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While writing an optimized ftol function I found some very odd behaviour in GCC 4.6.1 . Let me show you the code first (for clarity I marked the differences): fast_trunc_one, C: int fast_trunc_one(int i) { int mantissa, exponent, sign, r; mantissa = (i & 0x07fffff) | 0x800000; exponent = 150 - ((i >> 23) & 0xff); sign = i & 0x80000000; if (exponent < 0) { r = mantissa << -exponent; /* diff */ } else { r = mantissa >> exponent; /* diff */ } return (r ^ -sign) + sign; /* diff */ } fast_trunc_two, C: int fast_trunc_two(int i) { int mantissa, exponent, sign, r; mantissa = (i & 0x07fffff) | 0x800000; exponent = 150 - ((i >> 23) & 0xff); sign = i & 0x80000000; if (exponent < 0) { r = (mantissa << -exponent) ^ -sign; /* diff */ } else { r = (mantissa >> exponent) ^ -sign; /* diff */ } return r + sign; /* diff */ } Seems the same right? Well GCC disagrees. After compiling with gcc -O3 -S -Wall -o test.s test.c this is the assembly output: fast_trunc_one, generated: _fast_trunc_one: LFB0: .cfi_startproc movl 4(%esp), %eax movl $150, %ecx movl %eax, %edx andl $8388607, %edx sarl $23, %eax orl $8388608, %edx andl $255, %eax subl %eax, %ecx movl %edx, %eax sarl %cl, %eax testl %ecx, %ecx js L5 rep ret .p2align 4,,7 L5: negl %ecx movl %edx, %eax sall %cl, %eax ret .cfi_endproc fast_trunc_two, generated: _fast_trunc_two: LFB1: .cfi_startproc pushl %ebx .cfi_def_cfa_offset 8 .

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