Allow alias to point to an arbitrary ConstantExpr.

This  patch changes GlobalAlias to point to an arbitrary ConstantExpr and it is
up to MC (or the system assembler) to decide if that expression is valid or not.

This reduces our ability to diagnose invalid uses and how early we can spot
them, but it also lets us do things like

@test5 = alias inttoptr(i32 sub (i32 ptrtoint (i32* @test2 to i32),
                                 i32 ptrtoint (i32* @bar to i32)) to i32*)

An important implication of this patch is that the notion of aliased global
doesn't exist any more. The alias has to encode the information needed to
access it in its metadata (linkage, visibility, type, etc).

Another consequence to notice is that getSection has to return a "const char *".
It could return a NullTerminatedStringRef if there was such a thing, but when
that was proposed the decision was to just uses "const char*" for that.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210062 91177308-0d34-0410-b5e6-96231b3b80d8

5 files changed, 76 insertions, 67 deletions

diff --git a/lib/IR/AsmWriter.cpp b/lib/IR/AsmWriter.cpp
index 8aee77ac07..14467137aa 100644
--- a/lib/IR/AsmWriter.cpp
+++ b/lib/IR/AsmWriter.cpp
@@ -1494,16 +1494,10 @@ void AssemblyWriter::printAlias(const GlobalAlias *GA) {
 
   PrintLinkage(GA->getLinkage(), Out);
 
-  PointerType *Ty = GA->getType();
   const Constant *Aliasee = GA->getAliasee();
-  if (!Aliasee || Ty != Aliasee->getType()) {
-    if (unsigned AddressSpace = Ty->getAddressSpace())
-      Out << "addrspace(" << AddressSpace << ") ";
-    TypePrinter.print(Ty->getElementType(), Out);
-    Out << ", ";
-  }
 
   if (!Aliasee) {
+    TypePrinter.print(GA->getType(), Out);
     Out << " <<NULL ALIASEE>>";
   } else {
     writeOperand(Aliasee, !isa<ConstantExpr>(Aliasee));
diff --git a/lib/IR/Core.cpp b/lib/IR/Core.cpp
index 27ce503e1c..aa373f602a 100644
--- a/lib/IR/Core.cpp
+++ b/lib/IR/Core.cpp
@@ -1286,7 +1286,7 @@ void LLVMSetLinkage(LLVMValueRef Global, LLVMLinkage Linkage) {
 }
 
 const char *LLVMGetSection(LLVMValueRef Global) {
-  return unwrap<GlobalValue>(Global)->getSection().c_str();
+  return unwrap<GlobalValue>(Global)->getSection();
 }
 
 void LLVMSetSection(LLVMValueRef Global, const char *Section) {
diff --git a/lib/IR/Globals.cpp b/lib/IR/Globals.cpp
index 344a08d8f3..d5723c226a 100644
--- a/lib/IR/Globals.cpp
+++ b/lib/IR/Globals.cpp
@@ -20,6 +20,7 @@
 #include "llvm/IR/GlobalVariable.h"
 #include "llvm/IR/LeakDetector.h"
 #include "llvm/IR/Module.h"
+#include "llvm/IR/Operator.h"
 #include "llvm/Support/ErrorHandling.h"
 using namespace llvm;
 
@@ -58,10 +59,22 @@ void GlobalValue::copyAttributesFrom(const GlobalValue *Src) {
   setDLLStorageClass(Src->getDLLStorageClass());
 }
 
-unsigned GlobalValue::getAlignment() const {
-  if (auto *GA = dyn_cast<GlobalAlias>(this))
-    return GA->getAliasee()->getAlignment();
+const GlobalObject *getBaseObject(const Constant &C) {
+  // FIXME: We should probably return a base + offset pair for non-zero GEPs.
+  return dyn_cast<GlobalObject>(C.stripPointerCasts());
+}
 
+unsigned GlobalValue::getAlignment() const {
+  if (auto *GA = dyn_cast<GlobalAlias>(this)) {
+    // In general we cannot compute this at the IR level, but we try.
+    if (const GlobalObject *GO = getBaseObject(*GA->getAliasee()))
+      return GO->getAlignment();
+
+    // FIXME: we should also be able to handle:
+    // Alias = Global + Offset
+    // Alias = Absolute
+    return 0;
+  }
   return cast<GlobalObject>(this)->getAlignment();
 }
 
@@ -80,9 +93,13 @@ void GlobalObject::copyAttributesFrom(const GlobalValue *Src) {
   setSection(GV->getSection());
 }
 
-const std::string &GlobalValue::getSection() const {
-  if (auto *GA = dyn_cast<GlobalAlias>(this))
-    return GA->getAliasee()->getSection();
+const char *GlobalValue::getSection() const {
+  if (auto *GA = dyn_cast<GlobalAlias>(this)) {
+    // In general we cannot compute this at the IR level, but we try.
+    if (const GlobalObject *GO = getBaseObject(*GA->getAliasee()))
+      return GO->getSection();
+    return "";
+  }
   return cast<GlobalObject>(this)->getSection();
 }
 
@@ -216,7 +233,7 @@ void GlobalVariable::copyAttributesFrom(const GlobalValue *Src) {
 //===----------------------------------------------------------------------===//
 
 GlobalAlias::GlobalAlias(Type *Ty, unsigned AddressSpace, LinkageTypes Link,
-                         const Twine &Name, GlobalObject *Aliasee,
+                         const Twine &Name, Constant *Aliasee,
                          Module *ParentModule)
     : GlobalValue(PointerType::get(Ty, AddressSpace), Value::GlobalAliasVal,
                   &Op<0>(), 1, Link, Name) {
@@ -229,7 +246,7 @@ GlobalAlias::GlobalAlias(Type *Ty, unsigned AddressSpace, LinkageTypes Link,
 
 GlobalAlias *GlobalAlias::create(Type *Ty, unsigned AddressSpace,
                                  LinkageTypes Link, const Twine &Name,
-                                 GlobalObject *Aliasee, Module *ParentModule) {
+                                 Constant *Aliasee, Module *ParentModule) {
   return new GlobalAlias(Ty, AddressSpace, Link, Name, Aliasee, ParentModule);
 }
 
@@ -241,18 +258,18 @@ GlobalAlias *GlobalAlias::create(Type *Ty, unsigned AddressSpace,
 
 GlobalAlias *GlobalAlias::create(Type *Ty, unsigned AddressSpace,
                                  LinkageTypes Linkage, const Twine &Name,
-                                 GlobalObject *Aliasee) {
+                                 GlobalValue *Aliasee) {
   return create(Ty, AddressSpace, Linkage, Name, Aliasee, Aliasee->getParent());
 }
 
 GlobalAlias *GlobalAlias::create(LinkageTypes Link, const Twine &Name,
-                                 GlobalObject *Aliasee) {
+                                 GlobalValue *Aliasee) {
   PointerType *PTy = Aliasee->getType();
   return create(PTy->getElementType(), PTy->getAddressSpace(), Link, Name,
                 Aliasee);
 }
 
-GlobalAlias *GlobalAlias::create(const Twine &Name, GlobalObject *Aliasee) {
+GlobalAlias *GlobalAlias::create(const Twine &Name, GlobalValue *Aliasee) {
   return create(Aliasee->getLinkage(), Name, Aliasee);
 }
 
@@ -272,4 +289,8 @@ void GlobalAlias::eraseFromParent() {
   getParent()->getAliasList().erase(this);
 }
 
-void GlobalAlias::setAliasee(GlobalObject *Aliasee) { setOperand(0, Aliasee); }
+void GlobalAlias::setAliasee(Constant *Aliasee) {
+  assert(!Aliasee || Aliasee->getType() == getType() &&
+         "Alias and aliasee types should match!");
+  setOperand(0, Aliasee);
+}
diff --git a/lib/IR/Value.cpp b/lib/IR/Value.cpp
index d734e4ea81..cc599745d5 100644
--- a/lib/IR/Value.cpp
+++ b/lib/IR/Value.cpp
@@ -301,27 +301,6 @@ void Value::takeName(Value *V) {
     ST->reinsertValue(this);
 }
 
-static GlobalObject &findReplacementForAliasUse(Value &C) {
-  if (auto *GO = dyn_cast<GlobalObject>(&C))
-    return *GO;
-  if (auto *GA = dyn_cast<GlobalAlias>(&C))
-    return *GA->getAliasee();
-  auto *CE = cast<ConstantExpr>(&C);
-  assert(CE->getOpcode() == Instruction::BitCast ||
-         CE->getOpcode() == Instruction::GetElementPtr ||
-         CE->getOpcode() == Instruction::AddrSpaceCast);
-  if (CE->getOpcode() == Instruction::GetElementPtr)
-    assert(cast<GEPOperator>(CE)->hasAllZeroIndices());
-  return findReplacementForAliasUse(*CE->getOperand(0));
-}
-
-static void replaceAliasUseWith(Use &U, Value *New) {
-  GlobalObject &Replacement = findReplacementForAliasUse(*New);
-  assert(&cast<GlobalObject>(*U) != &Replacement &&
-         "replaceAliasUseWith cannot form an alias cycle");
-  U.set(&Replacement);
-}
-
 #ifndef NDEBUG
 static bool contains(SmallPtrSet<ConstantExpr *, 4> &Cache, ConstantExpr *Expr,
                      Constant *C) {
@@ -373,10 +352,6 @@ void Value::replaceAllUsesWith(Value *New) {
     // Must handle Constants specially, we cannot call replaceUsesOfWith on a
     // constant because they are uniqued.
     if (auto *C = dyn_cast<Constant>(U.getUser())) {
-      if (isa<GlobalAlias>(C)) {
-        replaceAliasUseWith(U, New);
-        continue;
-      }
       if (!isa<GlobalValue>(C)) {
         C->replaceUsesOfWithOnConstant(this, New, &U);
         continue;
diff --git a/lib/IR/Verifier.cpp b/lib/IR/Verifier.cpp
index bcc38c1ad5..1719cb2759 100644
--- a/lib/IR/Verifier.cpp
+++ b/lib/IR/Verifier.cpp
@@ -241,6 +241,9 @@ private:
   void visitGlobalValue(const GlobalValue &GV);
   void visitGlobalVariable(const GlobalVariable &GV);
   void visitGlobalAlias(const GlobalAlias &GA);
+  void visitAliaseeSubExpr(const GlobalAlias &A, const Constant &C);
+  void visitAliaseeSubExpr(SmallPtrSet<const GlobalAlias *, 4> &Visited,
+                           const GlobalAlias &A, const Constant &C);
   void visitNamedMDNode(const NamedMDNode &NMD);
   void visitMDNode(MDNode &MD, Function *F);
   void visitModuleIdents(const Module &M);
@@ -474,36 +477,52 @@ void Verifier::visitGlobalVariable(const GlobalVariable &GV) {
   visitGlobalValue(GV);
 }
 
+void Verifier::visitAliaseeSubExpr(const GlobalAlias &GA, const Constant &C) {
+  SmallPtrSet<const GlobalAlias*, 4> Visited;
+  Visited.insert(&GA);
+  visitAliaseeSubExpr(Visited, GA, C);
+}
+
+void Verifier::visitAliaseeSubExpr(SmallPtrSet<const GlobalAlias *, 4> &Visited,
+                                   const GlobalAlias &GA, const Constant &C) {
+  if (const auto *GV = dyn_cast<GlobalValue>(&C)) {
+    Assert1(!GV->isDeclaration(), "Alias must point to a definition", &GA);
+
+    if (const auto *GA2 = dyn_cast<GlobalAlias>(GV)) {
+      Assert1(Visited.insert(GA2), "Aliases cannot form a cycle", &GA);
+
+      Assert1(!GA2->mayBeOverridden(), "Alias cannot point to a weak alias",
+              &GA);
+    }
+  }
+
+  if (const auto *CE = dyn_cast<ConstantExpr>(&C))
+    VerifyConstantExprBitcastType(CE);
+
+  for (const Use &U : C.operands()) {
+    Value *V = &*U;
+    if (const auto *GA2 = dyn_cast<GlobalAlias>(V))
+      visitAliaseeSubExpr(Visited, GA, *GA2->getAliasee());
+    else if (const auto *C2 = dyn_cast<Constant>(V))
+      visitAliaseeSubExpr(Visited, GA, *C2);
+  }
+}
+
 void Verifier::visitGlobalAlias(const GlobalAlias &GA) {
   Assert1(!GA.getName().empty(),
           "Alias name cannot be empty!", &GA);
   Assert1(GlobalAlias::isValidLinkage(GA.getLinkage()),
           "Alias should have external or external weak linkage!", &GA);
-  Assert1(GA.getAliasee(),
-          "Aliasee cannot be NULL!", &GA);
-  Assert1(!GA.hasUnnamedAddr(), "Alias cannot have unnamed_addr!", &GA);
-
   const Constant *Aliasee = GA.getAliasee();
-  const GlobalValue *GV = dyn_cast<GlobalValue>(Aliasee);
-
-  if (!GV) {
-    const ConstantExpr *CE = dyn_cast<ConstantExpr>(Aliasee);
-    if (CE && (CE->getOpcode() == Instruction::BitCast ||
-               CE->getOpcode() == Instruction::AddrSpaceCast ||
-               CE->getOpcode() == Instruction::GetElementPtr))
-      GV = dyn_cast<GlobalValue>(CE->getOperand(0));
+  Assert1(Aliasee, "Aliasee cannot be NULL!", &GA);
+  Assert1(GA.getType() == Aliasee->getType(),
+          "Alias and aliasee types should match!", &GA);
+  Assert1(!GA.hasUnnamedAddr(), "Alias cannot have unnamed_addr!", &GA);
 
-    Assert1(GV, "Aliasee should be either GlobalValue, bitcast or "
-                "addrspacecast of GlobalValue",
-            &GA);
+  Assert1(isa<GlobalValue>(Aliasee) || isa<ConstantExpr>(Aliasee),
+          "Aliasee should be either GlobalValue or ConstantExpr", &GA);
 
-    VerifyConstantExprBitcastType(CE);
-  }
-  Assert1(!GV->isDeclaration(), "Alias must point to a definition", &GA);
-  if (const GlobalAlias *GAAliasee = dyn_cast<GlobalAlias>(GV)) {
-    Assert1(!GAAliasee->mayBeOverridden(), "Alias cannot point to a weak alias",
-            &GA);
-  }
+  visitAliaseeSubExpr(GA, *Aliasee);
 
   visitGlobalValue(GA);
 }