https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97503
Bug ID: 97503
Summary: Suboptimal use of cntlzw and cntlzd
Product: gcc
Version: 10.1.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: christophe.leroy at csgroup dot eu
Target Milestone: ---
int f(int x)
{
return x ? __builtin_clz(x) : 32;
}
Is built as
00000000 <f>:
0: 2c 03 00 00 cmpwi r3,0
4: 40 82 00 0c bne 10 <f+0x10>
8: 38 60 00 20 li r3,32
c: 4e 80 00 20 blr
10: 7c 63 00 34 cntlzw r3,r3
14: 4e 80 00 20 blr
I would expect
00000000 <f>:
0: 7c 63 00 34 cntlzw r3,r3
4: 4e 80 00 20 blr
Because cntlzw (Count Leading Zeros Word) is documentated in powerpc
instruction set as returning 0 to 32 inclusive
The same applies to the 64 bits version:
long f(long x)
{
return x ? __builtin_clzll(x) : 64;
}
0000000000000000 <.f>:
0: 2c 23 00 00 cmpdi r3,0
4: 41 82 00 0c beq 10 <.f+0x10>
8: 7c 63 00 74 cntlzd r3,r3
c: 4e 80 00 20 blr
10: 38 60 00 40 li r3,64
14: 4e 80 00 20 blr
