[PATCH] Move recursive GlobalAlias handling to be after the max depth check in computeKnownBits()

[Michael Kuperstein]

REPOSITORY
  rL LLVM

http://reviews.llvm.org/D6758

Files:
  llvm/trunk/lib/Analysis/ValueTracking.cpp
  llvm/trunk/test/Transforms/InstCombine/alias-recursion.ll

Index: llvm/trunk/lib/Analysis/ValueTracking.cpp
===================================================================
--- llvm/trunk/lib/Analysis/ValueTracking.cpp
+++ llvm/trunk/lib/Analysis/ValueTracking.cpp
@@ -793,22 +793,11 @@
     return;
   }
 
-  // A weak GlobalAlias is totally unknown. A non-weak GlobalAlias has
-  // the bits of its aliasee.
-  if (GlobalAlias *GA = dyn_cast<GlobalAlias>(V)) {
-    if (GA->mayBeOverridden()) {
-      KnownZero.clearAllBits(); KnownOne.clearAllBits();
-    } else {
-      computeKnownBits(GA->getAliasee(), KnownZero, KnownOne, TD, Depth+1, Q);
-    }
-    return;
-  }
-
   // The address of an aligned GlobalValue has trailing zeros.
-  if (GlobalValue *GV = dyn_cast<GlobalValue>(V)) {
-    unsigned Align = GV->getAlignment();
+  if (auto *GO = dyn_cast<GlobalObject>(V)) {
+    unsigned Align = GO->getAlignment();
     if (Align == 0 && TD) {
-      if (GlobalVariable *GVar = dyn_cast<GlobalVariable>(GV)) {
+      if (auto *GVar = dyn_cast<GlobalVariable>(GO)) {
         Type *ObjectType = GVar->getType()->getElementType();
         if (ObjectType->isSized()) {
           // If the object is defined in the current Module, we'll be giving
@@ -852,8 +841,18 @@
   // Start out not knowing anything.
   KnownZero.clearAllBits(); KnownOne.clearAllBits();
 
+  // Limit search depth.
+  // All recursive calls that increase depth must come after this.
   if (Depth == MaxDepth)
-    return;  // Limit search depth.
+    return;  
+
+  // A weak GlobalAlias is totally unknown. A non-weak GlobalAlias has
+  // the bits of its aliasee.
+  if (GlobalAlias *GA = dyn_cast<GlobalAlias>(V)) {
+    if (!GA->mayBeOverridden())
+      computeKnownBits(GA->getAliasee(), KnownZero, KnownOne, TD, Depth + 1, Q);
+    return;
+  }
 
   // Check whether a nearby assume intrinsic can determine some known bits.
   computeKnownBitsFromAssume(V, KnownZero, KnownOne, TD, Depth, Q);
Index: llvm/trunk/test/Transforms/InstCombine/alias-recursion.ll
===================================================================
--- llvm/trunk/test/Transforms/InstCombine/alias-recursion.ll
+++ llvm/trunk/test/Transforms/InstCombine/alias-recursion.ll
@@ -0,0 +1,24 @@
+; RUN: opt < %s -instcombine -S | FileCheck %s
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-pc-windows-msvc"
+
+%class.A = type { i32 (...)** }
+
+ at 0 = constant [1 x i8*] zeroinitializer
+
+ at vtbl = alias getelementptr inbounds ([1 x i8*]* @0, i32 0, i32 0)
+
+define i32 (%class.A*)* @test() {
+; CHECK-LABEL: test
+entry:
+  br i1 undef, label %for.body, label %for.end
+
+for.body:                                         ; preds = %for.body, %entry
+  br i1 undef, label %for.body, label %for.end
+
+for.end:                                          ; preds = %for.body, %entry
+  %A = phi i32 (%class.A*)** [ bitcast (i8** @vtbl to i32 (%class.A*)**), %for.body ], [ null, %entry ]
+  %B = load i32 (%class.A*)** %A
+  ret i32 (%class.A*)* %B
+}

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