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

[David Majnemer]

LGTM with tweaks.


================
Comment at: lib/Analysis/ValueTracking.cpp:797
@@ -807,3 +796,3 @@
   // The address of an aligned GlobalValue has trailing zeros.
-  if (GlobalValue *GV = dyn_cast<GlobalValue>(V)) {
-    unsigned Align = GV->getAlignment();
+  if (GlobalObject *GO = dyn_cast<GlobalObject>(V)) {
+    unsigned Align = GO->getAlignment();
----------------
You could make this more concise with `auto *GO = dyn_cast<GlobalObject>(V))`.

================
Comment at: lib/Analysis/ValueTracking.cpp:800
@@ -810,3 +799,3 @@
     if (Align == 0 && TD) {
-      if (GlobalVariable *GVar = dyn_cast<GlobalVariable>(GV)) {
+      if (GlobalVariable *GVar = dyn_cast<GlobalVariable>(GO)) {
         Type *ObjectType = GVar->getType()->getElementType();
----------------
Might as well make this `auto *GV = dyn_cast<GlobalVariable>(GO))` now.

================
Comment at: lib/Analysis/ValueTracking.cpp:850-851
@@ +849,4 @@
+  if (GlobalAlias *GA = dyn_cast<GlobalAlias>(V)) {
+    if (GA->mayBeOverridden()) {
+      KnownZero.clearAllBits(); KnownOne.clearAllBits();
+    } else {
----------------
We just cleared out all the bits on line 842, we probably don't need to do it again.  Instead, I'd replace it with an assert that `KnownZero` and `KnownOne` are both zero.

================
Comment at: test/Transforms/InstCombine/alias-recursion.ll:3-43
@@ +2,43 @@
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-pc-windows-msvc"
+
+%rtti.CompleteObjectLocator = type { i32, i32, i32, i32, i32, i32 }
+%rtti.TypeDescriptor = type { i8**, i8*, [8 x i8] }
+%rtti.ClassHierarchyDescriptor = type { i32, i32, i32, i32 }
+%rtti.BaseClassDescriptor = type { i32, i32, i32, i32, i32, i32, i32 }
+%class.A = type { i32 (...)** }
+
+ at 0 = private unnamed_addr constant [2 x i8*] [i8* bitcast (%rtti.CompleteObjectLocator* @"\01??_R4A@@6B@" to i8*), i8* bitcast (i32 (%class.A*)* @"\01?virt at A@@UEAAHXZ" to i8*)]
+@"\01??_R4A@@6B@" = linkonce_odr constant %rtti.CompleteObjectLocator { i32 1, i32 0, i32 0, i32 trunc (i64 sub nuw nsw (i64 ptrtoint (%rtti.TypeDescriptor* @"\01??_R0?AVA@@@8" to i64), i64 ptrtoint (i8* @__ImageBase to i64)) to i32), i32 trunc (i64 sub nuw nsw (i64 ptrtoint (%rtti.ClassHierarchyDescriptor* @"\01??_R3A@@8" to i64), i64 ptrtoint (i8* @__ImageBase to i64)) to i32), i32 trunc (i64 sub nuw nsw (i64 ptrtoint (%rtti.CompleteObjectLocator* @"\01??_R4A@@6B@" to i64), i64 ptrtoint (i8* @__ImageBase to i64)) to i32) }
+@"\01??_7type_info@@6B@" = external constant i8*
+@"\01??_R0?AVA@@@8" = linkonce_odr global %rtti.TypeDescriptor { i8** @"\01??_7type_info@@6B@", i8* null, [8 x i8] c".?AVA@@\00" }
+ at __ImageBase = external constant i8
+@"\01??_R3A@@8" = linkonce_odr constant %rtti.ClassHierarchyDescriptor { i32 0, i32 0, i32 1, i32 trunc (i64 sub nuw nsw (i64 ptrtoint ([2 x i32]* @"\01??_R2A@@8" to i64), i64 ptrtoint (i8* @__ImageBase to i64)) to i32) }
+@"\01??_R2A@@8" = linkonce_odr constant [2 x i32] [i32 trunc (i64 sub nuw nsw (i64 ptrtoint (%rtti.BaseClassDescriptor* @"\01??_R1A@?0A at EA@A@@8" to i64), i64 ptrtoint (i8* @__ImageBase to i64)) to i32), i32 0]
+@"\01??_R1A@?0A at EA@A@@8" = linkonce_odr constant %rtti.BaseClassDescriptor { i32 trunc (i64 sub nuw nsw (i64 ptrtoint (%rtti.TypeDescriptor* @"\01??_R0?AVA@@@8" to i64), i64 ptrtoint (i8* @__ImageBase to i64)) to i32), i32 0, i32 0, i32 -1, i32 0, i32 64, i32 trunc (i64 sub nuw nsw (i64 ptrtoint (%rtti.ClassHierarchyDescriptor* @"\01??_R3A@@8" to i64), i64 ptrtoint (i8* @__ImageBase to i64)) to i32) }
+@"\01??_7A@@6B@" = unnamed_addr alias getelementptr inbounds ([2 x i8*]* @0, i32 0, i32 1)
+
+define void @test() {
+; CHECK-LABEL: test
+entry:
+  br i1 undef, label %for.body.lr.ph, label %for.end
+
+for.body.lr.ph:                                   ; preds = %entry
+  br label %for.body
+
+for.body:                                         ; preds = %for.body.for.body_crit_edge, %for.body.lr.ph
+  %vtable = phi i32 (%class.A*)** [ bitcast (i8** @"\01??_7A@@6B@" to i32 (%class.A*)**), %for.body.lr.ph ], [ null, %for.body.for.body_crit_edge ]
+  %0 = load i32 (%class.A*)** %vtable, align 8
+  %call2 = tail call i32 %0(%class.A* undef)
+  br i1 undef, label %for.body.for.body_crit_edge, label %for.end
+
+for.body.for.body_crit_edge:                      ; preds = %for.body
+  br label %for.body
+
+for.end:                                          ; preds = %for.body, %entry
+  ret void
+}
+
+declare i32 @"\01?virt at A@@UEAAHXZ"(%class.A*) unnamed_addr align 2
+
----------------
I believe the following is more minimal:

```
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 (...)** }

@0 = constant [1 x i8*] zeroinitializer

@vtbl = alias getelementptr inbounds ([1 x i8*]* @0, i32 0, i32 0)

define i32 (%class.A*)* @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
}
```

http://reviews.llvm.org/D6758

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