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GMP Itemized Development Tasks
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<!-- 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>
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