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List:       gcc
Subject:    Suboptimal code (gcc 2.95.1, i386-unknown-openbsd)
From:       Marc Espie <espie () schutzenberger ! liafa ! jussieu ! fr>
Date:       1999-09-09 13:49:10
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Fiddling with optimization...

#define VAL 10
void f(char *s)
	{
	char *u = s;

	while (u < s+VAL)
		*u++='a';
	}


Using -O3, I get:

	.file	"a.c"
gcc2_compiled.:
___gnu_compiled_c:
.text
	.align 2,0x90
.globl _f
	.type	_f , @function
_f:
	pushl %ebp
	movl %esp,%ebp
	movl 8(%ebp),%eax
	leal 10(%eax),%edx
	cmpl %edx,%eax
	jae L4
	.align 2,0x90
L5:
	movb $97,(%eax)
	incl %eax
	cmpl %edx,%eax
	jb L5
L4:
	leave
	ret
	.size	_f , . - _f

Notice the comparison and jump to L4 that will never occur.

Is gcc required to assume that pointer arithmetic may wrap around,
or is it just some optimization it misses ?

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