Memory Dependence Analysis: fix a miscompile that uses DT to approxmiate the

reachablity.

We conservatively approximate the reachability analysis by saying it is not
reachable if there is a single path starting from "From" and the path does not
reach "To".

rdar://12801584


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171512 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Analysis/AliasAnalysis.cpp

@@ -361,8 +361,28 @@ AliasAnalysis::getModRefInfo(const AtomicRMWInst *RMW, const Location &Loc) {
 }
 
 namespace {
+  // Conservatively return true. Return false, if there is a single path
+  // starting from "From" and the path does not reach "To".
+  static bool hasPath(const BasicBlock *From, const BasicBlock *To) {
+    const unsigned MaxCheck = 5;
+    const BasicBlock *Current = From;
+    for (unsigned I = 0; I < MaxCheck; I++) {
+      unsigned NumSuccs = Current->getTerminator()->getNumSuccessors();
+      if (NumSuccs > 1)
+        return true;
+      if (NumSuccs == 0)
+        return false;
+      Current = Current->getTerminator()->getSuccessor(0);
+      if (Current == To)
+        return true;
+    }
+    return true;
+  }
+
   /// Only find pointer captures which happen before the given instruction. Uses
   /// the dominator tree to determine whether one instruction is before another.
+  /// Only support the case where the Value is defined in the same basic block
+  /// as the given instruction and the use.
   struct CapturesBefore : public CaptureTracker {
     CapturesBefore(const Instruction *I, DominatorTree *DT)
       : BeforeHere(I), DT(DT), Captured(false) {}
@@ -372,8 +392,15 @@ namespace {
     bool shouldExplore(Use *U) {
       Instruction *I = cast<Instruction>(U->getUser());
       BasicBlock *BB = I->getParent();
-      if (BeforeHere != I &&
+      // We explore this usage only if the usage can reach "BeforeHere".
-          (!DT->isReachableFromEntry(BB) || DT->dominates(BeforeHere, I)))
+      // If use is not reachable from entry, there is no need to explore.
+      if (BeforeHere != I && !DT->isReachableFromEntry(BB))
+        return false;
+      // If the value is defined in the same basic block as use and BeforeHere,
+      // there is no need to explore the use if BeforeHere dominates use.
+      // Check whether there is a path from I to BeforeHere.
+      if (BeforeHere != I && DT->dominates(BeforeHere, I) &&
+          !hasPath(BB, BeforeHere->getParent()))
         return false;
       return true;
     }
@@ -381,8 +408,11 @@ namespace {
     bool captured(Use *U) {
       Instruction *I = cast<Instruction>(U->getUser());
       BasicBlock *BB = I->getParent();
-      if (BeforeHere != I &&
+      // Same logic as in shouldExplore.
-          (!DT->isReachableFromEntry(BB) || DT->dominates(BeforeHere, I)))
+      if (BeforeHere != I && !DT->isReachableFromEntry(BB))
+        return false;
+      if (BeforeHere != I && DT->dominates(BeforeHere, I) &&
+          !hasPath(BB, BeforeHere->getParent()))
         return false;
       Captured = true;
       return true;

test/Transforms/GVN/MemdepMiscompile.ll

@@ -0,0 +1,54 @@
+; RUN: opt < %s -basicaa -gvn -S | FileCheck %s
+target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64"
+target triple = "x86_64-apple-macosx10.7.0"
+
+; rdar://12801584
+; Value of %shouldExit can be changed by RunInMode.
+; Make sure we do not replace load %shouldExit in while.cond.backedge
+; with a phi node where the value from while.body is 0.
+define i32 @test() nounwind ssp {
+entry:
+; CHECK: test()
+; CHECK: while.body:
+; CHECK: call void @RunInMode
+; CHECK: br i1 %tobool, label %while.cond.backedge, label %if.then
+; CHECK: while.cond.backedge:
+; CHECK: load i32* %shouldExit
+; CHECK: br i1 %cmp, label %while.body
+  %shouldExit = alloca i32, align 4
+  %tasksIdle = alloca i32, align 4
+  store i32 0, i32* %shouldExit, align 4
+  store i32 0, i32* %tasksIdle, align 4
+  call void @CTestInitialize(i32* %tasksIdle) nounwind
+  %0 = load i32* %shouldExit, align 4
+  %cmp1 = icmp eq i32 %0, 0
+  br i1 %cmp1, label %while.body.lr.ph, label %while.end
+
+while.body.lr.ph:
+  br label %while.body
+
+while.body:
+  call void @RunInMode(i32 100) nounwind
+  %1 = load i32* %tasksIdle, align 4
+  %tobool = icmp eq i32 %1, 0
+  br i1 %tobool, label %while.cond.backedge, label %if.then
+
+if.then:
+  store i32 0, i32* %tasksIdle, align 4
+  call void @TimerCreate(i32* %shouldExit) nounwind
+  br label %while.cond.backedge
+
+while.cond.backedge:
+  %2 = load i32* %shouldExit, align 4
+  %cmp = icmp eq i32 %2, 0
+  br i1 %cmp, label %while.body, label %while.cond.while.end_crit_edge
+
+while.cond.while.end_crit_edge:
+  br label %while.end
+
+while.end:
+  ret i32 0
+}
+declare void @CTestInitialize(i32*)
+declare void @RunInMode(i32)
+declare void @TimerCreate(i32*)