ELF: Add support for relocation type R_X86_64_8.

[pcc]

Summary

Although this relocation type is not part of the x86-64 psABI, I intend to
use it internally as part of the ThinLTO implementation.

[davide]

This is obviously correct, but can you please explain how do you plan to use it in ThinLTO? Thanks.

[pcc]

This is needed for the ThinLTO implementation of control flow integrity, see http://lists.llvm.org/pipermail/llvm-dev/2016-October/106490.html for more details.

The R_X86_64_8 relocation type in particular is needed for 8-bit immediate operands, such as the rorl instruction's rotate count operand.

[davide]

In D28841#649125, @pcc wrote:

This is needed for the ThinLTO implementation of control flow integrity, see http://lists.llvm.org/pipermail/llvm-dev/2016-October/106490.html for more details.

The R_X86_64_8 relocation type in particular is needed for 8-bit immediate operands, such as the rorl instruction's rotate count operand.

LGTM.

lld/ELF/Target.cpp

template <class ELFT> X86_64TargetInfo<ELFT>::X86_64TargetInfo() {
   // FreeBSD automatically promotes large, superpage-aligned allocations.
   DefaultImageBase = 0x200000;
 }
 
 template <class ELFT>
 RelExpr X86_64TargetInfo<ELFT>::getRelExpr(uint32_t Type,
                                            const SymbolBody &S) const {
   switch (Type) {
+  case R_X86_64_8:
   case R_X86_64_32:
   case R_X86_64_32S:
   case R_X86_64_64:
   case R_X86_64_DTPOFF32:
   case R_X86_64_DTPOFF64:
     return R_ABS;
   case R_X86_64_TPOFF32:
     return R_TLS;
void X86_64TargetInfo<ELFT>::relaxTlsLdToLe(uint8_t *Loc, uint32_t Type,
   };
   memcpy(Loc - 3, Inst, sizeof(Inst));
 }
 
 template <class ELFT>
 void X86_64TargetInfo<ELFT>::relocateOne(uint8_t *Loc, uint32_t Type,
                                          uint64_t Val) const {
   switch (Type) {
+  case R_X86_64_8:
+    checkUInt<8>(Loc, Val, Type);
+    *Loc = Val;
+    break;
   case R_X86_64_32:
     checkUInt<32>(Loc, Val, Type);
     write32le(Loc, Val);
     break;
   case R_X86_64_32S:
   case R_X86_64_TPOFF32:
   case R_X86_64_GOT32:
   case R_X86_64_GOTPCREL:

lld/test/ELF/Inputs/x86-64-reloc-8-error.s

+.globl foo
+.hidden foo
+foo = 256

lld/test/ELF/Inputs/x86-64-reloc-8.s

+.globl foo
+.hidden foo
+foo = 0x42

lld/test/ELF/x86-64-reloc-8.s

+// REQUIRES: x86
+
+// RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %S/Inputs/x86-64-reloc-8.s -o %t1
+// RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %S/Inputs/x86-64-reloc-8-error.s -o %t2
+// RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t
+// RUN: ld.lld -shared %t %t1 -o %t3
+
+// CHECK:      Contents of section .text:
+// CHECK-NEXT:   200000 42
+
+// RUN: not ld.lld -shared %t %t2 -o %t4 2>&1 | FileCheck --check-prefix=ERROR %s
+// ERROR: relocation R_X86_64_8 out of range
+
+.byte foo