[Bug target/120813] New: [15/16 Regression] RISC-V: Miscompile at -O[23]

[ewlu at rivosinc dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120813

            Bug ID: 120813
           Summary: [15/16 Regression] RISC-V: Miscompile at -O[23]
           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:
short s (short t, short u) { return u == 0 ? 0 : t / u; }
int x[6];
int y;
unsigned ak = 1;
unsigned short al = 65527;
unsigned *am = &ak;
int main() {
  for (int i = 0; i < 6; i++) {
    x[i] = i;
  }
  for (;;) {
    unsigned ar = 2080554998;
    char as = 4;
    if (s(34, al++) < ar)
      if (*am)
        break;
  }
  y = x[al & 5];
  __builtin_printf("...checksum after hashing %lX\n", y ^ 5UL);
}

Commands:
# -O3
> /scratch/ewlu/daily-upstream-build/build-gcv/bin/riscv64-unknown-linux-gnu-gcc
>  -O3 ./runtime-bisect/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
...checksum after hashing 5
# -O2
> /scratch/ewlu/daily-upstream-build/build-gcv/bin/riscv64-unknown-linux-gnu-gcc
>  -O2 ./runtime-bisect/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
...checksum after hashing 5
# -O1
> /scratch/ewlu/daily-upstream-build/build-gcv/bin/riscv64-unknown-linux-gnu-gcc
>  -O1 ./runtime-bisect/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
...checksum after hashing 4

Bisected to r15-9239-g4d7a634f6d4 as first bad commit. I'm not sure if this is
the same error as pr120242 or if it's a different issue. Posting it just in
case. If it is a dupe, this is another testcase

Found via fuzzer.