https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98961
Bug ID: 98961
Summary: Failure to optimize successive comparisons with 0 into
clz
Product: gcc
Version: 11.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: gabravier at gmail dot com
Target Milestone: ---
int f(int i, int j)
{
return i == 0 || j == 0;
}
This can be optimized to `return (__builtin_clz(i) | __builtin_clz(j)) >> 5;`
(if `clz(0)` returns 0). LLVM does this transformation, but GCC does not.
On x86, for example, with lzcnt, while this does not seem to be a net win in
terms of performance (at least, not for this code alone), it also simply not a
loss. As it is a win in terms of code size (which should make it a net win in
most situations), I think that should make it a net win overall in actual code.
See also https://gcc.gnu.org/bugzilla/show_bug.cgi?id=10588
