[InstCombine] Simplify separate_storage assumptions

Before this change, we call getUnderlyingObject on each separate_storage
operand on every alias() call (potentially requiring lots of pointer
chasing). Instead, we rewrite the assumptions in instcombine to do this
pointer-chasing once.

We still leave the getUnderlyingObject calls in alias(), just expecting
them to be no-ops much of the time. This is relatively fast (just a
couple dyn_casts with no pointer chasing) and avoids making alias
analysis results depend on whether or not instcombine has been run.

Differential Revision: https://reviews.llvm.org/D144933

1 files changed, 2 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/BasicAliasAnalysis.cpp b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
index b677eae..f806e37 100644
--- a/llvm/lib/Analysis/BasicAliasAnalysis.cpp
+++ b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
@@ -1516,6 +1516,8 @@ AliasResult BasicAAResult::aliasCheck(const Value *V1, LocationSize V1Size,
           assert(OBU.Inputs.size() == 2);
           const Value *Hint1 = OBU.Inputs[0].get();
           const Value *Hint2 = OBU.Inputs[1].get();
+          // This is often a no-op; instcombine rewrites this for us. No-op
+          // getUnderlyingObject calls are fast, though.
           const Value *HintO1 = getUnderlyingObject(Hint1);
           const Value *HintO2 = getUnderlyingObject(Hint2);