https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85560
Bug ID: 85560
Summary: Missed optimization in bit-by-bit variable zeroing
Product: gcc
Version: 9.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: vlad at ivanov dot email
Target Milestone: ---
The following C++ code loops until a variable is cleared bit-by-bit:
#include <cstdint>
uint32_t b;
uint32_t test() {
uint32_t a = b;
uint32_t bits = 1;
while (a != 0) {
a &= ~bits;
bits <<= 1;
}
return 0;
}
After looping, it returns 0. While the loop is useless, it still exists in the
assembly even with -O3.
GCC version: 9.0.0 20180427
Demo: https://godbolt.org/g/7gRMqu
Notably, the loop is optimized away on ICC 18
