https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101653
Bug ID: 101653
Summary: Bad code generated when optimizing nested for loops
Product: gcc
Version: 11.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: phd at phd dot re
Target Milestone: ---
Not sure if this was already reported.
Bad code is generated when optimizing nested for loops:
int main(void)
{
unsigned a, b, c = 0; // issue only occurs for unsigned
for (a = 0; a < 10; a++) // upper limit for variable a doesn't really
matter
{
for (b = 0; b < 2; b++) // bug only for b < 2, no issue for other
limits
{
c++; // first iteration: a == 0, b == 0, c == 1
if (c < a) // c will *never* be smaller than a from now on
{
return 123; // and yet this is somehow reachable with -O1/2/3
}
}
}
return 0; // with -O0 this returns 0 properly
}
Affected gcc versions: 9.1+ (including 11.1 and trunk)
Not affected versions: 8.5-
Bug occurs for -O1, -O2, -O3 (return 123)
No issue for -O0 (properly returns 0)
Both C and C++ are affected.
Live demo:
https://godbolt.org/z/nanc9osq5
