https://gcc.gnu.org/bugzilla/show_bug.cgi?id=73434
Bug ID: 73434
Summary: [6 Regression] Wrong code with casting, branches and
aliasing
Product: gcc
Version: 6.1.1
Status: UNCONFIRMED
Keywords: wrong-code
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: gcc at thecybershadow dot net
Target Milestone: ---
/////////////////// test.c //////////////////
#include <assert.h>
typedef struct { unsigned char x; } A;
typedef struct { unsigned char x; } B;
int idx = 0;
A objs[1] = {{0}};
int main()
{
B *b = (B*)&objs[idx];
b->x++;
if (b->x) // Always true
b->x = 0; // Always executed
assert(!b->x); // b->x should always be 0
return 0;
}
/////////////////////////////////////////////
Works fine with:
gcc -O1
gcc -O2 -fno-strict-aliasing
Assert fails with:
gcc -O2
gcc -O1 -fstrict-aliasing
Even though -fno-strict-aliasing makes a difference, the variable is accessed
through a single pointer.
Reproducible with:
- 6.1.0 on i686-w64-mingw32 from msys2
- 6.1.1 on x86_64-pc-linux-gnu from Arch Linux
- 6.1.1 on arm-linux-gnueabihf from ppa:ubuntu-toolchain-r/test
Not reproducible with 5.4.0.
