[SimpifyCFG] Remove recursion from FoldCondBranchOnPHI. NFCI.

Avoid stack overflow errors on systems with small stack sizes
by removing recursion in FoldCondBranchOnPHI.

This is a simple change as the recursion was only iteratively
calling the function again on the same arguments.
Ideally this would be compiled to a tail call, but there is
no guarantee.

Reviewed By: lebedev.ri

Differential Revision: https://reviews.llvm.org/D107803

diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index a72af2b..e581dfd 100644 (file)
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -2606,8 +2606,10 @@ static bool BlockIsSimpleEnoughToThreadThrough(BasicBlock *BB) {
 /// If we have a conditional branch on a PHI node value that is defined in the
 /// same block as the branch and if any PHI entries are constants, thread edges
 /// corresponding to that entry to be branches to their ultimate destination.
-static bool FoldCondBranchOnPHI(BranchInst *BI, DomTreeUpdater *DTU,
-                                const DataLayout &DL, AssumptionCache *AC) {
+static Optional<bool> FoldCondBranchOnPHIImpl(BranchInst *BI,
+                                              DomTreeUpdater *DTU,
+                                              const DataLayout &DL,
+                                              AssumptionCache *AC) {
   BasicBlock *BB = BI->getParent();
   PHINode *PN = dyn_cast<PHINode>(BI->getCondition());
   // NOTE: we currently cannot transform this case if the PHI node is used
@@ -2721,13 +2723,25 @@ static bool FoldCondBranchOnPHI(BranchInst *BI, DomTreeUpdater *DTU,
       DTU->applyUpdates(Updates);
     }
 
-    // Recurse, simplifying any other constants.
-    return FoldCondBranchOnPHI(BI, DTU, DL, AC) || true;
+    // Signal repeat, simplifying any other constants.
+    return None;
   }
 
   return false;
 }
 
+static bool FoldCondBranchOnPHI(BranchInst *BI, DomTreeUpdater *DTU,
+                                const DataLayout &DL, AssumptionCache *AC) {
+  Optional<bool> Result;
+  bool EverChanged = false;
+  do {
+    // Note that None means "we changed things, but recurse further."
+    Result = FoldCondBranchOnPHIImpl(BI, DTU, DL, AC);
+    EverChanged |= Result == None || *Result;
+  } while (Result == None);
+  return EverChanged;
+}
+
 /// Given a BB that starts with the specified two-entry PHI node,
 /// see if we can eliminate it.
 static bool FoldTwoEntryPHINode(PHINode *PN, const TargetTransformInfo &TTI,