Factor code for testing whether replacing one value with another

preserves LCSSA form out of ScalarEvolution and into the LoopInfo
class.  Use it to check that SimplifyInstruction simplifications
are not breaking LCSSA form.  Fixes PR8622.

llvm-svn: 119727

1 files changed, 7 insertions, 6 deletions

diff --git a/llvm/lib/Transforms/Scalar/LoopUnswitch.cpp b/llvm/lib/Transforms/Scalar/LoopUnswitch.cpp
index 009ee7b95ea..2eaeb438e06 100644
--- a/llvm/lib/Transforms/Scalar/LoopUnswitch.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopUnswitch.cpp
@@ -990,15 +990,16 @@ void LoopUnswitch::SimplifyCode(std::vector<Instruction*> &Worklist, Loop *L) {
       ++NumSimplify;
       continue;
     }
-    
+
     // See if instruction simplification can hack this up.  This is common for
     // things like "select false, X, Y" after unswitching made the condition be
     // 'false'.
-    if (Value *V = SimplifyInstruction(I, 0, DT)) {
-      ReplaceUsesOfWith(I, V, Worklist, L, LPM);
-      continue;
-    }
-    
+    if (Value *V = SimplifyInstruction(I, 0, DT))
+      if (LI->replacementPreservesLCSSAForm(I, V)) {
+        ReplaceUsesOfWith(I, V, Worklist, L, LPM);
+        continue;
+      }
+
     // Special case hacks that appear commonly in unswitched code.
     if (BranchInst *BI = dyn_cast<BranchInst>(I)) {
       if (BI->isUnconditional()) {