[MergeFunc] Erase unused duplicate functions if they are discardable

[vsk]

Summary

MergeFunc only deletes unused duplicate functions if they have local
linkage, but it should be safe to relax this to any "discardable if
unused" linkage type.

[whitequark]

LGTM, thanks!

llvm/lib/Transforms/IPO/MergeFunctions.cpp

if (!G->isInterposable() && !MergeFunctionsPDI) {
         // above).
         replaceDirectCallers(G, F);
       }
     }
 
     // If G was internal then we may have replaced all uses of G with F. If so,
     // stop here and delete G. There's no need for a thunk. (See note on
     // MergeFunctionsPDI above).
-    if (G->hasLocalLinkage() && G->use_empty() && !MergeFunctionsPDI) {
+    if (G->isDiscardableIfUnused() && G->use_empty() && !MergeFunctionsPDI) {
       G->eraseFromParent();
       ++NumFunctionsMerged;
       return;
     }
 
     if (writeThunkOrAlias(F, G)) {
       ++NumFunctionsMerged;
     }

llvm/test/Transforms/MergeFunc/linkonce_odr.ll

-; RUN: opt -S -mergefunc < %s | FileCheck %s
+; RUN: opt -S -mergefunc < %s | FileCheck %s -implicit-check-not=funC
 
 ; Replacments should be totally ordered on the function name.
 ; If we don't do this we  can end up with one module defining a thunk for @funA
 ; and another module defining a thunk for @funB.
 ;
 ; The problem with this is that the linker could then choose these two stubs
 ; each of the two modules and we end up with two stubs calling each other.
 
 ; CHECK-LABEL: define linkonce_odr i32 @funA
 ; CHECK-NEXT:    add
 ; CHECK:         ret
 
 ; CHECK-LABEL: define linkonce_odr i32 @funB
 ; CHECK-NEXT:    tail call i32 @funA(i32 %0, i32 %1)
 ; CHECK-NEXT:    ret
 
+define linkonce_odr i32 @funC(i32 %x, i32 %y) {
+  %sum = add i32 %x, %y
+  %sum2 = add i32 %x, %sum
+  %sum3 = add i32 %x, %sum2
+  ret i32 %sum3
+}
+
 define linkonce_odr i32 @funB(i32 %x, i32 %y) {
   %sum = add i32 %x, %y
   %sum2 = add i32 %x, %sum
   %sum3 = add i32 %x, %sum2
   ret i32 %sum3
 }
 
 define linkonce_odr i32 @funA(i32 %x, i32 %y) {
   %sum = add i32 %x, %y
   %sum2 = add i32 %x, %sum
   %sum3 = add i32 %x, %sum2
   ret i32 %sum3
 }
+
+; This creates a use of @funB, preventing -mergefunc from deleting it.
+; @funC, however, can safely be deleted as it has no uses, and is discardable
+; if unused.
+@take_addr_of_funB = global i8* bitcast (i32 (i32, i32)* @funB to i8*)