https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114809
Bug ID: 114809
Summary: [RISC-V RVV] Counting elements might be simpler
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: wojciech_mula at poczta dot onet.pl
Target Milestone: ---
Consider this simple procedure
---
#include <cstdint>
#include <cstdlib>
size_t count_chars(const char *src, size_t len, char c) {
size_t count = 0;
for (size_t i=0; i < len; i++) {
count += src[i] == c;
}
return count;
}
---
Assembly for it (GCC 14.0, -march=rv64gcv -O3):
---
count_chars(char const*, unsigned long, char):
beq a1,zero,.L4
vsetvli a4,zero,e8,mf8,ta,ma
vmv.v.x v2,a2
vsetvli zero,zero,e64,m1,ta,ma
vmv.v.i v1,0
.L3:
vsetvli a5,a1,e8,mf8,ta,ma
vle8.v v0,0(a0)
sub a1,a1,a5
add a0,a0,a5
vmseq.vv v0,v0,v2
vsetvli zero,zero,e64,m1,tu,mu
vadd.vi v1,v1,1,v0.t
bne a1,zero,.L3
vsetvli a5,zero,e64,m1,ta,ma
li a4,0
vmv.s.x v2,a4
vredsum.vs v1,v1,v2
vmv.x.s a0,v1
ret
.L4:
li a0,0
ret
---
The counting procedure might use `vcpop.m` instead of updating vector of
counters (`v1`) and summing them in the end. This would move all mode switches
outside the loop.
And there's a missing peephole optimization:
li a4,0
vmv.s.x v2,a4
It can be:
vmv.s.x v2,zero
