[JumpThreading][LVI] Invalidate LVI information after blocks are merged

Summary:
After a single predecessor is merged into a basic block, we need to invalidate
the LVI information for the new merged block, when LVI is not provably true for
all of instructions in the new block.
The test cases added show the correct LVI information using the LVI printer
pass.

Reviewers: reames, dberlin, davide, sanjoy

Reviewed by: dberlin, davide

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D34108

llvm-svn: 305699

1 files changed, 31 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Scalar/JumpThreading.cpp b/llvm/lib/Transforms/Scalar/JumpThreading.cpp
index c120036464d..c25dd8d0e16 100644
--- a/llvm/lib/Transforms/Scalar/JumpThreading.cpp
+++ b/llvm/lib/Transforms/Scalar/JumpThreading.cpp
@@ -752,6 +752,37 @@ bool JumpThreadingPass::ProcessBlock(BasicBlock *BB) {
       LVI->eraseBlock(SinglePred);
       MergeBasicBlockIntoOnlyPred(BB);
 
+      // Now that BB is merged into SinglePred (i.e. SinglePred Code followed by
+      // BB code within one basic block `BB`), we need to invalidate the LVI
+      // information associated with BB, because the LVI information need not be
+      // true for all of BB after the merge. For example,
+      // Before the merge, LVI info and code is as follows:
+      // SinglePred: <LVI info1 for %p val>
+      // %y = use of %p
+      // call @exit() // need not transfer execution to successor.
+      // assume(%p) // from this point on %p is true
+      // br label %BB
+      // BB: <LVI info2 for %p val, i.e. %p is true>
+      // %x = use of %p
+      // br label exit
+      //
+      // Note that this LVI info for blocks BB and SinglPred is correct for %p
+      // (info2 and info1 respectively). After the merge and the deletion of the
+      // LVI info1 for SinglePred. We have the following code:
+      // BB: <LVI info2 for %p val>
+      // %y = use of %p
+      // call @exit()
+      // assume(%p)
+      // %x = use of %p <-- LVI info2 is correct from here onwards.
+      // br label exit
+      // LVI info2 for BB is incorrect at the beginning of BB.
+
+      // Invalidate LVI information for BB if the LVI is not provably true for
+      // all of BB.
+      if (any_of(*BB, [](Instruction &I) {
+            return !isGuaranteedToTransferExecutionToSuccessor(&I);
+          }))
+        LVI->eraseBlock(BB);
       return true;
     }
   }