MachO: don't emit L... private symbols in do_not_dead_strip sections.

[t.p.northover]

Summary

The linker can sometimes drop the do_not_dead_strip if it can't associate the atom with a symbol (the other place to specify no dead-stripping in MachO files).

[steven_wu]

I think the reason for linker to drop that is macho linker do not care about no_dead_strip sections and it only looks at the no_dead_strip attribute on the atom. Once it turned into private label, the no_dead_strip attribute can be gone on the atom. Even ld -r preserves attribute on the section, it might not work.

[t.p.northover]

Thanks. Committed as 5d7632ee72c5.

llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp

const MCExpr *TargetLoweringObjectFileMachO::getIndirectSymViaGOTPCRel(
   return MCBinaryExpr::createSub(LHS, RHS, Ctx);
 }
 
 static bool canUsePrivateLabel(const MCAsmInfo &AsmInfo,
                                const MCSection &Section) {
   if (!AsmInfo.isSectionAtomizableBySymbols(Section))
     return true;
 
-  // If it is not dead stripped, it is safe to use private labels.
-  const MCSectionMachO &SMO = cast<MCSectionMachO>(Section);
-  if (SMO.hasAttribute(MachO::S_ATTR_NO_DEAD_STRIP))
-    return true;
-
+  // FIXME: we should be able to use private labels for sections that can't be
+  // dead-stripped (there's no issue with blocking atomization there), but `ld
+  // -r` sometimes drops the no_dead_strip attribute from sections so for safety
+  // we don't allow it.
   return false;
 }
 
 void TargetLoweringObjectFileMachO::getNameWithPrefix(
     SmallVectorImpl<char> &OutName, const GlobalValue *GV,
     const TargetMachine &TM) const {
   bool CannotUsePrivateLabel = true;
   if (auto *GO = GV->getBaseObject()) {

llvm/test/CodeGen/X86/osx-private-labels.ll

 @private12 = private global i32* null, section "__DATA,__foobar,interposing"
 ; CHECK: .section __DATA,__foobar,interposing
 ; CHECK-NEXT: .p2align  3
 ; CHECK-NEXT: L_private12:
 
 @private13 = private global i32 42, section "__DATA, __objc_classlist, regular, no_dead_strip"
 ; CHECK: .section __DATA,__objc_classlist,regular,no_dead_strip
 ; CHECK-NEXT: .p2align  2
-; CHECK-NEXT: L_private13:
+; CHECK-NEXT: _private13:
 
 @private14 = private global [4 x i8] c"zed\00", section "__TEXT,__objc_classname,cstring_literals"
 ; CHECK: .section __TEXT,__objc_classname,cstring_literals
 ; CHECK-NEXT: L_private14:
 
 @private15 = private global [4 x i8] c"zed\00", section "__TEXT,__objc_methname,cstring_literals"
 ; CHECK: .section __TEXT,__objc_methname,cstring_literals
 ; CHECK-NEXT: L_private15:
 
 @private16 = private global [4 x i8] c"zed\00", section "__TEXT,__objc_methtype,cstring_literals"
 ; CHECK: .section __TEXT,__objc_methtype,cstring_literals
 ; CHECK-NEXT: L_private16: