Reapply "[GVN] Prevent LoadPRE from hoisting across instructions that don't pass control flow to successors"

This patch fixes the miscompile that happens when PRE hoists loads across guards and
other instructions that don't always pass control flow to their successors. PRE is now prohibited
to hoist across such instructions because there is no guarantee that the load standing after such
instruction is still valid before such instruction. For example, a load from under a guard may be
invalid before the guard in the following case:
  int array[LEN];
  ...
  guard(0 <= index && index < LEN);
  use(array[index]);

Differential Revision: https://reviews.llvm.org/D37460


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@316975 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 7 insertions, 0 deletions

diff --git a/include/llvm/Transforms/Scalar/GVN.h b/include/llvm/Transforms/Scalar/GVN.h
index 251492a4e5d..440d3f67c35 100644
--- a/include/llvm/Transforms/Scalar/GVN.h
+++ b/include/llvm/Transforms/Scalar/GVN.h
@@ -18,6 +18,7 @@
 
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/MapVector.h"
+#include "llvm/ADT/PostOrderIterator.h"
 #include "llvm/ADT/SetVector.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/Analysis/AliasAnalysis.h"
@@ -27,6 +28,7 @@
 #include "llvm/IR/PassManager.h"
 #include "llvm/Support/Allocator.h"
 #include "llvm/Support/Compiler.h"
+#include "llvm/Transforms/Utils/OrderedInstructions.h"
 #include <cstdint>
 #include <utility>
 #include <vector>
@@ -156,7 +158,11 @@ private:
   AssumptionCache *AC;
   SetVector<BasicBlock *> DeadBlocks;
   OptimizationRemarkEmitter *ORE;
+  // Maps a block to the topmost instruction with implicit control flow in it.
+  DenseMap<const BasicBlock *, const Instruction *>
+      FirstImplicitControlFlowInsts;
 
+  OrderedInstructions *OI;
   ValueTable VN;
 
   /// A mapping from value numbers to lists of Value*'s that
@@ -268,6 +274,7 @@ private:
                                  BasicBlock *Curr, unsigned int ValNo);
   Value *findLeader(const BasicBlock *BB, uint32_t num);
   void cleanupGlobalSets();
+  void fillImplicitControlFlowInfo(BasicBlock *BB);
   void verifyRemoved(const Instruction *I) const;
   bool splitCriticalEdges();
   BasicBlock *splitCriticalEdges(BasicBlock *Pred, BasicBlock *Succ);