https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99046
Bug ID: 99046
Summary: [[gnu::const]] function needs noexcept to be
recognized as loop invariant
Product: gcc
Version: 10.2.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: glisse at gcc dot gnu.org
Target Milestone: ---
(from https://stackoverflow.com/q/66100945/1918193)
double x[1000] = {};
[[gnu::const]] double* g(double* var);
void f() {
for (int i = 1; i < 1000; i++) {
g(x)[i] = (g(x)[i-1] + 1.0) * 1.001;
}
}
g++ -O3 eliminates half of the calls to g, but fails to move it to a single
call before the loop, while llvm does just that. Gcc does manage it if I mark f
as noexcept or nothrow. Whether const functions may throw seems debatable, but
if they do throw, I expect them to do so consistently, and since the loop has
at least one iteration and starts with this call, the transformation seems safe
to me.
