[Bug target/23451] New: [3.4 4.0 regression] Redundant reloading from stack frame

[debian-gcc at lists dot debian dot org]

[forwarded from http://bugs.debian.org/301746]

  Matthias

Rechecked with gcc-4.0, bug submitter writes:

This is a regression from gcc-3.2 in that gcc-3.2 does not have this
problem bug gcc-3.3 does.

The attached program generates the following code on i386 with
gcc -S -O2:

...

.L2:
	movl	-16(%ebp), %eax
	xorl	%esi, %esi
	testl	%eax, %eax
	je	.L8

As you can see %eax now contains the value in -16(%ebp).

	subl	$12, %esp
	movl	-16(%ebp), %eax

gcc-3.4 decides to reload for no reason at all.

	pushl	%eax
	call	strlen
	movl	%eax, %esi
	addl	$16, %esp
.L5:
	pushl	%eax
	pushl	%edi
	pushl	%ebx
	leal	2(%edi,%esi), %eax
	pushl	%eax
	call	malloc
	movl	%eax, (%esp)
	movl	%eax, %ebx
	call	mempcpy
	movl	-16(%ebp), %edi

Again the value of -16(%ebp) is in %edi.

	addl	$16, %esp
	testl	%edi, %edi
	je	.L6
	movb	$61, (%eax)
	pushl	%ecx
	pushl	%esi
	movl	-16(%ebp), %edx

gcc-3.4 decides to reload it yet again.

	pushl	%edx
	incl	%eax
	pushl	%eax
	call	mempcpy
	addl	$16, %esp


-----------------------------------------------

typedef unsigned int size_t;
extern size_t strlen (__const char *__s) __attribute__ ((__pure__));
extern char *strchrnul (__const char *__s, int __c) __attribute__ ((__pure__));
extern void *mempcpy (void *__restrict __dest,
        __const void *__restrict __src, size_t __n) ;
extern void *malloc(size_t);

extern void setvareq(char *s, int flags);
extern char *endofname(const char *);
extern void sh_error(const char *, ...) __attribute__ ((__noreturn__));

void
setvar(const char *name, const char *val, int flags)
{
 char *p, *q;
 size_t namelen;
 char *nameeq;
 size_t vallen;

 q = endofname(name);
 p = strchrnul(q, '=');
 namelen = p - name;
 if (!namelen || p != q)
  sh_error("%.*s: bad variable name", namelen, name);
 vallen = 0;
 if (val == ((void *)0)) {
  flags |= 2;
 } else {
  vallen = strlen(val);
 }
 p = mempcpy(nameeq = malloc(namelen + vallen + 2), name, namelen);
 if (val) {
  *p++ = '=';
  p = mempcpy(p, val, vallen);
 }
 *p = '\0';
 setvareq(nameeq, flags | 1);
}

-- 
           Summary: [3.4 4.0 regression] Redundant reloading from stack
                    frame
           Product: gcc
           Version: 4.0.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: target
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: debian-gcc at lists dot debian dot org
                CC: gcc-bugs at gcc dot gnu dot org,herbert at gondor dot
                    apana dot org dot au
GCC target triplet: i486-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23451