Optimizer's mistake

Bug #1250883 reported by Shkarnikov Sergey on 2013-11-13
This bug affects 1 person
Affects Status Importance Assigned to Milestone
gcc-4.6 (Ubuntu)

Bug Description

OS: Ubuntu 12.04.3 LTS
gcc version: 4.6.3-1ubuntu5

It's a slightly modificated test case from gcc-4.6.3 testsuite (920612-1.c):

int main(){
 int a = 0;
 if ((int)((~0u) >> (1 + a))+1 > 0)

It operates fine after compilation with -O0 or -O1 option (exit is executed), but changes behavior if compiled with -O2 or -O3 (abort is executed). Decompilation shows that optimizer reduces the code down to abort(); . Besides, if variable "a" is absent, bug is absent even with O3 optimization. So in simple case the optimizer takes into account the overflow, but misses it after a trivial change of the code.

description: updated
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