https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88972
Bug ID: 88972
Summary: popcnt of limited 128-bit number with unnecessary
zeroing
Product: gcc
Version: 9.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: drepper.fsp+rhbz at gmail dot com
Target Milestone: ---
Compile the following code on x86-64 with -Ofast -march=haswell:
int f(__uint128_t m)
{
if (m < 64000)
return __builtin_popcount(m);
return -1;
}
The generated code with the trunk gcc looks like this:
0: b8 ff f9 00 00 mov $0xf9ff,%eax
5: 48 39 f8 cmp %rdi,%rax
8: b8 00 00 00 00 mov $0x0,%eax
d: 48 19 f0 sbb %rsi,%rax
10: 72 0e jb 20 <f+0x20>
12: 31 c0 xor %eax,%eax
14: f3 0f b8 c7 popcnt %edi,%eax
18: c3 retq
19: 0f 1f 80 00 00 00 00 nopl 0x0(%rax)
20: b8 ff ff ff ff mov $0xffffffff,%eax
25: c3 retq
The instruction at offset 12 is unnecessary. I guess this is a left-over from
the popcnt of the upper half which is recognized to be unnecessary and left
out. There is no addition anymore but somehow the register clearing survived.
