Re: GCC optimizes integer overflow: bug or feature?

[Denis Vlasenko]

On Tuesday 19 December 2006 23:39, Denis Vlasenko wrote:
> There are a lot of 100.00% safe optimizations which gcc
> can do. Value range propagation for bitwise operations, for one

Or this, absolutely typical C code. i386 arch can compare
16 bits at a time here (luckily, no alighment worries on this arch):

# cat tt.c
int f(char *p)
{
    if (p[0] == 1 && p[1] == 2) return 1;
    return 0;
}

# gcc -O2 -S -fomit-frame-pointer tt.c

# cat tt.s
        .file   "tt.c"
        .text
        .p2align 2,,3
.globl f
        .type   f, @function
f:
        movl    4(%esp), %eax
        cmpb    $1, (%eax)
        je      .L2
        xorl    %eax, %eax
        ret
        .p2align 2,,3
.L2:
        cmpb    $2, 1(%eax)
        sete    %al
        movzbl  %al, %eax
        ret
        .size   f, .-f
        .ident  "GCC: (GNU) 4.2.0 20061128 (prerelease)"
        .section        .note.GNU-stack,"",@progbits