[Bug target/117953] New: [15 Regression] GCC miscompile rvv intrinsics at `-O2` and `-O3`, missing `csrwi` to modify `vxrm`

[rvismith1115 at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117953

            Bug ID: 117953
           Summary: [15 Regression] GCC miscompile rvv intrinsics at `-O2`
                    and `-O3`, missing `csrwi` to modify `vxrm`
           Product: gcc
           Version: 15.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rvismith1115 at gmail dot com
  Target Milestone: ---

Created attachment 59818
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=59818&action=edit
reduced testcase

An test case example:
```
#include <riscv_vector.h>
void abort(void);
#define dataLen 10
int16_t a[dataLen]; int16_t b[dataLen];

int main(){
  for (int i = 0; i < dataLen; ++i) { a[i] = i * 5; }
  for (int i = 0; i < dataLen; ++i) { b[i] = 0; }
  int avl = dataLen;
  int16_t* ptr_a = a; int16_t* ptr_b = b;
  for (size_t vl; avl > 0; avl -= vl){
    vl = __riscv_vsetvl_e64m1(avl);
    vint16mf4_t va = __riscv_vle16_v_i16mf4(ptr_a, vl);
    va = __riscv_vasub_vx_i16mf4(va, 0, __RISCV_VXRM_RDN, vl); //round-down
(truncate) 
    __riscv_vse16_v_i16mf4(ptr_b, va, vl);
    ptr_a += vl; ptr_b += vl;
  }
  for(int i=0; i<dataLen; ++i) { 
    if(  (int)a[i]/2 != b[i]  ) abort();  
    __builtin_printf("%d ", b[i]); 
  } __builtin_printf("\n");
  return 0;
}
```

Flag and version info:
```
$ riscv64-unknown-elf-gcc --version
gcc () 15.0.0 20241117 (experimental)
Copyright (C) 2024 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
$ riscv64-unknown-elf-gcc -O3 -march=rv64gcv_zvfh -mabi=lp64d -Wno-psabi
-static bug.c
```

By saving intermediate files using `-save-temps`, I find there are no
instructions to modify `vxrm` register like `csrwi        vxrm,2`. The assembly
file is uploaded (by snapshot 20241117 at -O3).