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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204781 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 7 insertions, 3 deletions

diff --git a/lib/IR/Verifier.cpp b/lib/IR/Verifier.cpp
index 50c6ae204e..f5c8ac623b 100644
--- a/lib/IR/Verifier.cpp
+++ b/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);
 }