[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 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());
     // 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

 
 @weak1 = weak alias void (), void ()* @bar2
 ; CHECK: @weak1 = weak alias void (), void ()* @bar2
 
 @bar4 = private unnamed_addr constant [2 x i8*] zeroinitializer
 @foo4 = weak_odr unnamed_addr alias i8*, getelementptr inbounds ([2 x i8*], [2 x i8*]* @bar4, i32 0, i32 1)
 ; CHECK: @foo4 = weak_odr unnamed_addr alias i8*, getelementptr inbounds ([2 x i8*], [2 x i8*]* @bar4, i32 0, i32 1)
 
+@priva  = private alias void (), void ()* @bar5
+; CHECK: @priva = private alias void (), void ()* @bar5
+
 define void @bar2() {
   ret void
 }
 ; CHECK: define void @bar2()
 
+define weak void @bar5() {
+  ret void
+}
+; CHECK: define weak void @bar5()
+
 define void @baz() {
 entry:
          call void @foo1()
 ; 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()
+
+         call void @priva()
+; CHECK: call void @priva()
+
          ret void
 }
 
 @foo3 = alias void (), void ()* @bar3
 ; CHECK-NOT: bar3
 
 define internal void @bar3() {
   ret void
 }
 ;CHECK: define void @foo3