[MergeFunctions] Replace all uses of unnamed_addr functions

[whitequark]

Summary

This reduces code size for constructs like vtables or interrupt
tables that refer to functions in global initializers.

[whitequark]

@nlewycky @jfb ping?

lib/Transforms/IPO/MergeFunctions.cpp

 // delete G. Under MergeFunctionsPDI, we use G itself for creating
 // the thunk as we preserve the debug info (and associated instructions)
 // from G's entry block pertaining to G's incoming arguments which are
 // passed on as corresponding arguments in the call that G makes to F.
 // For better debugability, under MergeFunctionsPDI, we do not modify G's
 // call sites to point to F even when within the same translation unit.
 void MergeFunctions::writeThunk(Function *F, Function *G) {
   if (!G->isInterposable() && !MergeFunctionsPDI) {
+    if (G->hasGlobalUnnamedAddr()) {
+      // If G's address is not significant, replace it entirely.
+      Constant *BitcastF = ConstantExpr::getBitCast(F, G->getType());
+      G->replaceAllUsesWith(BitcastF);
+    } else {
       // Redirect direct callers of G to F. (See note on 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) {
     G->eraseFromParent();
     return;
   }

test/Transforms/MergeFunc/merge-unnamed-addr-bitcast.ll

+; RUN: opt -S -mergefunc < %s | FileCheck %s
+
+%A = type { i32 }
+%B = type { i32 }
+
+; CHECK-NOT: @b
+
+@x = constant { i32 (i32)*, i32 (i32)* } 
+  { i32 (i32)* bitcast (i32 (%A)* @a to i32 (i32)*), 
+    i32 (i32)* bitcast (i32 (%B)* @b to i32 (i32)*) }
+; CHECK: { i32 (i32)* bitcast (i32 (%A)* @a to i32 (i32)*), i32 (i32)* bitcast (i32 (%A)* @a to i32 (i32)*) }
+
+define internal i32 @a(%A) unnamed_addr {
+  extractvalue %A %0, 0
+  xor i32 %2, 0
+  ret i32 %3
+}
+
+define internal i32 @b(%B) unnamed_addr {
+  extractvalue %B %0, 0
+  xor i32 %2, 0
+  ret i32 %3
+}
+
+define i32 @c(i32) {
+  insertvalue %B undef, i32 %0, 0
+  call i32 @b(%B %2)
+; CHECK: call i32 bitcast (i32 (%A)* @a to i32 (%B)*)(%B %2)
+  ret i32 %3
+}

test/Transforms/MergeFunc/merge-unnamed-addr.ll

+; RUN: opt -S -mergefunc < %s | FileCheck %s
+
+; CHECK-NOT: @b
+
+@x = constant { i32 (i32)*, i32 (i32)* } { i32 (i32)* @a, i32 (i32)* @b }
+; CHECK: { i32 (i32)* @a, i32 (i32)* @a }
+
+define internal i32 @a(i32 %a) unnamed_addr {
+  %b = xor i32 %a, 0
+  %c = xor i32 %b, 0
+  ret i32 %c
+}
+
+define internal i32 @b(i32 %a) unnamed_addr {
+  %b = xor i32 %a, 0
+  %c = xor i32 %b, 0
+  ret i32 %c
+}