[Bug target/120651] New: [15/16 Regression] RISC-V: Miscompile at -O3 with -flto since r15-3228-g771256bcb9d

[ewlu at rivosinc dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120651

            Bug ID: 120651
           Summary: [15/16 Regression] RISC-V: Miscompile at -O3 with
                    -flto since r15-3228-g771256bcb9d
           Product: gcc
           Version: 16.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ewlu at rivosinc dot com
  Target Milestone: ---

Testcase:
long long s;
short a = 3;
_Bool b[20];
unsigned long long c[20][20];
int main() {
  for (signed p = 0; p < 18; p += 4)
    for (unsigned q = 0; q < 20; q++) {
      a *= (unsigned char)2057594037927935;
      b[q] = 0;
      c[p][q] = 0;
    }
  s ^= a;
  for (int p = 0; p < 20;)
    for (int u = 0; u < 20;)
      for (; p < 20; ++p)
        for (; u < 20; ++u) {
          s ^= b[p];
          s ^= c[p][u];
        }
  __builtin_printf("%llu\n", s);
}

Commands:
# -O3
> /scratch/ewlu/daily-upstream-build/build-gcv/bin/riscv64-unknown-linux-gnu-gcc
>  -march=rv64gcv -flto -O3 -mrvv-vector-bits=zvl red.c -o user-config.out 
> -fsigned-char -fno-strict-aliasing -fwrapv 
> QEMU_CPU=rv64,vlen=128,rvv_ta_all_1s=true,rvv_ma_all_1s=true,v=true,vext_spec=v1.0,zve32f=true,zve64f=true
>  timeout --verbose -k 0.1 4 
> /scratch/ewlu/daily-upstream-build/build-gcv/bin/qemu-riscv64 user-config.out 
> 1
15875
# -O2
> /scratch/ewlu/daily-upstream-build/build-gcv/bin/riscv64-unknown-linux-gnu-gcc
>  -march=rv64gcv -flto -O2 -mrvv-vector-bits=zvl red.c -o user-config.out 
> -fsigned-char -fno-strict-aliasing -fwrapv 
> QEMU_CPU=rv64,vlen=128,rvv_ta_all_1s=true,rvv_ma_all_1s=true,v=true,vext_spec=v1.0,zve32f=true,zve64f=true
>  timeout --verbose -k 0.1 4 
> /scratch/ewlu/daily-upstream-build/build-gcv/bin/qemu-riscv64 user-config.out 
> 1
18446744073709540355

Also appears when swapping -flto for -fwhole-program

Found via fuzzer