IR: Do not consider available_externally linkage to be linker-weak.

From the linker's perspective, an available_externally global is equivalent
to an external declaration (per isDeclarationForLinker()), so it is incorrect
to consider it to be a weak definition.

Also clean up some logic in the dead argument elimination pass and clarify
its comments to better explain how its behavior depends on linkage,
introduce GlobalValue::isStrongDefinitionForLinker() and start using
it throughout the optimizers and backend.

Differential Revision: http://reviews.llvm.org/D10941

llvm-svn: 241413

1 files changed, 12 insertions, 14 deletions

diff --git a/llvm/lib/Transforms/IPO/DeadArgumentElimination.cpp b/llvm/lib/Transforms/IPO/DeadArgumentElimination.cpp
index 76898f27505..d0447640259 100644
--- a/llvm/lib/Transforms/IPO/DeadArgumentElimination.cpp
+++ b/llvm/lib/Transforms/IPO/DeadArgumentElimination.cpp
@@ -326,7 +326,18 @@ bool DAE::DeleteDeadVarargs(Function &Fn) {
 /// instead.
 bool DAE::RemoveDeadArgumentsFromCallers(Function &Fn)
 {
-  if (Fn.isDeclaration() || Fn.mayBeOverridden())
+  // We cannot change the arguments if this TU does not define the function or
+  // if the linker may choose a function body from another TU, even if the
+  // nominal linkage indicates that other copies of the function have the same
+  // semantics. In the below example, the dead load from %p may not have been
+  // eliminated from the linker-chosen copy of f, so replacing %p with undef
+  // in callers may introduce undefined behavior.
+  //
+  // define linkonce_odr void @f(i32* %p) {
+  //   %v = load i32 %p
+  //   ret void
+  // }
+  if (!Fn.isStrongDefinitionForLinker())
     return false;
 
   // Functions with local linkage should already have been handled, except the
@@ -334,19 +345,6 @@ bool DAE::RemoveDeadArgumentsFromCallers(Function &Fn)
   if (Fn.hasLocalLinkage() && !Fn.getFunctionType()->isVarArg())
     return false;
 
-  // If a function seen at compile time is not necessarily the one linked to
-  // the binary being built, it is illegal to change the actual arguments
-  // passed to it. These functions can be captured by isWeakForLinker().
-  // *NOTE* that mayBeOverridden() is insufficient for this purpose as it
-  // doesn't include linkage types like AvailableExternallyLinkage and
-  // LinkOnceODRLinkage. Take link_odr* as an example, it indicates a set of
-  // *EQUIVALENT* globals that can be merged at link-time. However, the
-  // semantic of *EQUIVALENT*-functions includes parameters. Changing
-  // parameters breaks this assumption.
-  //
-  if (Fn.isWeakForLinker())
-    return false;
-
   if (Fn.use_empty())
     return false;