!fixup Document and sort by weak/linkince ODR.

Author: fhahn

llvm/lib/Transforms/IPO/MergeFunctions.cpp

@@ -891,6 +891,10 @@ bool MergeFunctions::writeThunkOrAlias(Function *F, Function *G) {
 
 // Merge two equivalent functions. Upon completion, Function G is deleted.
 void MergeFunctions::mergeTwoFunctions(Function *F, Function *G) {
+
+  // Create a new thunk that both F and G can call, if F cannot call G directly.
+  // That is the case if F is either interposable or if G is either weak_odr or
+  // linkonce_odr.
   if (F->isInterposable() || G->hasWeakODRLinkage() ||
       G->hasLinkOnceODRLinkage()) {
     assert(G->isInterposable() || G->hasWeakODRLinkage() ||
@@ -993,11 +997,20 @@ static bool isFuncOrderCorrect(const Function *F, const Function *G) {
     // one, but not the other way around.
     return !F->isInterposable();
   }
+
+  if (F->hasWeakODRLinkage() != G->hasWeakODRLinkage() ||
+      F->hasLinkOnceODRLinkage() != G->hasLinkOnceODRLinkage()) {
+    // ODR functions before non-ODR functions. A ODR function can call a non-ODR
+    // function if it is not interposable, but not the other way around.
+    return F->hasWeakODRLinkage() || F->hasLinkOnceODRLinkage();
+  }
+
   if (F->hasLocalLinkage() != G->hasLocalLinkage()) {
     // External before local, because we definitely have to keep the external
     // function, but may be able to drop the local one.
     return !F->hasLocalLinkage();
   }
+
   // Impose a total order (by name) on the replacement of functions. This is
   // important when operating on more than one module independently to prevent
   // cycles of thunks calling each other when the modules are linked together.

llvm/test/Transforms/MergeFunc/merge-linkonce-odr.ll

@@ -63,7 +63,7 @@ declare void @zar(ptr)
 ; CHECK-NEXT:    ret void
 ;
 ;
-; CHECK-LABEL: define private void @1(
+; CHECK-LABEL: define linkonce_odr hidden void @non_linkonce_caller_of_bar_2(
 ; CHECK-SAME: ptr [[P:%.*]]) {
 ; CHECK-NEXT:  [[ENTRY:.*:]]
 ; CHECK-NEXT:    tail call void @bar(ptr [[P]])
@@ -93,15 +93,9 @@ declare void @zar(ptr)
 ; CHECK-NEXT:    ret void
 ;
 ;
-; CHECK-LABEL: define linkonce_odr hidden void @non_linkonce_caller_of_bar_2(
-; CHECK-SAME: ptr [[TMP0:%.*]]) {
-; CHECK-NEXT:    tail call void @[[GLOB1:[0-9]+]](ptr [[TMP0]])
-; CHECK-NEXT:    ret void
-;
-;
 ; CHECK-LABEL: define hidden void @linkonce_odr_caller_of_bar_1(
 ; CHECK-SAME: ptr [[TMP0:%.*]]) {
-; CHECK-NEXT:    tail call void @[[GLOB1]](ptr [[TMP0]])
+; CHECK-NEXT:    tail call void @non_linkonce_caller_of_bar_2(ptr [[TMP0]])
 ; CHECK-NEXT:    ret void
 ;
 ;

llvm/test/Transforms/MergeFunc/merge-weak-odr.ll

@@ -72,7 +72,7 @@ declare void @zar(ptr)
 ; CHECK-NEXT:    ret void
 ;
 ;
-; CHECK-LABEL: define private void @1(
+; CHECK-LABEL: define weak_odr hidden void @weak_odr_caller_of_zar_2(
 ; CHECK-SAME: ptr [[P:%.*]]) {
 ; CHECK-NEXT:  [[ENTRY:.*:]]
 ; CHECK-NEXT:    tail call void @zar(ptr [[P]])
@@ -99,14 +99,8 @@ declare void @zar(ptr)
 ; CHECK-NEXT:    ret void
 ;
 ;
-; CHECK-LABEL: define weak_odr hidden void @weak_odr_caller_of_zar_2(
-; CHECK-SAME: ptr [[TMP0:%.*]]) {
-; CHECK-NEXT:    tail call void @[[GLOB1:[0-9]+]](ptr [[TMP0]])
-; CHECK-NEXT:    ret void
-;
-;
 ; CHECK-LABEL: define hidden void @non_weak_caller_of_zar_1(
 ; CHECK-SAME: ptr [[TMP0:%.*]]) {
-; CHECK-NEXT:    tail call void @[[GLOB1]](ptr [[TMP0]])
+; CHECK-NEXT:    tail call void @weak_odr_caller_of_zar_2(ptr [[TMP0]])
 ; CHECK-NEXT:    ret void
 ;