https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121344
Bug ID: 121344
Summary: New __builtin_clz(0) when compiling with -Os
Product: gcc
Version: 16.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: kristerw at gcc dot gnu.org
Target Milestone: ---
Compiling the following on x86_64 with -Os introduces new undefined behavior.
int
foo (unsigned char b)
{
int c = 0;
while (b) {
b >>= 1;
c++;
}
return c;
}
The issue is that the loop has been transformed to use __builtin_clz, which is
UB when called with 0.
Interestingly, -O1, -O2, and -O3 also transform this to use __builtin_clz, but
they correctly protect the code with:
if (b_3(D) != 0)
So the compiler knows how to handle it correctly, but is failing to do so with
-Os.
