[llvm] r280218 - [SimplifyCFG] Improve FoldValueComparisonIntoPredecessors to handle more cases

[James Molloy via llvm-commits]

Author: jamesm
Date: Wed Aug 31 05:46:39 2016
New Revision: 280218

URL: http://llvm.org/viewvc/llvm-project?rev=280218&view=rev
Log:
[SimplifyCFG] Improve FoldValueComparisonIntoPredecessors to handle more cases

A very important case is not handled here: multiple arcs to a single block with a PHI. Consider:

    a:
      %1 = icmp %b, 1
      br %1, label %c, label %e
    c:
      %2 = icmp %b, 2
      br %2, label %d, label %e
    d:
      br %e
    e:
      phi [0, %a], [1, %c], [2, %d]

FoldValueComparisonIntoPredecessors will refuse to fold this, as it doesn't know how to deal with two arcs to a common destination with different PHI values. The answer is obvious - just split all conflicting arcs.

Modified:
    llvm/trunk/lib/Transforms/Utils/SimplifyCFG.cpp

Modified: llvm/trunk/lib/Transforms/Utils/SimplifyCFG.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Utils/SimplifyCFG.cpp?rev=280218&r1=280217&r2=280218&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Utils/SimplifyCFG.cpp (original)
+++ llvm/trunk/lib/Transforms/Utils/SimplifyCFG.cpp Wed Aug 31 05:46:39 2016
@@ -174,7 +174,8 @@ public:
 
 /// Return true if it is safe to merge these two
 /// terminator instructions together.
-static bool SafeToMergeTerminators(TerminatorInst *SI1, TerminatorInst *SI2) {
+static bool SafeToMergeTerminators(TerminatorInst *SI1, TerminatorInst *SI2,
+                                   std::vector<BasicBlock*> *FailBlocks = nullptr) {
   if (SI1 == SI2)
     return false; // Can't merge with self!
 
@@ -183,18 +184,22 @@ static bool SafeToMergeTerminators(Termi
   // conflicting incoming values from the two switch blocks.
   BasicBlock *SI1BB = SI1->getParent();
   BasicBlock *SI2BB = SI2->getParent();
-  SmallPtrSet<BasicBlock *, 16> SI1Succs(succ_begin(SI1BB), succ_end(SI1BB));
 
+  SmallPtrSet<BasicBlock *, 16> SI1Succs(succ_begin(SI1BB), succ_end(SI1BB));
+  bool Fail = false;
   for (BasicBlock *Succ : successors(SI2BB))
     if (SI1Succs.count(Succ))
       for (BasicBlock::iterator BBI = Succ->begin(); isa<PHINode>(BBI); ++BBI) {
         PHINode *PN = cast<PHINode>(BBI);
         if (PN->getIncomingValueForBlock(SI1BB) !=
-            PN->getIncomingValueForBlock(SI2BB))
-          return false;
+            PN->getIncomingValueForBlock(SI2BB)) {
+          if (FailBlocks)
+            FailBlocks->push_back(Succ);
+          Fail = true;
+        }
       }
 
-  return true;
+  return !Fail;
 }
 
 /// Return true if it is safe and profitable to merge these two terminator
@@ -954,7 +959,16 @@ bool SimplifyCFGOpt::FoldValueComparison
     TerminatorInst *PTI = Pred->getTerminator();
     Value *PCV = isValueEqualityComparison(PTI); // PredCondVal
 
-    if (PCV == CV && SafeToMergeTerminators(TI, PTI)) {
+    if (PCV == CV && TI != PTI) {
+      std::vector<BasicBlock*> FailBlocks;
+      if (!SafeToMergeTerminators(TI, PTI, &FailBlocks)) {
+        for (auto *Succ : FailBlocks) {
+          std::vector<BasicBlock*> Blocks = { TI->getParent() };
+          if (!SplitBlockPredecessors(Succ, Blocks, ".fold.split"))
+            return false;
+        }
+      }
+
       // Figure out which 'cases' to copy from SI to PSI.
       std::vector<ValueEqualityComparisonCase> BBCases;
       BasicBlock *BBDefault = GetValueEqualityComparisonCases(TI, BBCases);