[SimplifyCFG] Don't SimplifyBranchOnICmpChain with ExtraCase

Summary:
Here we try to avoid issues with "explicit branch" with SimplifyBranchOnICmpChain
which can check on undef. Msan by design reports branches on uninitialized
memory and undefs, so we have false report here.

In general msan does not like when we convert

```
// If at least one of them is true we can MSAN is ok if another is undefs
if (a || b)
  return;
```
into
```
// If 'a' is undef MSAN will complain even if 'b' is true
if (a)
  return;
if (b)
  return;
```

Example

Before optimization we had something like this:
```
while (true) {
  bool maybe_undef = doStuff();

  while (true) {
    char c = getChar();
    if (c != 10 && c != 13)
     continue
    break;
  }

  // we know that c == 10 || c == 13 if we get here,
  // so msan know that branch is not affected by maybe_undef
  if (maybe_undef || c == 10 || c == 13)
    continue;
  return;
}
```

SimplifyBranchOnICmpChain will convert that into
```
while (true) {
  bool maybe_undef = doStuff();

  while (true) {
    char c = getChar();
    if (c != 10 && c != 13)
      continue;
    break;
  }

  // however msan will complain here:
  if (maybe_undef)
    continue;

  // we know that c == 10 || c == 13, so either way we will get continue
  switch(c) {
    case 10: continue;
    case 13: continue;
  }
  return;
}
```

Reviewers: eugenis, efriedma

Reviewed By: eugenis, efriedma

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67205

llvm-svn: 371138

1 files changed, 6 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index 3a18b52..bcf5219 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -3729,12 +3729,17 @@ static bool SimplifyBranchOnICmpChain(BranchInst *BI, IRBuilder<> &Builder,
 
   BasicBlock *BB = BI->getParent();
 
+  // MSAN does not like undefs as branch condition which can be introduced
+  // with "explicit branch".
+  if (ExtraCase && BB->getParent()->hasFnAttribute(Attribute::SanitizeMemory))
+    return false;
+
   LLVM_DEBUG(dbgs() << "Converting 'icmp' chain with " << Values.size()
                     << " cases into SWITCH.  BB is:\n"
                     << *BB);
 
   // If there are any extra values that couldn't be folded into the switch
-  // then we evaluate them with an explicit branch first.  Split the block
+  // then we evaluate them with an explicit branch first. Split the block
   // right before the condbr to handle it.
   if (ExtraCase) {
     BasicBlock *NewBB =