[llvm] r282337 - Analysis: Return early for UndefValue in computeKnownBits

[Duncan P. N. Exon Smith via llvm-commits]

Author: dexonsmith
Date: Sat Sep 24 15:42:02 2016
New Revision: 282337

URL: http://llvm.org/viewvc/llvm-project?rev=282337&view=rev
Log:
Analysis: Return early for UndefValue in computeKnownBits

There is no benefit in looking through assumptions on UndefValue to
guess known bits.  Return early to avoid walking their use-lists, and
assert that all instances of ConstantData are handled here for similar
reasons (UndefValue was the only integer/pointer holdout).

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=282337&r1=282336&r2=282337&view=diff
==============================================================================
--- llvm/trunk/lib/Analysis/ValueTracking.cpp (original)
+++ llvm/trunk/lib/Analysis/ValueTracking.cpp Sat Sep 24 15:42:02 2016
@@ -1502,6 +1502,14 @@ void computeKnownBits(const Value *V, AP
   // Start out not knowing anything.
   KnownZero.clearAllBits(); KnownOne.clearAllBits();
 
+  // We can't imply anything about undefs.
+  if (isa<UndefValue>(V))
+    return;
+
+  // There's no point in looking through other users of ConstantData for
+  // assumptions.  Confirm that we've handled them all.
+  assert(!isa<ConstantData>(V) && "Unhandled constant data!");
+
   // Limit search depth.
   // All recursive calls that increase depth must come after this.
   if (Depth == MaxDepth)