https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120141
Bug ID: 120141
Summary: [RVV] Noop are not removed
Product: gcc
Version: 16.0
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: ---
I observed that RVV noops, like shifting by 0 or adding 0, are not removed from
the program.
I fully understand that a compiler cannot do it when `vsetvli` changes the mode
between operations. But in the sample program `v8` is written and then shifted
under the same vector mode.
Is there any reason that comes from the RVV spec which is not obvious?
Sample program:
---
#include <riscv_vector.h>
vuint16m1_t naive_avg(vuint16m1_t x, vuint16m1_t y) {
const auto vl = __riscv_vsetvlmax_e16m1();
const auto a = __riscv_vadd(x, y, vl);
return __riscv_vsrl(a, 0, vl);
}
---
Compiled with `-O3 -march=rv64gcv` yield the following assembly:
---
naive_avg(__rvv_uint16m1_t, __rvv_uint16m1_t):
vsetvli a5,zero,e16,m1,ta,ma
vadd.vv v8,v8,v9
vsrl.vi v8,v8,0
ret
---
