[LLD][COFF] Enable linking of __declspec(selectany) symbols from Clang and GCC

When annotating a symbol with __declspec(selectany), Clang assigns it
comdat 2 while GCC assigns it comdat 3. This patch enables two object
files that contain a __declspec(selectany) symbol, one created by gcc
and the other by clang, to be linked together instead of issuing a
duplicate symbol error.

Differential Revision: https://reviews.llvm.org/D73139

Author: zero9178

lld/COFF/InputFiles.cpp

@@ -500,6 +500,17 @@ void ObjFile::handleComdatSelection(COFFSymbolRef sym, COMDATType &selection,
     leaderSelection = selection = IMAGE_COMDAT_SELECT_LARGEST;
   }
 
+  // GCCs __declspec(selectany) doesn't actually pick "any" but "same size as".
+  // Clang on the other hand picks "any". To be able to link two object files
+  // with a __declspec(selectany) declaration, one compiled with gcc and the
+  // other with clang, we merge them as proper "same size as"
+  if (config->mingw && ((selection == IMAGE_COMDAT_SELECT_ANY &&
+                         leaderSelection == IMAGE_COMDAT_SELECT_SAME_SIZE) ||
+                        (selection == IMAGE_COMDAT_SELECT_SAME_SIZE &&
+                         leaderSelection == IMAGE_COMDAT_SELECT_ANY))) {
+    leaderSelection = selection = IMAGE_COMDAT_SELECT_SAME_SIZE;
+  }
+
   // Other than that, comdat selections must match.  This is a bit more
   // strict than link.exe which allows merging "any" and "largest" if "any"
   // is the first symbol the linker sees, and it allows merging "largest"

lld/test/COFF/comdat-gcc-compatibility.s

@@ -0,0 +1,13 @@
+# REQUIRES: x86
+# RUN: llvm-mc %s -triple x86_64-pc-win32 -defsym obj=0 -filetype=obj -o %t1.obj
+# RUN: llvm-mc %s -triple x86_64-pc-win32 -defsym obj=1 -filetype=obj -o %t2.obj
+# RUN: lld-link /lldmingw /noentry /dll %t1.obj %t2.obj /out:%t3.dll
+# RUN: not lld-link /noentry /dll %t1.obj %t2.obj /out:%t3.dll
+.if obj==0
+        .section .text$nm, "", discard, symbol
+.else
+        .section .text$nm, "", same_size, symbol
+.endif
+        .globl symbol
+symbol:
+        .long 1