[AArch64][MachO] Drop a 2015 ld64 workaround avoiding local relocations

[MaskRay]

Summary

Commit d6223a24718df3c30b67422c2eb6d0e93af9c4e8 introduced an ld64 workaround
that disables local relocations for many pointer-sized relocations (e.g.
relocations referencing __DATA,__literal*).

ld64 has been fixed long ago (2015). Drop this workaround so that the
behavior matches x86-64.

This change fixes the following assembler error by emitting an ARM64_RELOC_UNSIGNED referencing the section xray_instr_map

// error: unsupported relocation of local symbol 'Lxray_sleds_end1'. Must have non-local symbol earlier in section.
        .section        __DATA,xray_instr_map
Lxray_sleds_start1:
        .space  13
Lxray_sleds_end1:

        .section        __DATA,xray_fn_idx
        .quad   Lxray_sleds_start1
        .quad   Lxray_sleds_end1

.subsections_via_symbols

(Though we will make xray_fn_idx entries PC-relative (D152661) to avoid the
issue in another way).

[MaskRay]

remove a stale comment

[MaskRay]

@ilammy This patch will fix the .quad Lxray_sleds_end1 (error: unsupported relocation of local symbol 'Lxray_sleds_end1'. Must have non-local symbol earlier in section.) issue for arm64-apple-darwin XRay.
Though D152661 (PC-relative entries) will avoid this construct in the first place.

[MaskRay]

Ping:)

[pete]

I don't even remember this bug from the ld64 side. But yeah, if things are good now then i don't see any issue with this. Thanks!

[MaskRay]

In D152831#4462187, @pete wrote:

I don't even remember this bug from the ld64 side. But yeah, if things are good now then i don't see any issue with this. Thanks!

Thanks! It may be a long-forgotten bug :)

llvm/lib/Target/AArch64/MCTargetDesc/AArch64MachObjectWriter.cpp

static bool canUseLocalRelocation(const MCSectionMachO &Section,
   // But only if they don't point to a few forbidden sections.
   if (!Symbol.isInSection())
     return true;
   const MCSectionMachO &RefSec = cast<MCSectionMachO>(Symbol.getSection());
   if (RefSec.getType() == MachO::S_CSTRING_LITERALS)
     return false;
 
   if (RefSec.getSegmentName() == "__DATA" &&
-      RefSec.getName() == "__objc_classrefs")
+      (RefSec.getName() == "__cfstring" ||
+       RefSec.getName() == "__objc_classrefs"))
     return false;
 
-  // FIXME: ld64 currently handles internal pointer-sized relocations
-  // incorrectly (applying the addend twice). We should be able to return true
-  // unconditionally by this point when that's fixed.
-  return false;
+  return true;
 }
 
 void AArch64MachObjectWriter::recordRelocation(
     MachObjectWriter *Writer, MCAssembler &Asm, const MCAsmLayout &Layout,
     const MCFragment *Fragment, const MCFixup &Fixup, MCValue Target,
     uint64_t &FixedValue) {
   unsigned IsPCRel = Writer->isFixupKindPCRel(Asm, Fixup.getKind());
 

llvm/test/MC/MachO/AArch64/ld64-workaround.s

 ; RUN: llvm-mc -triple arm64-apple-darwin10 %s -filetype=obj -o - | llvm-readobj -r --expand-relocs - | FileCheck %s
 
-; Test that we produce an external relocation. This is a known and temporary bug
-; in ld64, where it mishandles pointer-sized internal relocations. We should be
-; able to remove this entirely soon.
+/// To work around an ld64 arm64 bug before 2015, we used to disable local
+/// relocations for many pointer-sized relocations. Test that the workaround
+/// is dropped and the behavior now matches x86-64.
 
 // CHECK:      Relocations [
 // CHECK-NEXT:   Section __data {
 // CHECK-NEXT:     Relocation {
 // CHECK-NEXT:       Offset: 0x18
 // CHECK-NEXT:       PCRel: 0
 // CHECK-NEXT:       Length: 3
 // CHECK-NEXT:       Type: ARM64_RELOC_UNSIGNED (0)
-// CHECK-NEXT:       Symbol: Llit16
+// CHECK-NEXT:       Section: __literal16
 // CHECK-NEXT:     }
 // CHECK-NEXT:     Relocation {
 // CHECK-NEXT:       Offset: 0x10
 // CHECK-NEXT:       PCRel: 0
 // CHECK-NEXT:       Length: 3
 // CHECK-NEXT:       Type: ARM64_RELOC_UNSIGNED (0)
-// CHECK-NEXT:       Symbol: Llit8
+// CHECK-NEXT:       Section: __literal8
 // CHECK-NEXT:     }
 // CHECK-NEXT:     Relocation {
 // CHECK-NEXT:       Offset: 0x8
 // CHECK-NEXT:       PCRel: 0
 // CHECK-NEXT:       Length: 3
 // CHECK-NEXT:       Type: ARM64_RELOC_UNSIGNED (0)
-// CHECK-NEXT:       Symbol: Llit4
+// CHECK-NEXT:       Section: __literal4
 // CHECK-NEXT:     }
 // CHECK-NEXT:     Relocation {
 // CHECK-NEXT:       Offset: 0x0
 // CHECK-NEXT:       PCRel: 0
 // CHECK-NEXT:       Length: 3
 // CHECK-NEXT:       Type: ARM64_RELOC_UNSIGNED (0)
 // CHECK-NEXT:       Symbol: Lcfstring
 // CHECK-NEXT:     }

llvm/test/MC/MachO/AArch64/mergeable.s

 // RUN: llvm-mc -triple aarch64-apple-darwin14 %s -filetype=obj -o - | llvm-readobj -r --expand-relocs - | FileCheck %s
-// FIXME: the final relocation should be internal, but the linker doesn't
-// currently handle the it correctly.
 
 // Test that we "S + K" produce a relocation with a symbol, but just S produces
 // a relocation with the section.
 
   .section  __TEXT,__literal4,4byte_literals
 L0:
   .long 42
 

 // CHECK-NEXT:       Type: ARM64_RELOC_UNSIGNED (0)
 // CHECK-NEXT:       Symbol: L0
 // CHECK-NEXT:     }
 // CHECK-NEXT:     Relocation {
 // CHECK-NEXT:       Offset: 0x0
 // CHECK-NEXT:       PCRel: 0
 // CHECK-NEXT:       Length: 3
 // CHECK-NEXT:       Type: ARM64_RELOC_UNSIGNED (0)
-// CHECK-NEXT:       Symbol: L0
+// CHECK-NEXT:       Section: __literal4
 // CHECK-NEXT:     }
 // CHECK-NEXT:   }
 // CHECK-NEXT: ]