[Bug target/108764] New: [RISCV] Cost model for RVB is too aggressive

[sinan.lin at linux dot alibaba.com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108764

            Bug ID: 108764
           Summary: [RISCV] Cost model for RVB is too aggressive
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: sinan.lin at linux dot alibaba.com
  Target Milestone: ---

The cost model of instructions like shNadd, add.uw, ... are set to
COSTS_N_INSNS (1), which brings bad codegen in some cases.


Here is a test case for the problem.

```
#include <stdint.h>
int64_t foo (void * ptr, int value, int64_t num);

int64_t clear1 (int64_t *array, int64_t *end, int64_t size)
{
    return array + size < end ? foo(array, 0, size * 8) : 0;
}
```

generates asm with -march=rv64gc_zba -O3
```
clear1:
        slli    a4,a2,3
        sh3add  a5,a2,a0
        bgtu    a1,a5,.L4
        li      a0,0
        ret
.L4:
        mv      a2,a4
        li      a1,0
        tail    foo
```

generates asm with -march=rv64gc -O3
```
clear1:
        slli    a2,a2,3
        add     a5,a0,a2
        bgtu    a1,a5,.L4
        li      a0,0
        ret
.L4:
        li      a1,0
        tail    foo
```