Optimizer bug or undefined behavior?

Question

I have the following code:

#include <math.h> #include <stdio.h> using u16 = unsigned short int; using s16 = signed short int; using s32 = signed int; s16 Algorithm(s16 sample) { s32 absSample = static_cast<s32>(abs(sample)); s32 sampleBits = absSample >> 7; s32 sampleMasked = absSample & 0x7F; s16 result = (u16)(sampleMasked << sampleBits) | (u16)(1 << (sampleBits - 2)); if (sample < 0) { result = -result; } return result; } int main() { s16 result = Algorithm(-63); if (result == -63) { printf("OK!\n"); return 0; } else { printf("BUG!?\n"); return 1; } }

In x64 debug it prints "OK!" but in x64 release it prints "BUG!?", is there a problem with my code or is the compiler doing something wrong here? How can I fix or workaround this issue?

Just making sure, do you have the hotfix for update 3 (assuming you are using MSVC 2015)? — user975989
– user975989, Commented Mar 23, 2017 at 22:54
I'm on update 2 currently, is this something fixed in update 3? — paulm
– paulm, Commented Mar 23, 2017 at 22:55
Those types should probably be uint16_t, int16_t, and int32_t, right? Since signed int isn't guaranteed to be 32 bits? — Mooing Duck
– Mooing Duck, Commented Mar 23, 2017 at 22:56
x64 Release (all the default settings) with MSVC 2015 Update 3 seems to give OK. — Rudolfs Bundulis
– Rudolfs Bundulis, Commented Mar 23, 2017 at 23:00

Rudolfs Bundulis · Accepted Answer · 2017-03-23 23:12:25Z

4

Since the value -63 triggers undefined behavior (sampleBits will be 0 thus you will be shifting -2 places) anything can happen:)

answered Mar 23, 2017 at 23:12

Rudolfs Bundulis

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paulm Over a year ago

Added a check so the shift value is limited to 0 and both now produce the correct value

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