SelectionDAG: target-specific simplification of generic nodes using demanded bits

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All,

The AMDGPU target has a 24-bit multiply instruction. In SimplifyDemandedBits I’d like to be able to turn a generic i32 ISD::MUL into AMDGPUISD::MUL_I24 if only the low order 24 bits are demanded. Currently it seems like there’s no way to do that, because the target hook SimplifyDemandedBitsForTargetNode is only called for target-specific nodes, not for generic nodes like MUL.

Would it be acceptable to call the target hook for generic nodes as well? Here’s a patch to show the general idea: https://reviews.llvm.org/D89964
(It probably needs a bit of polish, e.g. the name “SimplifyDemandedBitsForTargetNode” is misleading now.)

In the future perhaps “demanded bits” could become a cached analysis that could be queried from anywhere. Then I could write a target-specific DAG combine for MUL that would query the demanded bits for the node to do this transformation.

Thanks,
Jay.

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X86 would definitely benefit from this as we often use generic opcodes as part of more complex patterns that could then be handled through SimplifyDemandedBits/SimplifyDemandedVectorElts/SimplifyMultipleUse.

Is this likely to slow compile times down?

I’ve tried but struggled to come up with an acceptable way to determine the accumulated demanded bits/elts across all users of a SDValue - I was always worried that caching would miss nodes that had recently been added/removed.

I’ve often thought it a shame that the standard combines don’t include the demandedbits/elts masks directly instead of having a separate SimplifyDemanded* mechanism.

Simon.