https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104375
Bug ID: 104375
Summary: [x86] Failure to recognize bzhi patter nwhen shr is
present
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: gabravier at gmail dot com
Target Milestone: ---
#include <stdint.h>
uint64_t bextr_u64(uint64_t w, unsigned off, unsigned int len)
{
return (w >> off) & ((1U << len) - 1U);
}
With -mbmi2, this can be optimized to using shrx followed by bzhi. This
transformation is done by LLVM, but not by GCC.
PS: Even in the case where the shr is removed and thus the bzhi pattern is
recognized (e.g. `return w & ((1U << len) - 1U);`), it is still not compiled
optimally as it for some reason decides to put the result of the bzhi in an
intermediary register before moving it to eax.
