https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111791
Bug ID: 111791
Summary: RISC-V: Strange loop vectorizaion on popcount function
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: kito at gcc dot gnu.org
Target Milestone: ---
Symptom:
A typical popcount implementation with Brian Kernighan’s algorithm, vectorizer
has recognized that as popcount, but...come with strange vectorization result,
I know that might because I add -fno-vect-cost-model, but I still don't
understand why it vectorized, so I guess maybe it's something worth to report.
NOTE:
Those bad/strange code gen will gone once scalar popcount instruction
available.
Case:
```
int popcount(unsigned long value)
{
int nbits;
for (nbits = 0; value != 0; value &= value - 1)
nbits++;
return nbits;
}
```
Command to reproduce:
```
$ riscv64-unknown-linux-gnu-gcc x.c -march=rv64gcv -o - -S -fno-vect-cost-model
-O3
```
Sha1: g:faae30c49560f1481f036061fa2f894b0f7257f8 (some random point of top of
trunk)
Current output:
```
.globl popcount
.type popcount, @function
popcount:
.LFB0:
.cfi_startproc
beq a0,zero,.L4
addi sp,sp,-16
.cfi_def_cfa_offset 16
sd ra,8(sp)
.cfi_offset 1, -8
call __popcountdi2
csrr a2,vlenb
sext.w a0,a0
srli a2,a2,2
vsetvli a3,zero,e32,m1,ta,ma
vid.v v1
.L3:
vsetvli a5,a0,e8,mf4,ta,ma
sub a0,a0,a5
vsetvli a3,zero,e32,m1,ta,ma
vmv1r.v v3,v1
vmv.v.x v2,a2
vadd.vv v1,v1,v2
bne a0,zero,.L3
ld ra,8(sp)
.cfi_restore 1
addi a5,a5,-1
vadd.vi v3,v3,1
vslidedown.vx v3,v3,a5
addi sp,sp,16
.cfi_def_cfa_offset 0
vmv.x.s a0,v3
jr ra
.L4:
li a0,0
ret
.cfi_endproc
.LFE0:
.size popcount, .-popcount
```
