[AMDGPU] `int64` modulo-constant `x % 3` and divide-by-constant `x / 3` compile to 80 instructions.

[bjacob]

This is observed with -xhip targeting AMD MI300 (gfx942).

Compiler Explorer link: https://godbolt.org/z/xrfhhaaeY. For completeness, the clang flags are -O3 --cuda-device-only -x hip -nogpuinc -nogpulib --offload-arch=gfx942.

Testcase:

__attribute__((device))
int64_t a(int64_t i) {
    return i % 3;
}

This compiles to 80 instructions.

By contrast, the same testcase with int64_t replaced by int32_t compiles to just 8 instructions.

I was expecting the int64 variant to generate slightly over 2x more instructions than the int32 variant (since the target requires rewriting int64 ops into pairs of int32 ops). Not 10x.

The above Compiler Explorer link shows the same happening with i / 3 instead of i % 3.

[llvmbot]

@llvm/issue-subscribers-backend-amdgpu

Author: Benoit Jacob (bjacob)

This is observed with `-xhip` targeting AMD MI300 (`gfx942`).

Compiler Explorer link: https://godbolt.org/z/xrfhhaaeY. For completeness, the clang flags are -O3 --cuda-device-only -x hip -nogpuinc -nogpulib --offload-arch=gfx942.

Testcase:

__attribute__((device))
int64_t a(int64_t i) {
    return i % 3;
}

This compiles to 80 instructions.

By contrast, the same testcase with int64_t replaced by int32_t compiles to just 8 instructions.

I was expecting the int64 variant to generate slightly over 2x more instructions than the int32 variant (since the target requires rewriting int64 ops into pairs of int32 ops). Not 10x.

The above Compiler Explorer link shows the same happening with i / 3 instead of i % 3.

[Pierre-vh]

cc @arsenm @jayfoad this appears to be due to LowerUDIVREM64, is this expected behavior?

[arsenm]

cc @arsenm @jayfoad this appears to be due to LowerUDIVREM64, is this expected behavior?

I'd expect all of the divide-by-constant cases to get caught by the generic combiner handling of them and never reach the custom lowering

[Artem-B]

I'd expect all of the divide-by-constant cases to get caught by the generic combiner handling of them and never reach the custom lowering

Agreed. E.g. NVPTX lowers those division/modulo ops to multiplication by reciprocal: https://godbolt.org/z/MWo9hsMaW

[topperc]

cc @arsenm @jayfoad this appears to be due to LowerUDIVREM64, is this expected behavior?

I'd expect all of the divide-by-constant cases to get caught by the generic combiner handling of them and never reach the custom lowering

Divide by constant might be blocked by MULHS and SMUL_LOHI not being legal.

[Pierre-vh]

cc @arsenm @jayfoad this appears to be due to LowerUDIVREM64, is this expected behavior?

I'd expect all of the divide-by-constant cases to get caught by the generic combiner handling of them and never reach the custom lowering

Divide by constant might be blocked by MULHS and SMUL_LOHI not being legal.

That seems to be the case, and it explains why we don't get that much code when using i32 (which is legal for MULHS).

I think there are two ways to enable this transform for i64:

• Change TLI so the decision to lower or not is deferred to a overridable hook. We could then force lowering for i64, which I think would then result in the illegal MULHS being added and expanded.
• Add custom lowering for i64 MULHS, which would just call the TLI lowering hook.

The first one is nice because it's a target fix with less chances of causing side effects, but it forces a change to TLI.
The second one is contained to the AMDGPU backend but might result in i64 MULHS (or MUL_LOHI) being generated in other places where it may be less desirable.

Thoughts @arsenm ?

[arsenm]

Can we just change or remove the legality check?