[BasicAA] Use GEP as context for computeKnownBits in aliasGEP.

In order to use assumptions, computeKnownBits needs a context
instruction. We can use the GEP, if it is an instruction. We already
pass the assumption cache, but it cannot be used without a context
instruction.

Reviewers: anemet, asbirlea, hfinkel, spatel

Reviewed By: asbirlea

Differential Revision: https://reviews.llvm.org/D71264

1 files changed, 2 insertions, 1 deletions

diff --git a/llvm/lib/Analysis/BasicAliasAnalysis.cpp b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
index 227b12d1b67..e852d663c6b 100644
--- a/llvm/lib/Analysis/BasicAliasAnalysis.cpp
+++ b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
@@ -1482,7 +1482,8 @@ AliasResult BasicAAResult::aliasGEP(
         // give up if we can't determine conditions that hold for every cycle:
         const Value *V = DecompGEP1.VarIndices[i].V;
 
-        KnownBits Known = computeKnownBits(V, DL, 0, &AC, nullptr, DT);
+        KnownBits Known =
+            computeKnownBits(V, DL, 0, &AC, dyn_cast<Instruction>(GEP1), DT);
         bool SignKnownZero = Known.isNonNegative();
         bool SignKnownOne = Known.isNegative();