ELF: Give automatically generated __start_* and __stop_* symbols hidd…

…en visibility.

These symbols describe a property of a linkage unit, so it seems reasonable
to limit their visibility to the linkage unit. Furthermore the use cases I
am aware of do not require more than hidden visibility.

This is a departure from the behavior of the bfd and gold linkers. However,
it is unclear that the decision to give these symbols default visibility
in those linkers was made deliberately. The __start_*/__stop_* feature
was added to the bfd linker in 1994 [1], while the visibility feature was
added about five years later [2], so it may have been that the visibility
of these symbols was not considered. The feature was implemented in gold
[3] in the same way; the behavior may have simply been copied from bfd.

The only related discussion I could find on the binutils mailing list [4]
was a user issue which would most likely not have occurred if the symbols
had hidden visibility.

[1] https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=commit;h=5efddb2e7c3229b569a862205f61d42860af678b
[2] https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=commit;h=0fc731e447cd01e7fc35197b487ff0e4fd25afca
[3] https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=commit;h=bfd58944a64b0997a310b95fbe0423338961e71c
[4] https://sourceware.org/ml/binutils/2014-05/msg00011.html

Differential Revision: http://reviews.llvm.org/D19024

llvm-svn: 266121

lld/ELF/Writer.cpp

@@ -1261,11 +1261,11 @@ void Writer<ELFT>::addStartStopSymbols(OutputSectionBase<ELFT> *Sec) {
   StringRef Stop = Saver.save("__stop_" + S);
   if (SymbolBody *B = Symtab.find(Start))
     if (B->isUndefined())
-      Symtab.addSynthetic(Start, *Sec, 0, STV_DEFAULT);
+      Symtab.addSynthetic(Start, *Sec, 0, STV_HIDDEN);
   if (SymbolBody *B = Symtab.find(Stop))
     if (B->isUndefined())
       Symtab.addSynthetic(Stop, *Sec, DefinedSynthetic<ELFT>::SectionEnd,
-                          STV_DEFAULT);
+                          STV_HIDDEN);
 }
 
 template <class ELFT> static bool needsPtLoad(OutputSectionBase<ELFT> *Sec) {

lld/test/ELF/startstop-shared.s

@@ -5,21 +5,21 @@
 
         .quad __start_foo
         .section foo,"a"
-// By default the symbol is visible and we need a dynamic reloc.
-// CHECK:  R_X86_64_64 __start_foo 0x0
+// By default the symbol is hidden.
+// CHECK:      R_X86_64_RELATIVE - 0x[[ADDR1:.*]]
 
         .hidden __start_bar
         .quad __start_bar
         .section bar,"a"
-// Test that we are able to hide the symbol.
-// CHECK:      R_X86_64_RELATIVE - 0x[[ADDR:.*]]
+// References do not affect the visibility.
+// CHECK:      R_X86_64_RELATIVE - 0x[[ADDR2:.*]]
 
 // CHECK:      Name: __start_bar
-// CHECK-NEXT: Value: 0x[[ADDR]]
+// CHECK-NEXT: Value: 0x[[ADDR2]]
 // CHECK-NEXT: Size:
 // CHECK-NEXT: Binding: Local
 
 // CHECK:      Name: __start_foo
-// CHECK-NEXT: Value:
+// CHECK-NEXT: Value: 0x[[ADDR1]]
 // CHECK-NEXT: Size:
-// CHECK-NEXT: Binding: Global
+// CHECK-NEXT: Binding: Local