[InstSimplify] select Cond, true, false --> Cond

This is step 1 of damage control assuming that we need to remove several
over-reaching folds for select-of-booleans because they can cause
miscompiles as shown in D72396.

The scalar case seems obviously safe:
https://rise4fun.com/Alive/jSj

And I don't think there's any danger for vectors either - if the
condition is poisoned, then the select must be poisoned too, so undef
elements don't make any difference.

Differential Revision: https://reviews.llvm.org/D72412

1 files changed, 9 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
index afcca2ab1fa..d7510c89910 100644
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -3996,6 +3996,15 @@ static Value *SimplifySelectInst(Value *Cond, Value *TrueVal, Value *FalseVal,
       return FalseVal;
   }
 
+  // select i1 Cond, i1 true, i1 false --> i1 Cond
+  assert(Cond->getType()->isIntOrIntVectorTy(1) &&
+         "Select must have bool or bool vector condition");
+  assert(TrueVal->getType() == FalseVal->getType() &&
+         "Select must have same types for true/false ops");
+  if (Cond->getType() == TrueVal->getType() &&
+      match(TrueVal, m_One()) && match(FalseVal, m_ZeroInt()))
+    return Cond;
+
   // select ?, X, X -> X
   if (TrueVal == FalseVal)
     return TrueVal;