[COFF] Don't export symbols that have corresponding __imp_ symbols

GNU ld has got an exception for such symbols, and mingw-w64
occasionally uses that exception to avoid exporting symbols in cases
where they otherwise aren't caught by the other exclusion mechanisms.

Differential Revision: https://reviews.llvm.org/D40553

llvm-svn: 319291

lld/COFF/MinGW.cpp

@@ -8,6 +8,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "MinGW.h"
+#include "SymbolTable.h"
 #include "lld/Common/ErrorHandler.h"
 #include "llvm/Object/COFF.h"
 #include "llvm/Support/Path.h"
@@ -100,6 +101,15 @@ bool AutoExporter::shouldExport(Defined *Sym) const {
   if (ExcludeSymbols.count(Sym->getName()))
     return false;
 
+  // Don't export anything that looks like an import symbol (which also can be
+  // a manually defined data symbol with such a name).
+  if (Sym->getName().startswith("__imp_"))
+    return false;
+
+  // If a corresponding __imp_ symbol exists and is defined, don't export it.
+  if (Symtab->find(("__imp_" + Sym->getName()).str()))
+    return false;
+
   // Check that file is non-null before dereferencing it, symbols not
   // originating in regular object files probably shouldn't be exported.
   if (!Sym->getFile())

lld/test/COFF/export-all.s

@@ -7,8 +7,10 @@
 # RUN: llvm-readobj %t.lib | FileCheck -check-prefix=IMPLIB %s
 
 # CHECK-NOT: Name: DllMainCRTStartup
+# CHECK-NOT: Name: _imp__unexported
 # CHECK: Name: dataSym
 # CHECK: Name: foobar
+# CHECK-NOT: Name: unexported
 
 # IMPLIB: Symbol: __imp__dataSym
 # IMPLIB-NOT: Symbol: _dataSym
@@ -18,14 +20,20 @@
 .global _foobar
 .global _DllMainCRTStartup@12
 .global _dataSym
+.global _unexported
+.global __imp__unexported
 .text
 _DllMainCRTStartup@12:
   ret
 _foobar:
   ret
+_unexported:
+  ret
 .data
 _dataSym:
   .int 4
+__imp__unexported:
+  .int _unexported
 
 # Test specifying -export-all-symbols, on an object file that contains
 # dllexport directive for some of the symbols.