StructurizeCFG: Directly invert cmp instructions

The most common case for a branch condition is
a single use compare. Directly invert the branch
predicate rather than adding a lot of xor i1 true
which the DAG will have to fold later.

This produces nicer to read structurizer output.

This produces some random changes in codegen
due to the DAG swapping branch conditions itself,
and then does a poor job of dealing with those
inverts.

llvm-svn: 300732

1 files changed, 12 insertions, 2 deletions

diff --git a/llvm/lib/Transforms/Scalar/StructurizeCFG.cpp b/llvm/lib/Transforms/Scalar/StructurizeCFG.cpp
index 49ce0262c97..659353e912f 100644
--- a/llvm/lib/Transforms/Scalar/StructurizeCFG.cpp
+++ b/llvm/lib/Transforms/Scalar/StructurizeCFG.cpp
@@ -352,10 +352,20 @@ Value *StructurizeCFG::invert(Value *Condition) {
   if (Instruction *Inst = dyn_cast<Instruction>(Condition)) {
     // Third: Check all the users for an invert
     BasicBlock *Parent = Inst->getParent();
-    for (User *U : Condition->users())
-      if (Instruction *I = dyn_cast<Instruction>(U))
+    for (User *U : Condition->users()) {
+      if (Instruction *I = dyn_cast<Instruction>(U)) {
         if (I->getParent() == Parent && match(I, m_Not(m_Specific(Condition))))
           return I;
+      }
+    }
+
+    // Avoid creating a new instruction in the common case of a compare.
+    if (CmpInst *Cmp = dyn_cast<CmpInst>(Inst)) {
+      if (Cmp->hasOneUse()) {
+        Cmp->setPredicate(Cmp->getInversePredicate());
+        return Cmp;
+      }
+    }
 
     // Last option: Create a new instruction
     return BinaryOperator::CreateNot(Condition, "", Parent->getTerminator());