From 3a884f5c17ac32e34e7e62b4602a0d73eeda1ce8 Mon Sep 17 00:00:00 2001
From: Chad Rosier <mcrosier@apple.com>
Date: Mon, 14 May 2012 20:35:04 +0000
Subject: 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
---
 lib/Analysis/AliasAnalysis.cpp | 83 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 83 insertions(+)

(limited to 'lib/Analysis/AliasAnalysis.cpp')

diff --git a/lib/Analysis/AliasAnalysis.cpp b/lib/Analysis/AliasAnalysis.cpp
index 95c834b451..3b6aab13a5 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 @@ AliasAnalysis::getModRefInfo(const AtomicRMWInst *RMW, const Location &Loc) {
   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
-- 
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