https://bugs.llvm.org/show_bug.cgi?id=40611
Bug ID: 40611
Summary: Missed optimization opportunity: transform comparisons
with constants to a mask test
Product: clang
Version: trunk
Hardware: All
OS: All
Status: NEW
Severity: enhancement
Priority: P
Component: LLVM Codegen
Assignee: unassignedclangb...@nondot.org
Reporter: nok.ra...@gmail.com
CC: llvm-bugs@lists.llvm.org, neeil...@live.com,
richard-l...@metafoo.co.uk
LLVM does not recognize that:
bool zot(unsigned i) {
return i == 0 || i == 2;
}
Can be rewritten as:
bool zot_handopt(unsigned i) {
return !(~2u & i);
}
https://rise4fun.com/Alive/PXsF
With more values the optimization even more useful:
bool ztfos(unsigned i) {
return i == 0 || i == 2 || i == 4 || i == 6;
}
bool ztfos_handopt(unsigned i) {
return !(~6u & i);
}
More complex example that greatly benefits from such optimization:
bool is_sign(char c) {
return c == '-' || c == '+';
}
bool is_sign_handopt(char c) {
unsigned char i = c;
i -= '+';
return !(~2u & i);
}
https://rise4fun.com/Alive/nKIp
GCC and MSVC recognize and optimize all the examples above
https://godbolt.org/z/_Pmy0S
The problem is not target specific (checked x86, x86-64, ppc32, ppc64, arm, and
arm64).
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