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, 16 insertions, 14 deletions

diff --git a/llvm/lib/Transforms/Utils/LoopSimplify.cpp b/llvm/lib/Transforms/Utils/LoopSimplify.cpp
index 320223c247f..f77a58300f4 100644
--- a/llvm/lib/Transforms/Utils/LoopSimplify.cpp
+++ b/llvm/lib/Transforms/Utils/LoopSimplify.cpp
@@ -269,11 +269,12 @@ ReprocessLoop:
   PHINode *PN;
   for (BasicBlock::iterator I = L->getHeader()->begin();
        (PN = dyn_cast<PHINode>(I++)); )
-    if (Value *V = SimplifyInstruction(PN, 0, DT)) {
-      if (AA) AA->deleteValue(PN);
-      PN->replaceAllUsesWith(V);
-      PN->eraseFromParent();
-    }
+    if (Value *V = SimplifyInstruction(PN, 0, DT))
+      if (LI->replacementPreservesLCSSAForm(PN, V)) {
+          if (AA) AA->deleteValue(PN);
+          PN->replaceAllUsesWith(V);
+          PN->eraseFromParent();
+      }
 
   // If this loop has multiple exits and the exits all go to the same
   // block, attempt to merge the exits. This helps several passes, such
@@ -445,17 +446,18 @@ static void AddBlockAndPredsToSet(BasicBlock *InputBB, BasicBlock *StopBlock,
 /// FindPHIToPartitionLoops - The first part of loop-nestification is to find a
 /// PHI node that tells us how to partition the loops.
 static PHINode *FindPHIToPartitionLoops(Loop *L, DominatorTree *DT,
-                                        AliasAnalysis *AA) {
+                                        AliasAnalysis *AA, LoopInfo *LI) {
   for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ) {
     PHINode *PN = cast<PHINode>(I);
     ++I;
-    if (Value *V = SimplifyInstruction(PN, 0, DT)) {
-      // This is a degenerate PHI already, don't modify it!
-      PN->replaceAllUsesWith(V);
-      if (AA) AA->deleteValue(PN);
-      PN->eraseFromParent();
-      continue;
-    }
+    if (Value *V = SimplifyInstruction(PN, 0, DT))
+      if (LI->replacementPreservesLCSSAForm(PN, V)) {
+        // This is a degenerate PHI already, don't modify it!
+        PN->replaceAllUsesWith(V);
+        if (AA) AA->deleteValue(PN);
+        PN->eraseFromParent();
+        continue;
+      }
 
     // Scan this PHI node looking for a use of the PHI node by itself.
     for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
@@ -523,7 +525,7 @@ void LoopSimplify::PlaceSplitBlockCarefully(BasicBlock *NewBB,
 /// created.
 ///
 Loop *LoopSimplify::SeparateNestedLoop(Loop *L, LPPassManager &LPM) {
-  PHINode *PN = FindPHIToPartitionLoops(L, DT, AA);
+  PHINode *PN = FindPHIToPartitionLoops(L, DT, AA, LI);
   if (PN == 0) return 0;  // No known way to partition.
 
   // Pull out all predecessors that have varying values in the loop.  This