[MergeFunctions] Merge global unnamed_addr functions as aliases

[hermord]

Summary

Even if an unnamed_addr function being merged has external linkage, it should be still alright to make it an alias instead of a thunk, as an optimization.

lib/Transforms/IPO/MergeFunctions.cpp

if (F->isInterposable()) {
     if (!G->isInterposable() && !MergeFunctionsPDI) {
       if (G->hasGlobalUnnamedAddr()) {
         // G might have been a key in our GlobalNumberState, and it's illegal
         // to replace a key in ValueMap<GlobalValue *> with a non-global.
         GlobalNumbers.erase(G);
         // If G's address is not significant, replace it entirely.
         Constant *BitcastF = ConstantExpr::getBitCast(F, G->getType());
         G->replaceAllUsesWith(BitcastF);
+        // If G is local, then we will eliminate it completely below.
+        // Otherwise, the next best thing is to make it an alias to F.
+        if (!G->hasLocalLinkage()) {
+          // Initialize alias with empty name as G is still holding onto its own.
+          auto Alias = GlobalAlias::create(G->getValueType(),
+                                           G->getType()->getAddressSpace(),
+                                           G->getLinkage(),
+                                           "", BitcastF, G->getParent());
+          Alias->takeName(G);
+          G->eraseFromParent();
+          ++NumFunctionsMerged;
+          return;
+        }
       } 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,

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

+; RUN: opt -S -mergefunc < %s | FileCheck %s
+
+%A = type { i32 }
+%B = type { i32 }
+
+; CHECK: @b = alias {{.*}} @a {{.*}}
+
+define i32 @a(%A) unnamed_addr {
+  extractvalue %A %0, 0
+  xor i32 %2, 0
+  ret i32 %3
+}
+
+define i32 @b(%B) unnamed_addr {
+  extractvalue %B %0, 0
+  xor i32 %2, 0
+  ret i32 %3
+}