[PATCH] D77453: [GlobalISel] Simplify G_ADD when it has (0-X) on the LHS or RHS

[Matt Arsenault via Phabricator via llvm-commits]

arsenm added inline comments.


================
Comment at: llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp:1634
+  std::tie(SubLHS, SubRHS) = MatchInfo;
+  Builder.buildSub(MI.getOperand(0).getReg(), SubLHS, SubRHS);
+  MI.eraseFromParent();
----------------
paquette wrote:
> arsenm wrote:
> > Should this preserve any flags?
> In the DAGCombiner, it doesn't seem to:
> 
> ```
>   // fold ((0-A) + B) -> B-A
>   if (N0.getOpcode() == ISD::SUB && isNullOrNullSplat(N0.getOperand(0)))
>     return DAG.getNode(ISD::SUB, DL, VT, N1, N0.getOperand(1));
> ```
> 
> but maybe we should? It would make sense.
> 
> 
Nobody ever really went back to preserve flags in DAGCombiner (to some degree for FP ops). I would check what InstCombine does


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  https://reviews.llvm.org/D77453/new/

https://reviews.llvm.org/D77453