[RISC-V] Miscompile on rv64gcv with -O[23]

[ewlu]

Testcase:

short a[12];
short b[12];
long long al;
int f;
short m = 31554;
long long q[12];
int main() {
  for (int i = 0; i < 2; ++i)
    q[i] = 6;
  for (short i = 0; i < 12; i += m - 31553) {
    a[i] = q[i];
    b[i] = f > q[i];
  }
  for (int i = 0; i < 12; ++i)
    al += a[i];
  for (int i = 0; i < 2; ++i)
    al += b[i];
  __builtin_printf("%llu\n", al);
}

Commands:

# riscv
$ ./bin/clang -march=rv64gcv_zvl256b -flto -O2 red.c -o user-config.out
$ QEMU_CPU=rv64,vlen=256,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 ./bin/qemu-riscv64 user-config.out 1
10

$ ./bin/clang -march=rv64gcv_zvl256b -flto -O3 red.c -o user-config.out
$ QEMU_CPU=rv64,vlen=256,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 ./bin/qemu-riscv64 user-config.out 1
10

# x86
$ ./native.out 1
12

Godbolt: https://godbolt.org/z/7hMW6eerT

Tried changing m - 31553 to 1 and the error disappears

Bisected to 9b7282e as the first bad commit

Found via fuzzer

[llvmbot]

@llvm/issue-subscribers-backend-risc-v

Author: Edwin Lu (ewlu)

Testcase: ```c short a[12]; short b[12]; long long al; int f; short m = 31554; long long q[12]; int main() { for (int i = 0; i < 2; ++i) q[i] = 6; for (short i = 0; i < 12; i += m - 31553) { a[i] = q[i]; b[i] = f > q[i]; } for (int i = 0; i < 12; ++i) al += a[i]; for (int i = 0; i < 2; ++i) al += b[i]; __builtin_printf("%llu\n", al); } ```

Commands:

# riscv
$ ./bin/clang -march=rv64gcv_zvl256b -flto -O2 red.c -o user-config.out
$ QEMU_CPU=rv64,vlen=256,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 ./bin/qemu-riscv64 user-config.out 1
10

$ ./bin/clang -march=rv64gcv_zvl256b -flto -O3 red.c -o user-config.out
$ QEMU_CPU=rv64,vlen=256,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 ./bin/qemu-riscv64 user-config.out 1
10

# x86
$ ./native.out 1
12

Godbolt: https://godbolt.org/z/7hMW6eerT

Tried changing m - 31553 to 1 and the error disappears

Bisected to 9b7282e as the first bad commit

Found via fuzzer

[mshockwave]

PR: #132123