[GlobalOpt] Don't replace alias with aliasee if aliasee is interposable

[aeubanks]

Summary

Both the alias and aliasee linkage are important.

PR27866 provides some background.

[aeubanks]

update

[aeubanks]

As far as I can tell regarding PR27866, it seems that the linkage type of an alias doesn't do anything. Let me know if I'm misunderstanding

[aeubanks]

ping

I can keep the check to see if the alias itself is interposable, although it sounds like that shouldn't matter

[rnk]

We chatted about this, and convinced ourselves to keep the original check

[aeubanks]

keep check of alias itself

[rnk]

lgtm

llvm/lib/Transforms/IPO/GlobalOpt.cpp

for (Module::alias_iterator I = M.alias_begin(), E = M.alias_end();
     if (!J->hasName() && !J->isDeclaration() && !J->hasLocalLinkage())
       J->setLinkage(GlobalValue::InternalLinkage);
 
     if (deleteIfDead(*J, NotDiscardableComdats)) {
       Changed = true;
       continue;
     }
 
-    // If the alias can change at link time, nothing can be done - bail out.
-    if (J->isInterposable())
-      continue;
-
     Constant *Aliasee = J->getAliasee();
-    GlobalValue *Target = dyn_cast<GlobalValue>(Aliasee->stripPointerCasts());
+    GlobalValue *Target =
+        dyn_cast<GlobalValue>(Aliasee->stripPointerCastsAndAliases());
     // We can't trivially replace the alias with the aliasee if the aliasee is
-    // non-trivial in some way.
+    // non-trivial in some way. We also can't replace the alias with the aliasee
+    // if the aliasee is interposable because aliases point to the local
+    // definition.
     // TODO: Try to handle non-zero GEPs of local aliasees.
-    if (!Target)
+    if (!Target || Target->isInterposable())
       continue;
     Target->removeDeadConstantUsers();
 
     // Make all users of the alias use the aliasee instead.
     bool RenameTarget;
     if (!hasUsesToReplace(*J, Used, RenameTarget))
       continue;
 

llvm/test/Transforms/GlobalOpt/alias-resolve.ll

; CHECK: call void @bar2()
 
          call void @foo2()
 ; CHECK: call void @bar2()
 
          call void @bar1()
 ; CHECK: call void @bar2()
 
          call void @weak1()
-; CHECK: call void @weak1()
+; CHECK: call void @bar2()
+
+         call void @bar6()
+; CHECK: call void @bar6()
          ret void
 }
 
 @foo3 = alias void (), void ()* @bar3
 ; CHECK-NOT: bar3
 
 define internal void @bar3() {
   ret void
 }
 ;CHECK: define void @foo3
+
+define weak void @bar5() {
+  ret void
+}
+
+@bar6 = alias void(), void ()* @bar5