[PATCH] D87936: [GISel] Add new combines for G_ADD

[Jay Foad via Phabricator via llvm-commits]

foad added inline comments.


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Comment at: llvm/include/llvm/CodeGen/GlobalISel/CombinerHelper.h:739
 
+  /// Transform G_ADD(x, -cst) to G_SUB(x, cst).
+  bool matchAddNegConstant(MachineInstr &MI, APInt &Cst);
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Does SelectionDAG do this? I suppose it can be useful to canonicalize, but my gut feeling is that it would be better to do the converse, i.e. canonicalize G_SUB(x, cst) -> G_ADD(x, -cst). That way you are canonicalizing on using G_ADD instead of G_SUB wherever possible, which feels more useful than using positive constants instead of negative constants wherever possible. E.g. if you're writing a pattern to match a "reg + simm" addressing mode you would only need to match G_ADD.


Repository:
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  https://reviews.llvm.org/D87936/new/

https://reviews.llvm.org/D87936