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.
