[llvm] r370576 - [DAGCombiner] Don't create illegal narrow stores

[Roman Lebedev via llvm-commits]

Test missing.

On Sat, Aug 31, 2019 at 1:44 PM James Molloy via llvm-commits
<llvm-commits at lists.llvm.org> wrote:
>
> Author: jamesm
> Date: Sat Aug 31 03:46:16 2019
> New Revision: 370576
>
> URL: http://llvm.org/viewvc/llvm-project?rev=370576&view=rev
> Log:
> [DAGCombiner] Don't create illegal narrow stores
>
> Narrowing stores when the target doesn't support the narrow version
> forces the target to expand into a load-modify-store sequence, which
> is highly suboptimal. The information narrowing throws away (legality
> of the inverse transform) is hard to re-analyze. If the target doesn't
> support a store of the narrow type, don't narrow even in pre-legalize
> mode.
>
> No test as this is DAGCombiner and depends on target bits.
>
> Modified:
>     llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
>
> Modified: llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp?rev=370576&r1=370575&r2=370576&view=diff
> ==============================================================================
> --- llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp (original)
> +++ llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp Sat Aug 31 03:46:16 2019
> @@ -14737,10 +14737,15 @@ ShrinkLoadReplaceStoreWithStore(const st
>
>    // Check that it is legal on the target to do this.  It is legal if the new
>    // VT we're shrinking to (i8/i16/i32) is legal or we're still before type
> -  // legalization.
> -  MVT VT = MVT::getIntegerVT(NumBytes*8);
> +  // legalization (and the target doesn't explicitly think this is a bad idea).
> +  MVT VT = MVT::getIntegerVT(NumBytes * 8);
> +  const TargetLowering &TLI = DAG.getTargetLoweringInfo();
>    if (!DC->isTypeLegal(VT))
>      return SDValue();
> +  if (St->getMemOperand() &&
> +      !TLI.allowsMemoryAccess(*DAG.getContext(), DAG.getDataLayout(), VT,
> +                              *St->getMemOperand()))
> +    return SDValue();
>
>    // Okay, we can do this!  Replace the 'St' store with a store of IVal that is
>    // shifted by ByteShift and truncated down to NumBytes.
>
>
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