Move the capture analysis from MemoryDependencyAnalysis to a more general place
so that it can be reused in MemCpyOptimizer. This analysis is needed to remove
an unnecessary memcpy when returning a struct into a local variable.
rdar://11341081
PR12686
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@156776 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Analysis/AliasAnalysis.cpp b/lib/Analysis/AliasAnalysis.cpp
index 95c834b..3b6aab1 100644
--- a/lib/Analysis/AliasAnalysis.cpp
+++ b/lib/Analysis/AliasAnalysis.cpp
@@ -25,6 +25,9 @@
//===----------------------------------------------------------------------===//
#include "llvm/Analysis/AliasAnalysis.h"
+#include "llvm/Analysis/CaptureTracking.h"
+#include "llvm/Analysis/Dominators.h"
+#include "llvm/Analysis/ValueTracking.h"
#include "llvm/Pass.h"
#include "llvm/BasicBlock.h"
#include "llvm/Function.h"
@@ -356,6 +359,86 @@
return ModRef;
}
+namespace {
+ /// Only find pointer captures which happen before the given instruction. Uses
+ /// the dominator tree to determine whether one instruction is before another.
+ struct CapturesBefore : public CaptureTracker {
+ CapturesBefore(const Instruction *I, DominatorTree *DT)
+ : BeforeHere(I), DT(DT), Captured(false) {}
+
+ void tooManyUses() { Captured = true; }
+
+ bool shouldExplore(Use *U) {
+ Instruction *I = cast<Instruction>(U->getUser());
+ BasicBlock *BB = I->getParent();
+ if (BeforeHere != I &&
+ (!DT->isReachableFromEntry(BB) || DT->dominates(BeforeHere, I)))
+ return false;
+ return true;
+ }
+
+ bool captured(Use *U) {
+ Instruction *I = cast<Instruction>(U->getUser());
+ BasicBlock *BB = I->getParent();
+ if (BeforeHere != I &&
+ (!DT->isReachableFromEntry(BB) || DT->dominates(BeforeHere, I)))
+ return false;
+ Captured = true;
+ return true;
+ }
+
+ const Instruction *BeforeHere;
+ DominatorTree *DT;
+
+ bool Captured;
+ };
+}
+
+// FIXME: this is really just shoring-up a deficiency in alias analysis.
+// BasicAA isn't willing to spend linear time determining whether an alloca
+// was captured before or after this particular call, while we are. However,
+// with a smarter AA in place, this test is just wasting compile time.
+AliasAnalysis::ModRefResult
+AliasAnalysis::callCapturesBefore(const Instruction *I,
+ const AliasAnalysis::Location &MemLoc,
+ DominatorTree *DT) {
+ if (!DT || !TD) return AliasAnalysis::ModRef;
+
+ const Value *Object = GetUnderlyingObject(MemLoc.Ptr, TD);
+ if (!isIdentifiedObject(Object) || isa<GlobalValue>(Object) ||
+ isa<Constant>(Object))
+ return AliasAnalysis::ModRef;
+
+ ImmutableCallSite CS(I);
+ if (!CS.getInstruction() || CS.getInstruction() == Object)
+ return AliasAnalysis::ModRef;
+
+ CapturesBefore CB(I, DT);
+ llvm::PointerMayBeCaptured(Object, &CB);
+ if (CB.Captured)
+ return AliasAnalysis::ModRef;
+
+ unsigned ArgNo = 0;
+ for (ImmutableCallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
+ CI != CE; ++CI, ++ArgNo) {
+ // Only look at the no-capture or byval pointer arguments. If this
+ // pointer were passed to arguments that were neither of these, then it
+ // couldn't be no-capture.
+ if (!(*CI)->getType()->isPointerTy() ||
+ (!CS.doesNotCapture(ArgNo) && !CS.isByValArgument(ArgNo)))
+ continue;
+
+ // If this is a no-capture pointer argument, see if we can tell that it
+ // is impossible to alias the pointer we're checking. If not, we have to
+ // assume that the call could touch the pointer, even though it doesn't
+ // escape.
+ if (!isNoAlias(AliasAnalysis::Location(*CI),
+ AliasAnalysis::Location(Object))) {
+ return AliasAnalysis::ModRef;
+ }
+ }
+ return AliasAnalysis::NoModRef;
+}
// AliasAnalysis destructor: DO NOT move this to the header file for
// AliasAnalysis or else clients of the AliasAnalysis class may not depend on