diff options
Diffstat (limited to 'gnulib/import/malloca.c')
| -rw-r--r-- | gnulib/import/malloca.c | 140 |
1 files changed, 48 insertions, 92 deletions
diff --git a/gnulib/import/malloca.c b/gnulib/import/malloca.c index a6132775b16..f60c5fb5d93 100644 --- a/gnulib/import/malloca.c +++ b/gnulib/import/malloca.c @@ -1,6 +1,6 @@ /* Safe automatic memory allocation. - Copyright (C) 2003, 2006-2007, 2009-2016 Free Software Foundation, Inc. - Written by Bruno Haible <bruno@clisp.org>, 2003. + Copyright (C) 2003, 2006-2007, 2009-2019 Free Software Foundation, Inc. + Written by Bruno Haible <bruno@clisp.org>, 2003, 2018. This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by @@ -13,7 +13,7 @@ GNU General Public License for more details. You should have received a copy of the GNU General Public License - along with this program; if not, see <http://www.gnu.org/licenses/>. */ + along with this program; if not, see <https://www.gnu.org/licenses/>. */ #define _GL_USE_STDLIB_ALLOC 1 #include <config.h> @@ -21,82 +21,49 @@ /* Specification. */ #include "malloca.h" -#include <stdint.h> - #include "verify.h" /* The speed critical point in this file is freea() applied to an alloca() result: it must be fast, to match the speed of alloca(). The speed of mmalloca() and freea() in the other case are not critical, because they - are only invoked for big memory sizes. */ - -#if HAVE_ALLOCA - -/* Store the mmalloca() results in a hash table. This is needed to reliably - distinguish a mmalloca() result and an alloca() result. - - Although it is possible that the same pointer is returned by alloca() and - by mmalloca() at different times in the same application, it does not lead - to a bug in freea(), because: - - Before a pointer returned by alloca() can point into malloc()ed memory, - the function must return, and once this has happened the programmer must - not call freea() on it anyway. - - Before a pointer returned by mmalloca() can point into the stack, it - must be freed. The only function that can free it is freea(), and - when freea() frees it, it also removes it from the hash table. */ - -#define MAGIC_NUMBER 0x1415fb4a -#define MAGIC_SIZE sizeof (int) -/* This is how the header info would look like without any alignment - considerations. */ -struct preliminary_header { void *next; int magic; }; -/* But the header's size must be a multiple of sa_alignment_max. */ -#define HEADER_SIZE \ - (((sizeof (struct preliminary_header) + sa_alignment_max - 1) / sa_alignment_max) * sa_alignment_max) -union header { - void *next; - struct { - char room[HEADER_SIZE - MAGIC_SIZE]; - int word; - } magic; -}; -verify (HEADER_SIZE == sizeof (union header)); -/* We make the hash table quite big, so that during lookups the probability - of empty hash buckets is quite high. There is no need to make the hash - table resizable, because when the hash table gets filled so much that the - lookup becomes slow, it means that the application has memory leaks. */ -#define HASH_TABLE_SIZE 257 -static void * mmalloca_results[HASH_TABLE_SIZE]; - -#endif + are only invoked for big memory sizes. + Here we use a bit in the address as an indicator, an idea by Ondřej Bílka. + malloca() can return three types of pointers: + - Pointers ≡ 0 mod 2*sa_alignment_max come from stack allocation. + - Pointers ≡ sa_alignment_max mod 2*sa_alignment_max come from heap + allocation. + - NULL comes from a failed heap allocation. */ + +/* Type for holding very small pointer differences. */ +typedef unsigned char small_t; +/* Verify that it is wide enough. */ +verify (2 * sa_alignment_max - 1 <= (small_t) -1); void * mmalloca (size_t n) { #if HAVE_ALLOCA - /* Allocate one more word, that serves as an indicator for malloc()ed - memory, so that freea() of an alloca() result is fast. */ - size_t nplus = n + HEADER_SIZE; + /* Allocate one more word, used to determine the address to pass to freea(), + and room for the alignment ≡ sa_alignment_max mod 2*sa_alignment_max. */ + size_t nplus = n + sizeof (small_t) + 2 * sa_alignment_max - 1; if (nplus >= n) { - void *p = malloc (nplus); + char *mem = (char *) malloc (nplus); - if (p != NULL) + if (mem != NULL) { - size_t slot; - union header *h = p; - - p = h + 1; - - /* Put a magic number into the indicator word. */ - h->magic.word = MAGIC_NUMBER; - - /* Enter p into the hash table. */ - slot = (uintptr_t) p % HASH_TABLE_SIZE; - h->next = mmalloca_results[slot]; - mmalloca_results[slot] = p; - + char *p = + (char *)((((uintptr_t)mem + sizeof (small_t) + sa_alignment_max - 1) + & ~(uintptr_t)(2 * sa_alignment_max - 1)) + + sa_alignment_max); + /* Here p >= mem + sizeof (small_t), + and p <= mem + sizeof (small_t) + 2 * sa_alignment_max - 1 + hence p + n <= mem + nplus. + So, the memory range [p, p+n) lies in the allocated memory range + [mem, mem + nplus). */ + ((small_t *) p)[-1] = p - mem; + /* p ≡ sa_alignment_max mod 2*sa_alignment_max. */ return p; } } @@ -115,35 +82,24 @@ mmalloca (size_t n) void freea (void *p) { - /* mmalloca() may have returned NULL. */ - if (p != NULL) + /* Check argument. */ + if ((uintptr_t) p & (sa_alignment_max - 1)) { - /* Attempt to quickly distinguish the mmalloca() result - which has - a magic indicator word - and the alloca() result - which has an - uninitialized indicator word. It is for this test that sa_increment - additional bytes are allocated in the alloca() case. */ - if (((int *) p)[-1] == MAGIC_NUMBER) - { - /* Looks like a mmalloca() result. To see whether it really is one, - perform a lookup in the hash table. */ - size_t slot = (uintptr_t) p % HASH_TABLE_SIZE; - void **chain = &mmalloca_results[slot]; - for (; *chain != NULL;) - { - union header *h = p; - if (*chain == p) - { - /* Found it. Remove it from the hash table and free it. */ - union header *p_begin = h - 1; - *chain = p_begin->next; - free (p_begin); - return; - } - h = *chain; - chain = &h[-1].next; - } - } - /* At this point, we know it was not a mmalloca() result. */ + /* p was not the result of a malloca() call. Invalid argument. */ + abort (); + } + /* Determine whether p was a non-NULL pointer returned by mmalloca(). */ + if ((uintptr_t) p & sa_alignment_max) + { + void *mem = (char *) p - ((small_t *) p)[-1]; + free (mem); } } #endif + +/* + * Hey Emacs! + * Local Variables: + * coding: utf-8 + * End: + */ |