Teach SimplifyCFG that (switch (select cond, X, Y)) is better expressed as a branch.

Based on a patch by Alistair Lynn.

llvm-svn: 126647

1 files changed, 26 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index c6708857cb5..72fa468db60 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -1800,6 +1800,26 @@ static bool SimplifyTerminatorOnSelect(TerminatorInst *OldTerm, Value *Cond,
   return true;
 }
 
+// SimplifySwitchOnSelect - Replaces
+//   (switch (select cond, X, Y)) on constant X, Y
+// with a branch - conditional if X and Y lead to distinct BBs,
+// unconditional otherwise.
+static bool SimplifySwitchOnSelect(SwitchInst *SI, SelectInst *Select) {
+  // Check for constant integer values in the select.
+  ConstantInt *TrueVal = dyn_cast<ConstantInt>(Select->getTrueValue());
+  ConstantInt *FalseVal = dyn_cast<ConstantInt>(Select->getFalseValue());
+  if (!TrueVal || !FalseVal)
+    return false;
+
+  // Find the relevant condition and destinations.
+  Value *Condition = Select->getCondition();
+  BasicBlock *TrueBB = SI->getSuccessor(SI->findCaseValue(TrueVal));
+  BasicBlock *FalseBB = SI->getSuccessor(SI->findCaseValue(FalseVal));
+
+  // Perform the actual simplification.
+  return SimplifyTerminatorOnSelect(SI, Condition, TrueBB, FalseBB);
+}
+
 // SimplifyIndirectBrOnSelect - Replaces
 //   (indirectbr (select cond, blockaddress(@fn, BlockA),
 //                             blockaddress(@fn, BlockB)))
@@ -2309,7 +2329,12 @@ bool SimplifyCFGOpt::SimplifySwitch(SwitchInst *SI) {
   if (BasicBlock *OnlyPred = BB->getSinglePredecessor())
     if (SimplifyEqualityComparisonWithOnlyPredecessor(SI, OnlyPred))
       return SimplifyCFG(BB) | true;
-  
+
+  Value *Cond = SI->getCondition();
+  if (SelectInst *Select = dyn_cast<SelectInst>(Cond))
+    if (SimplifySwitchOnSelect(SI, Select))
+      return SimplifyCFG(BB) | true;
+
   // If the block only contains the switch, see if we can fold the block
   // away into any preds.
   BasicBlock::iterator BBI = BB->begin();