MergeFunc: preserve COMDAT information when creating a thunk

[compnerd]

Summary

We would previously drop the COMDAT on the thunk we generated when replacing a function body with the forwarding thunk. This would result in a function that may have been multiply emitted and multiply merged to be emitted with the same name without the COMDAT. This is a hard error with PE/COFF where the COMDAT is used for the deduplication of Value Witness functions for Swift.

[rjmccall]

LGTM.

[compnerd]

SVN r358728

lib/Transforms/IPO/MergeFunctions.cpp

   } else {
     NewG = Function::Create(G->getFunctionType(), G->getLinkage(),
                             G->getAddressSpace(), "", G->getParent());
+    NewG->setComdat(G->getComdat());
     BB = BasicBlock::Create(F->getContext(), "", NewG);
   }
 

test/Transforms/MergeFunc/comdat.ll

+; RUN: opt -S -mergefunc %s | FileCheck %s
+
+@symbols = linkonce_odr global <{ i8*, i8* }> <{ i8* bitcast (i32 (i32, i32)* @f to i8*), i8* bitcast (i32 (i32, i32)* @g to i8*) }>
+
+$f = comdat any
+$g = comdat any
+
+define linkonce_odr hidden i32 @f(i32 %x, i32 %y) comdat {
+  %sum = add i32 %x, %y
+  %sum2 = add i32 %x, %sum
+  %sum3 = add i32 %x, %sum
+  ret i32 %sum3
+}
+
+define linkonce_odr hidden i32 @g(i32 %x, i32 %y) comdat {
+  %sum = add i32 %x, %y
+  %sum2 = add i32 %x, %sum
+  %sum3 = add i32 %x, %sum
+  ret i32 %sum3
+}
+
+; CHECK-DAG: define linkonce_odr hidden i32 @f(i32 %x, i32 %y) comdat
+; CHECK-DAG: define linkonce_odr hidden i32 @g(i32 %x, i32 %y) comdat
+