[llvm] [GlobalIsel] Combine selects with constants (PR #76089)

[Amara Emerson via llvm-commits]

Thorsten =?utf-8?q?Schütt?= <schuett at gmail.com>
Message-ID:
In-Reply-To: <llvm.org/llvm/llvm-project/pull/76089 at github.com>


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@@ -6352,16 +6352,22 @@ bool CombinerHelper::tryFoldSelectOfConstants(GSelect *Select,
   LLT CondTy = MRI.getType(Select->getCondReg());
   LLT TrueTy = MRI.getType(Select->getTrueReg());
 
-  // Either both are scalars or both are vectors.
-  std::optional<APInt> TrueOpt = getConstantOrConstantSplatVector(True);
-  std::optional<APInt> FalseOpt = getConstantOrConstantSplatVector(False);
+  // Only do this before legalization to avoid conflicting with target-specific
+  // transforms in the other direction.
+  if (CondTy != LLT::scalar(1))
+    return false;
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aemerson wrote:

To implement your comment this check only works for AArch64, you should use `isPreLegalize()` instead.

However I think you actually want what your code does here, i.e. the `s1` check. So probably rewrite the comment to remove the part about before-legalization. For non-s1 types you might need to care about getBooleanContents() etc so easier to do it for s1 only.

https://github.com/llvm/llvm-project/pull/76089