[InstCombine] if the condition of a select may be known via assumes, eliminate the select

This is a limited solution for PR31512:
https://llvm.org/bugs/show_bug.cgi?id=31512

The motivation is that we will need to increase usage of llvm.assume and/or metadata to solve PR28430:
https://llvm.org/bugs/show_bug.cgi?id=28430

...and this kind of simplification is needed to take advantage of that extra information.

The 'not' test case would be handled by:
https://reviews.llvm.org/D28485

Differential Revision:
https://reviews.llvm.org/D28337

llvm-svn: 291915

1 files changed, 14 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
index 36644845352..74ad7110a2f 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -1450,6 +1450,20 @@ Instruction *InstCombiner::visitSelectInst(SelectInst &SI) {
     }
   }
 
+  // If we can compute the condition, there's no need for a select.
+  // Like the above fold, we are attempting to reduce compile-time cost by
+  // putting this fold here with limitations rather than in InstSimplify.
+  // The motivation for this call into value tracking is to take advantage of
+  // the assumption cache, so make sure that is populated.
+  if (!CondVal->getType()->isVectorTy() && !AC.assumptions().empty()) {
+    APInt KnownOne(1, 0), KnownZero(1, 0);
+    computeKnownBits(CondVal, KnownZero, KnownOne, 0, &SI);
+    if (KnownOne == 1)
+      return replaceInstUsesWith(SI, TrueVal);
+    if (KnownZero == 1)
+      return replaceInstUsesWith(SI, FalseVal);
+  }
+
   if (Instruction *BitCastSel = foldSelectCmpBitcasts(SI, *Builder))
     return BitCastSel;