[Bug target/95861] New: popcnt --- unnecessary leading xor instruction

[zero at smallinteger dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95861

            Bug ID: 95861
           Summary: popcnt --- unnecessary leading xor instruction
           Product: gcc
           Version: 9.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: zero at smallinteger dot com
  Target Milestone: ---

Created attachment 48778
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=48778&action=edit
sample code

Consider the attached code, compiled with -O3 -mpopcnt.  The assembly produced
in gcc 9.3 is below (the same code is generated by gcc 10.x at godbolt).

foo(unsigned long long):
        xor     eax, eax
        popcnt  rax, rdi
        ret


The popcnt instruction will overwrite rax.  Whether eax is cleared with xor has
no effect.  No xor instruction should be emitted in this case.  For reference,
trunk clang emits this code instead:

foo(unsigned long long):                                # @foo(unsigned long
long)
        popcnt  rax, rdi
        ret


This second code sample is as expected.