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Old 03-31-2010, 11:38 PM
Pierre Habouzit
Default Bug#576111: gcc-4.4 miscompiles __builtin_expect in -O0

Package: gcc-4.4
Version: 4.4.3-4
Severity: grave

Since gcc-4.4 version 4.4.3-4 (and yes -5 is still affected), gcc miscompiles
__builtin_expect when no optimization is set (at least).

Test case:

int foo(int t) {
if (__builtin_expect(t & 0x100, 0))
return 0;
return 1;

Bad assembly:

gcc -O0 -S -o /dev/stdout a.c
.file "a.c"
.globl foo
.type foo, @function
pushq %rbp
.cfi_def_cfa_offset 16
movq %rsp, %rbp
.cfi_offset 6, -16
.cfi_def_cfa_register 6
movl %edi, -4(%rbp)
movl -4(%rbp), %eax
andl $256, %eax
movzbl %al, %eax <---------------------
testq %rax, %rax
je .L2
movl $0, %eax
jmp .L3
movl $1, %eax
.size foo, .-foo
.ident "GCC: (Debian 4.4.3-5) 4.4.3"
.section .note.GNU-stack,"",@progbits

The buggy line is marked with the arrow.
gcc-4.4 version 4.4.3-3 is correct and doesn't perform the buggy movzbl.

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