From: Duncan P. N. Exon Smith <dexonsmith@apple.com>
Date: Sat, 24 Sep 2016 20:42:02 +0000 (+0000)
Subject: Analysis: Return early for UndefValue in computeKnownBits
X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=5cb29541603a97167a07afe8ac6bd59e863bf2d1;p=llvm

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).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@282337 91177308-0d34-0410-b5e6-96231b3b80d8
---

diff --git a/lib/Analysis/ValueTracking.cpp b/lib/Analysis/ValueTracking.cpp
index 827e2524915..7983240c3dd 100644
--- a/lib/Analysis/ValueTracking.cpp
+++ b/lib/Analysis/ValueTracking.cpp
@@ -1502,6 +1502,14 @@ void computeKnownBits(const Value *V, APInt &KnownZero, APInt &KnownOne,
   // 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)