[llvm] r256735 - Fix a horrible infloop in value tracking in the face of dead code.

[Chandler Carruth via llvm-commits]

Author: chandlerc
Date: Mon Jan  4 01:23:12 2016
New Revision: 256735

URL: http://llvm.org/viewvc/llvm-project?rev=256735&view=rev
Log:
Fix a horrible infloop in value tracking in the face of dead code.

Amazingly, we just never triggered this without:
1) Moving code around for MetadataTracking so that a certain *different*
   amount of inlining occurs in the per-TU compile step.
2) Then you LTO opt or clang with a bootstrap, and get inlining, loop
   opts, and GVN line up everything *just* right.

I don't really know how we didn't hit this before. We really need to be
fuzz testing stuff, it shouldn't be hard to trigger. I'm working on
crafting a reduced nice test case, and will submit that when I have it,
but I want to get LTO build bots going again.

Modified:
    llvm/trunk/lib/Analysis/ValueTracking.cpp

Modified: llvm/trunk/lib/Analysis/ValueTracking.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/ValueTracking.cpp?rev=256735&r1=256734&r2=256735&view=diff
==============================================================================
--- llvm/trunk/lib/Analysis/ValueTracking.cpp (original)
+++ llvm/trunk/lib/Analysis/ValueTracking.cpp Mon Jan  4 01:23:12 2016
@@ -2830,7 +2830,12 @@ Value *llvm::GetPointerBaseWithConstantO
                                               const DataLayout &DL) {
   unsigned BitWidth = DL.getPointerTypeSizeInBits(Ptr->getType());
   APInt ByteOffset(BitWidth, 0);
-  while (1) {
+
+  // We walk up the defs but use a visited set to handle unreachable code. In
+  // that case, we stop after accumulating the cycle once (not that it
+  // matters).
+  SmallPtrSet<Value *, 16> Visited;
+  while (Visited.insert(Ptr).second) {
     if (Ptr->getType()->isVectorTy())
       break;