Prevent alias from pointing to weak aliases.

Aliases are just another name for a position in a file. As such, the
regular symbol resolutions are not applied. For example, given

define void @my_func() {
  ret void
}
@my_alias = alias weak void ()* @my_func
@my_alias2 = alias void ()* @my_alias

We produce without this patch:

        .weak   my_alias
my_alias = my_func
        .globl  my_alias2
my_alias2 = my_alias

That is, in the resulting ELF file my_alias, my_func and my_alias are
just 3 names pointing to offset 0 of .text. That is *not* the
semantics of IR linking. For example, linking in a

@my_alias = alias void ()* @other_func

would require the strong my_alias to override the weak one and
my_alias2 would end up pointing to other_func.

There is no way to represent that with aliases being just another
name, so the best solution seems to be to just disallow it, converting
a miscompile into an error.

llvm-svn: 204781

2 files changed, 21 insertions, 24 deletions

diff --git a/llvm/lib/IR/Globals.cpp b/llvm/lib/IR/Globals.cpp
index 11152d5d6c2..f338dd76aaa 100644
--- a/llvm/lib/IR/Globals.cpp
+++ b/llvm/lib/IR/Globals.cpp
@@ -236,10 +236,10 @@ void GlobalAlias::setAliasee(Constant *Aliasee) {
   setOperand(0, Aliasee);
 }
 
-GlobalValue *GlobalAlias::getAliasedGlobal() {
-  Constant *C = getAliasee();
-  if (C == 0) return 0;
-  
+static GlobalValue *getAliaseeGV(GlobalAlias *GA) {
+  Constant *C = GA->getAliasee();
+  assert(C && "Must alias something");
+
   if (GlobalValue *GV = dyn_cast<GlobalValue>(C))
     return GV;
 
@@ -248,30 +248,23 @@ GlobalValue *GlobalAlias::getAliasedGlobal() {
           CE->getOpcode() == Instruction::AddrSpaceCast ||
           CE->getOpcode() == Instruction::GetElementPtr) &&
          "Unsupported aliasee");
-  
+
   return cast<GlobalValue>(CE->getOperand(0));
 }
 
-GlobalValue *GlobalAlias::resolveAliasedGlobal(bool stopOnWeak) {
+GlobalValue *GlobalAlias::getAliasedGlobal() {
   SmallPtrSet<GlobalValue*, 3> Visited;
 
-  // Check if we need to stop early.
-  if (stopOnWeak && mayBeOverridden())
-    return this;
-
-  GlobalValue *GV = getAliasedGlobal();
-  Visited.insert(GV);
-
-  // Iterate over aliasing chain, stopping on weak alias if necessary.
-  while (GlobalAlias *GA = dyn_cast<GlobalAlias>(GV)) {
-    if (stopOnWeak && GA->mayBeOverridden())
-      break;
-
-    GV = GA->getAliasedGlobal();
+  GlobalAlias *GA = this;
 
+  for (;;) {
+    GlobalValue *GV = getAliaseeGV(GA);
     if (!Visited.insert(GV))
       return 0;
-  }
 
-  return GV;
+    // Iterate over aliasing chain.
+    GA = dyn_cast<GlobalAlias>(GV);
+    if (!GA)
+      return GV;
+  }
 }
diff --git a/llvm/lib/IR/Verifier.cpp b/llvm/lib/IR/Verifier.cpp
index 50c6ae204e9..f5c8ac623b1 100644
--- a/llvm/lib/IR/Verifier.cpp
+++ b/llvm/lib/IR/Verifier.cpp
@@ -502,10 +502,14 @@ void Verifier::visitGlobalAlias(const GlobalAlias &GA) {
     }
   }
   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);
+  }
 
-  const GlobalValue* Resolved = GA.resolveAliasedGlobal(/*stopOnWeak*/ false);
-  Assert1(Resolved,
-          "Aliasing chain should end with function or global variable", &GA);
+  const GlobalValue *AG = GA.getAliasedGlobal();
+  Assert1(AG, "Aliasing chain should end with function or global variable",
+          &GA);
 
   visitGlobalValue(GA);
 }