From 5671e8e4cdc3551b87a516d6130afb2243701ebd Mon Sep 17 00:00:00 2001
From: Thomas Voss <mail@thomasvoss.com>
Date: Fri, 14 Jun 2024 21:12:07 +0200
Subject: Add a basic arena allocator

---
 src/alloc.h | 18 ++++++++++-
 src/arena.c | 99 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 116 insertions(+), 1 deletion(-)
 create mode 100644 src/arena.c

diff --git a/src/alloc.h b/src/alloc.h
index 80e2384..004e6ec 100644
--- a/src/alloc.h
+++ b/src/alloc.h
@@ -1,13 +1,29 @@
 #ifndef ORYX_ALLOC_H
 #define ORYX_ALLOC_H
 
+#include <stdalign.h>
 #include <stddef.h>
 
 #include "common.h"
 
+typedef struct _arena *arena;
+
 /* Allocate a buffer of NMEMB elements of size SIZE.  If PTR is non-null then
    reallocate the buffer it points to.  Aborts on out-of-memory or overflow. */
 void *bufalloc(void *ptr, size_t nmemb, size_t size)
-	__attribute__((returns_nonnull, alloc_size(2, 3)));
+	__attribute__((returns_nonnull, warn_unused_result, alloc_size(2, 3)));
+
+/* Allocate a buffer of NMEMB elements of size SIZE with alignment ALIGN using
+   the arena-allocator A. */
+void *arena_alloc(arena *a, size_t nmemb, size_t size, size_t align)
+	__attribute__((returns_nonnull, warn_unused_result, malloc,
+                   alloc_size(2, 3), alloc_align(4)));
+
+/* Deallocate all memory associated with the arena A. */
+void arena_free(arena *a)
+	__attribute__((nonnull));
+
+/* Allocate a buffer of N elements of type T using the arena-allocator A. */
+#define arena_new(a, T, n) ((T *)arena_alloc((a), (n), sizeof(T), alignof(T)))
 
 #endif /* !ORYX_ALLOC_H */
diff --git a/src/arena.c b/src/arena.c
new file mode 100644
index 0000000..c9b20aa
--- /dev/null
+++ b/src/arena.c
@@ -0,0 +1,99 @@
+#include <sys/mman.h>
+
+#include <assert.h>
+#include <errno.h>
+#include <stddef.h>
+#include <stdint.h>
+#include <stdlib.h>
+
+#include "alloc.h"
+#include "errors.h"
+
+/* TODO: Support implementations without MAP_ANON? */
+#ifndef MAP_ANON
+static_assert(NULL, "MAP_ANON not available on this system");
+#endif
+
+#if DEBUG
+#	define ARENA_DFLT_CAP (8)
+#else
+#	define ARENA_DFLT_CAP (2048)
+#endif
+
+#define MAX(x, y)   ((x) > (y) ? (x) : (y))
+#define IS_POW_2(n) ((n) != 0 && ((n) & ((n)-1)) == 0)
+
+struct _arena {
+	size_t len, cap;
+	void *data, *last;
+	struct _arena *next;
+};
+
+static struct _arena *mkblk(size_t) __attribute__((returns_nonnull));
+static inline size_t pad(size_t, size_t) __attribute__((const, always_inline));
+
+void *
+arena_alloc(struct _arena **a, size_t nmemb, size_t size, size_t align)
+{
+	assert(IS_POW_2(align));
+	assert(nmemb * size != 0);
+
+	if (size > SIZE_MAX / nmemb) {
+		errno = EOVERFLOW;
+		err("%s:", __func__);
+	}
+
+	size *= nmemb;
+
+	for (struct _arena *p = *a; p != NULL; p = p->next) {
+		size_t nlen, off;
+		off = pad(p->len, align);
+		nlen = size + off;
+
+		if (nlen <= p->cap) {
+			void *ret = (char *)p->data + off;
+			p->len = nlen;
+			p->last = ret;
+			return ret;
+		}
+	}
+
+	/* No page exists with enough space */
+	struct _arena *p = mkblk(MAX(size, ARENA_DFLT_CAP));
+	p->len = size;
+	p->next = *a;
+	*a = p;
+	return p->data;
+}
+
+void
+arena_free(struct _arena **a)
+{
+	struct _arena *cur, *next;
+	for (cur = *a; cur != NULL; cur = next) {
+		next = cur->next;
+		munmap(cur->data, cur->cap);
+		free(cur);
+	}
+	*a = NULL;
+}
+
+struct _arena *
+mkblk(size_t cap)
+{
+	struct _arena *a = malloc(sizeof(*a));
+	if (a == NULL)
+		err("malloc:");
+	a->cap = cap;
+	a->data = mmap(NULL, cap, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANON, -1, 0);
+	if (a->data == MAP_FAILED)
+		err("mmap:");
+	a->last = a->data;
+	return a;
+}
+
+size_t
+pad(size_t len, size_t align)
+{
+	return (len + align - 1) & ~(align - 1);
+}
-- 
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