https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104444
Bug ID: 104444
Summary: Missing constant folding in shift expression.
Product: gcc
Version: 11.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: cassio.neri at gmail dot com
Target Milestone: ---
#include <cstdint>
inline bool f(uint32_t m, int n) {
return (m >> n) != 0;
}
bool g(int n) {
return f(1 << 24, n);
}
g can be optimised to "return n <= 24". LLVM does that but gcc doesn't.
The example above drove me to another missing optimisation opportunity based on
undefined behaviour. (Perhaps a matter for other report?)
bool h(uint32_t m, int n) {
return (n >= 0 && n < 32) || (m >> n) != 0;
}
If (n >= 0 && n < 32) is false, then (m >> n) is UB (in C++, probably also in
C). Therefore, h can be optimised to "return true" but gcc doesn't do that
(neither does LLVM).
See here: https://godbolt.org/z/hx9vGe6Kj
If confirmed, these bugs could be added to
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=19987
Potentially related:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95817
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94789#c1
