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Gcc 3.1 (-O) now generates broken inline code for memset in some cases.

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This broke newfs (newfs left some garbage in a bitmap).

The ASM for:

	#include <string.h>
	int x, foo[100];
	main()
	{
		memset(&foo[0], 0, x);
	}

is (at least if you have fixed function alignment):

	.file	"z.c"
	.text
	.p2align 2,,3
.globl main
	.type	main,@function
main:
	pushl	%ebp
	movl	%esp, %ebp
	pushl	%edi
	pushl	%eax
	movl	x, %ecx
	xorl	%eax, %eax
	shrl	$2, %ecx
	movl	$foo, %edi
	cld
	rep
	stosl
	andl	$-16, %esp
				<-- the lower bits of `len' should be loaded
				    near here
	testl	$2, %edi	<-- this seems to be meant to test the 2^1
				    bit in `len' (not alignment of the pointer
				    like it actually does).  %edi is the wrong
				    register for holding the bits, since it is
				    still needed for the pointer.
	je	.L2
	stosw
.L2:
	testl	$1, %edi	<-- similarly for the 2^0 bit.
	je	.L3
	stosb
.L3:
	movl	-4(%ebp), %edi
	leave
	ret
.Lfe1:
	.size	main,.Lfe1-main
	.comm	foo,400,32
	.comm	x,4,4
	.ident	"GCC: (GNU) 3.1 [FreeBSD] 20020509 (prerelease)"

This seems to only result in (len % 3) bytes not being cleared, since gcc
doesn't seem to use the builtin memset unless it knows that the pointer is
aligned.  If %edi could be misaligned, then too many bytes would be set.

Submitted by:	BDE
This commit is contained in:
obrien 2002-06-04 18:04:27 +00:00
parent 845c7be0e8
commit d047bea9fd
1 changed files with 7 additions and 0 deletions
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7
contrib/gcc/builtins.c
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@ -19,6 +19,8 @@ along with GCC; see the file COPYING. If not, write to the Free
Software Foundation, 59 Temple Place - Suite 330, Boston, MA
02111-1307, USA. */
/* $FreeBSD$ */
#include "config.h"
#include "system.h"
#include "machmode.h"
@ -2194,6 +2196,11 @@ expand_builtin_memset (exp, target, mode)
len_rtx = expand_expr (len, NULL_RTX, VOIDmode, 0);
/* Give up for non-constant lengths. They are broken on at least
i386's. */
if (GET_CODE (len_rtx) != CONST_INT)
return 0;
dest_mem = get_memory_rtx (dest);
set_mem_align (dest_mem, dest_align);
dest_addr = clear_storage (dest_mem, len_rtx);