longlong.h). This is unlikely to + save more than a couple of cycles, so perhaps isn't worth bothering with. +<li> UltraSPARC/32: On Solaris gcc doesn't give us <code>__sparc_v9__</code> + or anything to indicate V9 support when -mcpu=v9 is selected. See + gcc/config/sol2-sld-64.h. Will need to pass something through from + ./configure to select the right code in longlong.h. (Currently nothing + is lost because <code>mulx</code> for multiplying is commented out.) +<li> UltraSPARC/32: <code>mpn_divexact_1</code> and + <code>mpn_modexact_1c_odd</code> can use a 64-bit inverse and take + 64-bits at a time from the dividend, as per the 32-bit divisor case in + mpn/sparc64/mode1o.c. This must be done in assembler, since the full + 64-bit registers (<code>%gN</code>) are not available from C. +<li> UltraSPARC/32: <code>mpn_divexact_by3c</code> can work 64-bits at a time + using <code>mulx</code>, in assembler. This would be the same as for + sparc64. +<li> UltraSPARC: <code>binvert_limb</code> might save a few cycles from + masking down to just the useful bits at each point in the calculation, + since <code>mulx</code> speed depends on the highest bit set. Either + explicit masks or small types like <code>short</code> and + <code>int</code> ought to work. +<li> Sparc64 HAL R1 <code>popc</code>: This chip reputedly implements + <code>popc</code> properly (see gcc sparc.md). Would need to recognise + it as <code>sparchalr1</code> or something in configure / config.sub / + config.guess. <code>popc_limb</code> in gmp-impl.h could use this (per + commented out code). <code>count_trailing_zeros</code> could use it too. +<li> PA64: Improve <code>mpn_addmul_1</code>, <code>mpn_submul_1</code>, and + <code>mpn_mul_1</code>. The current code runs at 11 cycles/limb. It + should be possible to saturate the cache, which will happen at 8 + cycles/limb (7.5 for mpn_mul_1). Write special loops for s2 < 2^32; + it should be possible to make them run at about 5 cycles/limb. +<li> PPC601: See which of the power or powerpc32 code runs better. Currently + the powerpc32 is used, but only because it's the default for + <code>powerpc*</code>. +<li> PPC630: Rewrite <code>mpn_addmul_1</code>, <code>mpn_submul_1</code>, and + <code>mpn_mul_1</code>. Use both integer and floating-point operations, + possibly two floating-point and one integer limb per loop. Split operands + into four 16-bit chunks for fast fp operations. Should easily reach 9 + cycles/limb (using one int + one fp), but perhaps even 7 cycles/limb + (using one int + two fp). +<li> PPC630: <code>mpn_rshift</code> could do the same sort of unrolled loop + as <code>mpn_lshift</code>. Some judicious use of m4 might let the two + share source code, or with a register to control the loop direction + perhaps even share object code. +<li> Implement <code>mpn_mul_basecase</code> and <code>mpn_sqr_basecase</code> + for important machines. Helping the generic sqr_basecase.c with an + <code>mpn_sqr_diagonal</code> might be enough for some of the RISCs. +<li> POWER2/POWER2SC: Schedule <code>mpn_lshift</code>/<code>mpn_rshift</code>. + Will bring time from 1.75 to 1.25 cycles/limb. +<li> X86: Optimize non-MMX <code>mpn_lshift</code> for shifts by 1. (See + Pentium code.) +<li> X86: Good authority has it that in the past an inline <code>rep + movs</code> would upset GCC register allocation for the whole function. + Is this still true in GCC 3? It uses <code>rep movs</code> itself for + <code>__builtin_memcpy</code>. Examine the code for some simple and + complex functions to find out. Inlining <code>rep movs</code> would be + desirable, it'd be both smaller and faster. +<li> Pentium P54: <code>mpn_lshift</code> and <code>mpn_rshift</code> can come + down from 6.0 c/l to 5.5 or 5.375 by paying attention to pairing after + <code>shrdl</code> and <code>shldl</code>, see mpn/x86/pentium/README. +<li> Pentium P55 MMX: <code>mpn_lshift</code> and <code>mpn_rshift</code> + might benefit from some destination prefetching. +<li> PentiumPro: <code>mpn_divrem_1</code> might be able to use a + mul-by-inverse, hoping for maybe 30 c/l. +<li> K7: <code>mpn_lshift</code> and <code>mpn_rshift</code> might be able to + do something branch-free for unaligned startups, and shaving one insn + from the loop with alternative indexing might save a cycle. +<li> PPC32: Try using fewer registers in the current <code>mpn_lshift</code>. + The pipeline is now extremely deep, perhaps unnecessarily deep. +<li> Fujitsu VPP: Vectorize main functions, perhaps in assembly language. +<li> Fujitsu VPP: Write <code>mpn_mul_basecase</code> and + <code>mpn_sqr_basecase</code>. This should use a "vertical multiplication + method", to avoid carry propagation. splitting one of the operands in + 11-bit chunks. +<li> Pentium: <code>mpn_lshift</code> by 31 should use the special rshift + by 1 code, and vice versa <code>mpn_rshift</code> by 31 should use the + special lshift by 1. This would be best as a jump across to the other + routine, could let both live in lshift.asm and omit rshift.asm on finding + <code>mpn_rshift</code> already provided. +<li> Cray T3E: Experiment with optimization options. In particular, + -hpipeline3 seems promising. We should at least up -O to -O2 or -O3. +<li> Cray: <code>mpn_com</code> and <code>mpn_and_n</code> etc very probably + wants a pragma like <code>MPN_COPY_INCR</code>. +<li> Cray vector systems: <code>mpn_lshift</code>, <code>mpn_rshift</code>, + <code>mpn_popcount</code> and <code>mpn_hamdist</code> are nice and small + and could be inlined to avoid function calls. +<li> Cray: Variable length arrays seem to be faster than the tal-notreent.c + scheme. Not sure why, maybe they merely give the compiler more + information about aliasing (or the lack thereof). Would like to modify + <code>TMP_ALLOC</code> to use them, or introduce a new scheme. Memory + blocks wanted unconditionally are easy enough, those wanted only + sometimes are a problem. Perhaps a special size calculation to ask for a + dummy length 1 when unwanted, or perhaps an inlined subroutine + duplicating code under each conditional. Don't really want to turn + everything into a dog's dinner just because Cray don't offer an + <code>alloca</code>. +<li> Cray: <code>mpn_get_str</code> on power-of-2 bases ought to vectorize. + Does it? <code>bits_per_digit</code> and the inner loop over bits in a + limb might prevent it. Perhaps special cases for binary, octal and hex + would be worthwhile (very possibly for all processors too). +<li> S390: <code>BSWAP_LIMB_FETCH</code> looks like it could be done with + <code>lrvg</code>, as per glibc sysdeps/s390/s390-64/bits/byteswap.h. + This is only for 64-bit mode or something is it, since 32-bit mode has + other code? Also, is it worth using for <code>BSWAP_LIMB</code> too, or + would that mean a store and re-fetch? Presumably that's what comes out + in glibc. +<li> Improve <code>count_leading_zeros</code> for 64-bit machines: + <pre> + if ((x >> 32) == 0) { x <<= 32; cnt += 32; } + if ((x >> 48) == 0) { x <<= 16; cnt += 16; } + ... </pre> +<li> IRIX 6 MIPSpro compiler has an <code>__inline</code> which could perhaps + be used in <code>__GMP_EXTERN_INLINE</code>. What would be the right way + to identify suitable versions of that compiler? +<li> IRIX <code>cc</code> is rumoured to have an <code>_int_mult_upper</code> + (in <code><intrinsics.h></code> like Cray), but it didn't seem to + exist on some IRIX 6.5 systems tried. If it does actually exist + somewhere it would very likely be an improvement over a function call to + umul.asm. +<li> <code>mpn_get_str</code> final divisions by the base with + <code>udiv_qrnd_unnorm</code> could use some sort of multiply-by-inverse + on suitable machines. This ends up happening for decimal by presenting + the compiler with a run-time constant, but the same for other bases would + be good. Perhaps use could be made of the fact base<256. +<li> <code>mpn_umul_ppmm</code>, <code>mpn_udiv_qrnnd</code>: Return a + structure like <code>div_t</code> to avoid going through memory, in + particular helping RISCs that don't do store-to-load forwarding. Clearly + this is only possible if the ABI returns a structure of two + <code>mp_limb_t</code>s in registers. + <br> + On PowerPC, structures are returned in memory on AIX and Darwin. In SVR4 + they're returned in registers, except that draft SVR4 had said memory, so + it'd be prudent to check which is done. We can jam the compiler into the + right mode if we know how, since all this is purely internal to libgmp. + (gcc has an option, though of course gcc doesn't matter since we use + inline asm there.) +</ul> + +<h4>New Functionality</h4> +<ul> +<li> Maybe add <code>mpz_crr</code> (Chinese Remainder Reconstruction). +<li> Let `0b' and `0B' mean binary input everywhere. +<li> <code>mpz_init</code> and <code>mpq_init</code> could do lazy allocation. + Set <code>ALLOC(var)</code> to 0 to indicate nothing allocated, and let + <code>_mpz_realloc</code> do the initial alloc. Set + <code>z->_mp_d</code> to a dummy that <code>mpz_get_ui</code> and + similar can unconditionally fetch from. Niels Möller has had a go at + this. + <br> + The advantages of the lazy scheme would be: + <ul> + <li> Initial allocate would be the size required for the first value + stored, rather than getting 1 limb in <code>mpz_init</code> and then + more or less immediately reallocating. + <li> <code>mpz_init</code> would only store magic values in the + <code>mpz_t</code> fields, and could be inlined. + <li> A fixed initializer could even be used by applications, like + <code>mpz_t z = MPZ_INITIALIZER;</code>, which might be convenient + for globals. + </ul> + The advantages of the current scheme are: + <ul> + <li> <code>mpz_set_ui</code> and other similar routines needn't check the + size allocated and can just store unconditionally. + <li> <code>mpz_set_ui</code> and perhaps others like + <code>mpz_tdiv_r_ui</code> and a prospective + <code>mpz_set_ull</code> could be inlined. + </ul> +<li> Add <code>mpf_out_raw</code> and <code>mpf_inp_raw</code>. Make sure + format is portable between 32-bit and 64-bit machines, and between + little-endian and big-endian machines. A format which MPFR can use too + would be good. +<li> <code>mpn_and_n</code> ... <code>mpn_copyd</code>: Perhaps make the mpn + logops and copys available in gmp.h, either as library functions or + inlines, with the availability of library functions instantiated in the + generated gmp.h at build time. +<li> <code>mpz_set_str</code> etc variants taking string lengths rather than + null-terminators. +<li> <code>mpz_andn</code>, <code>mpz_iorn</code>, <code>mpz_nand</code>, + <code>mpz_nior</code>, <code>mpz_xnor</code> might be useful additions, + if they could share code with the current such functions (which should be + possible). +<li> <code>mpz_and_ui</code> etc might be of use sometimes. Suggested by + Niels Möller. +<li> <code>mpf_set_str</code> and <code>mpf_inp_str</code> could usefully + accept 0x, 0b etc when base==0. Perhaps the exponent could default to + decimal in this case, with a further 0x, 0b etc allowed there. + Eg. 0xFFAA@0x5A. A leading "0" for octal would match the integers, but + probably something like "0.123" ought not mean octal. +<li> <code>GMP_LONG_LONG_LIMB</code> or some such could become a documented + feature of gmp.h, so applications could know whether to + <code>printf</code> a limb using <code>%lu</code> or <code>%Lu</code>. +<li> <code>GMP_PRIdMP_LIMB</code> and similar defines following C99 + <inttypes.h> might be of use to applications printing limbs. But + if <code>GMP_LONG_LONG_LIMB</code> or whatever is added then perhaps this + can easily enough be left to applications. +<li> <code>gmp_printf</code> could accept <code>%b</code> for binary output. + It'd be nice if it worked for plain <code>int</code> etc too, not just + <code>mpz_t</code> etc. +<li> <code>gmp_printf</code> in fact could usefully accept an arbitrary base, + for both integer and float conversions. A base either in the format + string or as a parameter with <code>*</code> should be allowed. Maybe + <code>&13b</code> (b for base) or something like that. +<li> <code>gmp_printf</code> could perhaps accept <code>mpq_t</code> for float + conversions, eg. <code>"%.4Qf"</code>. This would be merely for + convenience, but still might be useful. Rounding would be the same as + for an <code>mpf_t</code> (ie. currently round-to-nearest, but not + actually documented). Alternately, perhaps a separate + <code>mpq_get_str_point</code> or some such might be more use. Suggested + by Pedro Gimeno. +<li> <code>mpz_rscan0</code> or <code>mpz_revscan0</code> or some such + searching towards the low end of an integer might match + <code>mpz_scan0</code> nicely. Likewise for <code>scan1</code>. + Suggested by Roberto Bagnara. +<li> <code>mpz_bit_subset</code> or some such to test whether one integer is a + bitwise subset of another might be of use. Some sort of return value + indicating whether it's a proper or non-proper subset would be good and + wouldn't cost anything in the implementation. Suggested by Roberto + Bagnara. +<li> <code>mpf_get_ld</code>, <code>mpf_set_ld</code>: Conversions between + <code>mpf_t</code> and <code>long double</code>, suggested by Dan + Christensen. Other <code>long double</code> routines might be desirable + too, but <code>mpf</code> would be a start. + <br> + <code>long double</code> is an ANSI-ism, so everything involving it would + need to be suppressed on a K&R compiler. + <br> + There'd be some work to be done by <code>configure</code> to recognise + the format in use, MPFR has a start on this. Often <code>long + double</code> is the same as <code>double</code>, which is easy but + pretty pointless. A single float format detector macro could look at + <code>double</code> then <code>long double</code> + <br> + Sometimes there's a compiler option for the size of a <code>long + double</code>, eg. xlc on AIX can use either 64-bit or 128-bit. It's + probably simplest to regard this as a compiler compatibility issue, and + leave it to users or sysadmins to ensure application and library code is + built the same. +<li> <code>mpz_sqrt_if_perfect_square</code>: When + <code>mpz_perfect_square_p</code> does its tests it calculates a square + root and then discards it. For some applications it might be useful to + return that root. Suggested by Jason Moxham. +<li> <code>mpz_get_ull</code>, <code>mpz_set_ull</code>, + <code>mpz_get_sll</code>, <code>mpz_get_sll</code>: Conversions for + <code>long long</code>. These would aid interoperability, though a + mixture of GMP and <code>long long</code> would probably not be too + common. Since <code>long long</code> is not always available (it's in + C99 and GCC though), disadvantages of using <code>long long</code> in + libgmp.a would be + <ul> + <li> Library contents vary according to the build compiler. + <li> gmp.h would need an ugly <code>#ifdef</code> block to decide if the + application compiler could take the <code>long long</code> + prototypes. + <li> Some sort of <code>LIBGMP_HAS_LONGLONG</code> might be wanted to + indicate whether the functions are available. (Applications using + autoconf could probe the library too.) + </ul> + It'd be possible to defer the need for <code>long long</code> to + application compile time, by having something like + <code>mpz_set_2ui</code> called with two halves of a <code>long + long</code>. Disadvantages of this would be, + <ul> + <li> Bigger code in the application, though perhaps not if a <code>long + long</code> is normally passed as two halves anyway. + <li> <code>mpz_get_ull</code> would be a rather big inline, or would have + to be two function calls. + <li> <code>mpz_get_sll</code> would be a worse inline, and would put the + treatment of <code>-0x10..00</code> into applications (see + <code>mpz_get_si</code> correctness above). + <li> Although having libgmp.a independent of the build compiler is nice, + it sort of sacrifices the capabilities of a good compiler to + uniformity with inferior ones. + </ul> + Plain use of <code>long long</code> is probably the lesser evil, if only + because it makes best use of gcc. In fact perhaps it would suffice to + guarantee <code>long long</code> conversions only when using GCC for both + application and library. That would cover free software, and we can + worry about selected vendor compilers later. + <br> + In C++ the situation is probably clearer, we demand fairly recent C++ so + <code>long long</code> should be available always. We'd probably prefer + to have the C and C++ the same in respect of <code>long long</code> + support, but it would be possible to have it unconditionally in gmpxx.h, + by some means or another. +<li> <code>mpz_strtoz</code> parsing the same as <code>strtol</code>. + Suggested by Alexander Kruppa. +</ul> + + +<h4>Configuration</h4> + +<ul> +<li> Alpha ev7, ev79: Add code to config.guess to detect these. Believe ev7 + will be "3-1307" in the current switch, but need to verify that. (On + OSF, current configfsf.guess identifies ev7 using psrinfo, we need to do + it ourselves for other systems.) +<li> Alpha OSF: Libtool (version 1.5) doesn't seem to recognise this system is + "pic always" and ends up running gcc twice with the same options. This + is wasteful, but harmless. Perhaps a newer libtool will be better. +<li> ARM: <code>umul_ppmm</code> in longlong.h always uses <code>umull</code>, + but is that available only for M series chips or some such? Perhaps it + should be configured in some way. +<li> HPPA: config.guess should recognize 7000, 7100, 7200, and 8x00. +<li> HPPA: gcc 3.2 introduces a <code>-mschedule=7200</code> etc parameter, + which could be driven by an exact hppa cpu type. +<li> Mips: config.guess should say mipsr3000, mipsr4000, mipsr10000, etc. + "hinv -c processor" gives lots of information on Irix. Standard + config.guess appends "el" to indicate endianness, but + <code>AC_C_BIGENDIAN</code> seems the best way to handle that for GMP. +<li> PowerPC: The function descriptor nonsense for AIX is currently driven by + <code>*-*-aix*</code>. It might be more reliable to do some sort of + feature test, examining the compiler output perhaps. It might also be + nice to merge the aix.m4 files into powerpc-defs.m4. +<li> config.m4 is generated only by the configure script, it won't be + regenerated by config.status. Creating it as an <code>AC_OUTPUT</code> + would work, but it might upset "make" to have things like <code>L$</code> + get into the Makefiles through <code>AC_SUBST</code>. + <code>AC_CONFIG_COMMANDS</code> would be the alternative. With some + careful m4 quoting the <code>changequote</code> calls might not be + needed, which might free up the order in which things had to be output. +<li> Automake: Latest automake has a <code>CCAS</code>, <code>CCASFLAGS</code> + scheme. Though we probably wouldn't be using its assembler support we + could try to use those variables in compatible ways. +<li> <code>GMP_LDFLAGS</code> could probably be done with plain + <code>LDFLAGS</code> already used by automake for all linking. But with + a bit of luck the next libtool will pass pretty much all + <code>CFLAGS</code> through to the compiler when linking, making + <code>GMP_LDFLAGS</code> unnecessary. +<li> mpn/Makeasm.am uses <code>-c</code> and <code>-o</code> together in the + .S and .asm rules, but apparently that isn't completely portable (there's + an autoconf <code>AC_PROG_CC_C_O</code> test for it). So far we've not + had problems, but perhaps the rules could be rewritten to use "foo.s" as + the temporary, or to do a suitable "mv" of the result. The only danger + from using foo.s would be if a compile failed and the temporary foo.s + then looked like the primary source. Hopefully if the + <code>SUFFIXES</code> are ordered to have .S and .asm ahead of .s that + wouldn't happen. Might need to check. +</ul> + + +<h4>Random Numbers</h4> +<ul> +<li> <code>_gmp_rand</code> is not particularly fast on the linear + congruential algorithm and could stand various improvements. + <ul> + <li> Make a second seed area within <code>gmp_randstate_t</code> (or + <code>_mp_algdata</code> rather) to save some copying. + <li> Make a special case for a single limb <code>2exp</code> modulus, to + avoid <code>mpn_mul</code> calls. Perhaps the same for two limbs. + <li> Inline the <code>lc</code> code, to avoid a function call and + <code>TMP_ALLOC</code> for every chunk. + <li> Perhaps the <code>2exp</code> and general LC cases should be split, + for clarity (if the general case is retained). + </ul> +<li> <code>gmp_randstate_t</code> used for parameters perhaps should become + <code>gmp_randstate_ptr</code> the same as other types. +<li> Some of the empirical randomness tests could be included in a "make + check". They ought to work everywhere, for a given seed at least. +</ul> + + +<h4>C++</h4> +<ul> +<li> <code>mpz_class(string)</code>, etc: Use the C++ global locale to + identify whitespace. + <br> + <code>mpf_class(string)</code>: Use the C++ global locale decimal point, + rather than the C one. + <br> + Consider making these variant <code>mpz_set_str</code> etc forms + available for <code>mpz_t</code> too, not just <code>mpz_class</code> + etc. +<li> <code>mpq_class operator+=</code>: Don't emit an unnecessary + <code>mpq_set(q,q)</code> before <code>mpz_addmul</code> etc. +<li> Put various bits of gmpxx.h into libgmpxx, to avoid excessive inlining. + Candidates for this would be, + <ul> + <li> <code>mpz_class(const char *)</code>, etc: since they're normally + not fast anyway, and we can hide the exception <code>throw</code>. + <li> <code>mpz_class(string)</code>, etc: to hide the <code>cstr</code> + needed to get to the C conversion function. + <li> <code>mpz_class string, char*</code> etc constructors: likewise to + hide the throws and conversions. + <li> <code>mpz_class::get_str</code>, etc: to hide the <code>char*</code> + to <code>string</code> conversion and free. Perhaps + <code>mpz_get_str</code> can write directly into a + <code>string</code>, to avoid copying. + <br> + Consider making such <code>string</code> returning variants + available for use with plain <code>mpz_t</code> etc too. + </ul> +</ul> + +<h4>Miscellaneous</h4> +<ul> +<li> <code>mpz_gcdext</code> and <code>mpn_gcdext</code> ought to document + what range of values the generated cofactors can take, and preferably + ensure the definition uniquely specifies the cofactors for given inputs. + A basic extended Euclidean algorithm or multi-step variant leads to + |x|<|b| and |y|<|a| or something like that, but there's probably + two solutions under just those restrictions. +<li> demos/factorize.c: use <code>mpz_divisible_ui_p</code> rather than + <code>mpz_tdiv_qr_ui</code>. (Of course dividing multiple primes at a + time would be better still.) +<li> The various test programs use quite a bit of the main + <code>libgmp</code>. This establishes good cross-checks, but it might be + better to use simple reference routines where possible. Where it's not + possible some attention could be paid to the order of the tests, so a + <code>libgmp</code> routine is only used for tests once it seems to be + good. +<li> <code>MUL_FFT_THRESHOLD</code> etc: the FFT thresholds should allow a + return to a previous k at certain sizes. This arises basically due to + the step effect caused by size multiples effectively used for each k. + Looking at a graph makes it fairly clear. +<li> <code>__gmp_doprnt_mpf</code> does a rather unattractive round-to-nearest + on the string returned by <code>mpf_get_str</code>. Perhaps some variant + of <code>mpf_get_str</code> could be made which would better suit. +</ul> + + +<h4>Aids to Development</h4> +<ul> +<li> Add <code>ASSERT</code>s at the start of each user-visible mpz/mpq/mpf + function to check the validity of each <code>mp?_t</code> parameter, in + particular to check they've been <code>mp?_init</code>ed. This might + catch elementary mistakes in user programs. Care would need to be taken + over <code>MPZ_TMP_INIT</code>ed variables used internally. If nothing + else then consistency checks like size<=alloc, ptr not + <code>NULL</code> and ptr+size not wrapping around the address space, + would be possible. A more sophisticated scheme could track + <code>_mp_d</code> pointers and ensure only a valid one is used. Such a + scheme probably wouldn't be reentrant, not without some help from the + system. +<li> tune/time.c could try to determine at runtime whether + <code>getrusage</code> and <code>gettimeofday</code> are reliable. + Currently we pretend in configure that the dodgy m68k netbsd 1.4.1 + <code>getrusage</code> doesn't exist. If a test might take a long time + to run then perhaps cache the result in a file somewhere. +<li> tune/time.c could choose the default precision based on the + <code>speed_unittime</code> determined, independent of the method in use. +<li> Cray vector systems: CPU frequency could be determined from + <code>sysconf(_SC_CLK_TCK)</code>, since it seems to be clock cycle + based. Is this true for all Cray systems? Would like some documentation + or something to confirm. +</ul> + + +<h4>Documentation</h4> +<ul> +<li> <code>mpz_inp_str</code> (etc) doesn't say when it stops reading digits. +<li> <code>mpn_get_str</code> isn't terribly clear about how many digits it + produces. It'd probably be possible to say at most one leading zero, + which is what both it and <code>mpz_get_str</code> currently do. But + want to be careful not to bind ourselves to something that might not suit + another implementation. +<li> <code>va_arg</code> doesn't do the right thing with <code>mpz_t</code> + etc directly, but instead needs a pointer type like <code>MP_INT*</code>. + It'd be good to show how to do this, but we'd either need to document + <code>mpz_ptr</code> and friends, or perhaps fallback on something + slightly nasty with <code>void*</code>. +</ul> + + +<h4>Bright Ideas</h4> + +<p> The following may or may not be feasible, and aren't likely to get done in the +near future, but are at least worth thinking about. + +<ul> +<li> Reorganize longlong.h so that we can inline the operations even for the + system compiler. When there is no such compiler feature, make calls to + stub functions. Write such stub functions for as many machines as + possible. +<li> longlong.h could declare when it's using, or would like to use, + <code>mpn_umul_ppmm</code>, and the corresponding umul.asm file could be + included in libgmp only in that case, the same as is effectively done for + <code>__clz_tab</code>. Likewise udiv.asm and perhaps cntlz.asm. This + would only be a very small space saving, so perhaps not worth the + complexity. +<li> longlong.h could be built at configure time by concatenating or + #including fragments from each directory in the mpn path. This would + select CPU specific macros the same way as CPU specific assembler code. + Code used would no longer depend on cpp predefines, and the current + nested conditionals could be flattened out. +<li> <code>mpz_get_si</code> returns 0x80000000 for -0x100000000, whereas it's + sort of supposed to return the low 31 (or 63) bits. But this is + undocumented, and perhaps not too important. +<li> <code>mpz_init_set*</code> and <code>mpz_realloc</code> could allocate + say an extra 16 limbs over what's needed, so as to reduce the chance of + having to do a reallocate if the <code>mpz_t</code> grows a bit more. + This could only be an option, since it'd badly bloat memory usage in + applications using many small values. +<li> <code>mpq</code> functions could perhaps check for numerator or + denominator equal to 1, on the assumption that integers or + denominator-only values might be expected to occur reasonably often. +<li> <code>count_trailing_zeros</code> is used on more or less uniformly + distributed numbers in a couple of places. For some CPUs + <code>count_trailing_zeros</code> is slow and it's probably worth handling + the frequently occurring 0 to 2 trailing zeros cases specially. +<li> <code>mpf_t</code> might like to let the exponent be undefined when + size==0, instead of requiring it 0 as now. It should be possible to do + size==0 tests before paying attention to the exponent. The advantage is + not needing to set exp in the various places a zero result can arise, + which avoids some tedium but is otherwise perhaps not too important. + Currently <code>mpz_set_f</code> and <code>mpf_cmp_ui</code> depend on + exp==0, maybe elsewhere too. +<li> <code>__gmp_allocate_func</code>: Could use GCC <code>__attribute__ + ((malloc))</code> on this, though don't know if it'd do much. GCC 3.0 + allows that attribute on functions, but not function pointers (see info + node "Attribute Syntax"), so would need a new autoconf test. This can + wait until there's a GCC that supports it. +<li> <code>mpz_add_ui</code> contains two <code>__GMPN_COPY</code>s, one from + <code>mpn_add_1</code> and one from <code>mpn_sub_1</code>. If those two + routines were opened up a bit maybe that code could be shared. When a + copy needs to be done there's no carry to append for the add, and if the + copy is non-empty no high zero for the sub. +</ul> + + +<h4>Old and Obsolete Stuff</h4> + +<p> The following tasks apply to chips or systems that are old and/or obsolete. +It's unlikely anything will be done about them unless anyone is actively using +them. + +<ul> +<li> Sparc32: The integer based udiv_nfp.asm used to be selected by + <code>configure --nfp</code> but that option is gone now that autoconf is + used. The file could go somewhere suitable in the mpn search if any + chips might benefit from it, though it's possible we don't currently + differentiate enough exact cpu types to do this properly. +<li> VAX D and G format <code>double</code> floats are straightforward and + could perhaps be handled directly in <code>__gmp_extract_double</code> + and maybe in <code>mpn_get_d</code>, rather than falling back on the + generic code. (Both formats are detected by <code>configure</code>.) +</ul> + + +<hr> + +</body> +</html> + +<!-- +Local variables: +eval: (add-hook 'write-file-hooks 'time-stamp) +time-stamp-start: "This file current as of " +time-stamp-format: "%:d %3b %:y" +time-stamp-end: "\\." +time-stamp-line-limit: 50 +End: +--> |