[llvm] [DAGCombiner] Add generic DAG combine for ISD::PARTIAL_REDUCE_MLA (PR #127083)

[James Chesterman via llvm-commits]

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@@ -12497,6 +12501,50 @@ SDValue DAGCombiner::visitMHISTOGRAM(SDNode *N) {
   return SDValue();
 }
 
+SDValue DAGCombiner::visitPARTIAL_REDUCE_MLA(SDNode *N) {
+  // Makes PARTIAL_REDUCE_MLA(Acc, MUL(EXT(MulOpLHS), EXT(MulOpRHS)), Splat(1))
+  // into PARTIAL_REDUCE_MLA(Acc, MulOpLHS, MulOpRHS)
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JamesChesterman wrote:

Nevermind sorry I've spotted a problem with what I've done.
At the start of `visitPARTIAL_REDUCE_MLA` I've checked whether `N->getOperand(1)` is legal or custom, when really I should be checking the unextended input type to the `MUL` operation, as this is what really matters when lowering.
The only issue is, to check this, I would need to make sure that there are extends on both of the MUL operands and that these are the same signedness etc before performing the `MUL` fold, which is repeating code a bit / I'd move a lot of code from the `foldExtendPARTIAL_REDUCE_MLA` function to before the `foldMulPARTIAL_REDUCE_MLA` function, which might defeat the purpose of it.
I would maybe prefer to just revert back to having the one function for the whole combine, as the `MUL` fold should not be done on its own now, would you be okay with this?

https://github.com/llvm/llvm-project/pull/127083