AMDGPU: Wrong code for fcanonicalize

[hvdijk]

Please consider this minimal LLVM IR:

define half @f(half %x) {
  %canonicalized = call half @llvm.canonicalize.f16(half %x)
  ret half %canonicalized
}

Run with llc -mtriple=amdgcn and we get:

f:                                      ; @f
        s_waitcnt vmcnt(0) expcnt(0) lgkmcnt(0)
        s_setpc_b64 s[30:31]

The canonicalize operation has been entirely optimised away.

The reason for this is we get during ISel:

  t0: ch,glue = EntryToken
          t2: f32,ch = CopyFromReg # D:1 t0, Register:f32 %0
        t4: f16 = fp_round # D:1 t2, TargetConstant:i64<1>
      t5: f16 = fcanonicalize # D:1 t4
    t6: f32 = fp_extend # D:1 t5
  t8: ch,glue = CopyToReg # D:1 t0, Register:f32 $vgpr0, t6
  t9: ch = RET_GLUE # D:1 t8, Register:f32 $vgpr0, t8:1

Here, fcanonicalize is optimised away because SITargetLowering::isCanonicalized determines that fp_round is guaranteed to return an already-canonicalised result, so no work is needed, but that then leaves us with fp_extend (fp_round x, /*strict=*/1) which is optimised to a no-op.

This prevents another optimisation from going in (#80520) which makes this problem show up in more cases than it currently does, and sadly I struggle to find a good way of ensuring we get correct code for this case without also making codegen for other tests worse.

@llvm/pr-subscribers-backend-amdgpu

[hvdijk]

(Incidentally: would AMDGPU's current handling of this operation make sense to move to target-independent stuff? No other target currently implements it and the same logic should work for all, I think?)

[llvmbot]

@llvm/issue-subscribers-backend-amdgpu

Author: Harald van Dijk (hvdijk)

Please consider this minimal LLVM IR: ```llvm define half @f(half %x) { %canonicalized = call half @llvm.canonicalize.f16(half %x) ret half %canonicalized } ``` Run with `llc -mtriple=amdgcn` and we get: ```asm f: ; @f s_waitcnt vmcnt(0) expcnt(0) lgkmcnt(0) s_setpc_b64 s[30:31] ``` The `canonicalize` operation has been entirely optimised away.

The reason for this is we get during ISel:

  t0: ch,glue = EntryToken
          t2: f32,ch = CopyFromReg # D:1 t0, Register:f32 %0
        t4: f16 = fp_round # D:1 t2, TargetConstant:i64<1>
      t5: f16 = fcanonicalize # D:1 t4
    t6: f32 = fp_extend # D:1 t5
  t8: ch,glue = CopyToReg # D:1 t0, Register:f32 $vgpr0, t6
  t9: ch = RET_GLUE # D:1 t8, Register:f32 $vgpr0, t8:1

Here, fcanonicalize is optimised away because SITargetLowering::isCanonicalized determines that fp_round is guaranteed to return an already-canonicalised result, so no work is needed, but that then leaves us with fp_extend (fp_round x, /*strict=*/1) which is optimised to a no-op.

This prevents another optimisation from going in (#80520) which makes this problem show up in more cases than it currently does, and sadly I struggle to find a good way of ensuring we get correct code for this case without also making codegen for other tests worse.

@llvm/pr-subscribers-backend-amdgpu

[jayfoad]

fp_extend (fp_round x, /*strict=*/1) which is optimised to a no-op

That sounds like the bug?

[arsenm]

Is this only a problem with the implicit ABI-promoted half-to-float case? Do you see the same issue if you manually write out the casts on a modern subtarget?

(Incidentally: would AMDGPU's current handling of this operation make sense to move to target-independent stuff? No other target currently implements it and the same logic should work for all, I think?)

Partially yes, and partially no. There are some broken-ish edge cases (e.g. we assume things that could later be selected to non-canonicalizing operations are canonicalized).

[hvdijk]

fp_extend (fp_round x, /*strict=*/1) which is optimised to a no-op

That sounds like the bug?

It's not. This is what the trunc parameter of fp_round (which I mistakenly called strict) is intended for. Quoting https://llvm.org/doxygen/namespacellvm_1_1ISD.html:

X = FP_ROUND(Y, TRUNC) - Rounding 'Y' from a larger floating point type down to the precision of the destination VT.

TRUNC is a flag, which is always an integer that is zero or one. If TRUNC is 0, this is a normal rounding, if it is 1, this FP_ROUND is known to not change the value of Y.

The TRUNC = 1 case is used in cases where we know that the value will not be modified by the node, because Y is not using any of the extra precision of source type. This allows certain transformations like FP_EXTEND(FP_ROUND(X,1)) -> X which are not safe for FP_EXTEND(FP_ROUND(X,0)) because the extra bits aren't removed.

This transformation is not safe when it comes to signalling NaNs, but generally, LLVM optimisations are not intended to be signalling-NaN-safe and may leave SNaN as SNaN. fcanonicalize is special in that regard in that it must get rid of it.

Is this only a problem with the implicit ABI-promoted half-to-float case? Do you see the same issue if you manually write out the casts on a modern subtarget?

It's not limited to half:

define float @f(float %x) {
  %ext = fpext float %x to double
  %canonicalized = call double @llvm.canonicalize.f64(double %ext)
  %trunc = fptrunc double %ext to float
  ret float %trunc
}

shows the same issue, and in this case, it persists even if I try different -mcpu values.

Partially yes, and partially no. There are some broken-ish edge cases (e.g. we assume things that could later be selected to non-canonicalizing operations are canonicalized).

Ah, but for generic target-independent codegen that is fine, that would only lead to redundant canonicalisation, right? That is, it would result in correct but suboptimal codegen. That should not be a problem, provided targets have enough hooks to make sure that operations that already canonicalise on their end aren't recanonicalised.

[arsenm]

Ah, but for generic target-independent codegen that is fine, that would only lead to redundant canonicalisation, right? That is, it would result in correct but suboptimal codegen. That should not be a problem, provided targets have enough hooks to make sure that operations that already canonicalise on their end aren't recanonicalised.

The main issue I've been meaning to fix is we specially treat the generic minnum/maxnum nodes based on the knowledge of the underlying instruction canonicalization behavior (which changed in gfx9+ IIRC)

[hvdijk]

I had a thought on how to possibly fix it. It seems to work so I created a PR for it, but I'm not 100% certain it covers all the cases, I would welcome a very close look.

[arsenm]

It's not limited to half:

I looked at this example, and this one actually breaks in the IR which I think is a more serious issue. InstSimplify folds it away to just ret %x

I also think addressing this from the trunc fold doesn't fundamentally address the issue; I think we shouldn't be trying to fold away canonicalizes during combining. We probably should only do this during the selection, and then there may still be risks based on selection patterns. It would be safest to do this during a post-selection machine pass