[JITLink][x86-64] Add x86_64::Pointer8 edge kind, ELF::R_X86_64_8 reloc support.

This commit adds an x86-64 Pointer8 edge kind (8-bit pointer), and uses it to
implement support for the ELF::R_X86_64_8 relocation kind.

Author: lhames

llvm/include/llvm/ExecutionEngine/JITLink/x86_64.h

@@ -64,6 +64,17 @@ enum EdgeKind_x86_64 : Edge::Kind {
   ///
   Pointer16,
 
+  /// A plain 8-bit pointer value relocation.
+  ///
+  /// Fixup expression:
+  ///   Fixup <- Target + Addend : uint8
+  ///
+  /// Errors:
+  ///   - The target must reside in the low 8-bits of the address space,
+  ///     otherwise an out-of-range error will be returned.
+  ///
+  Pointer8,
+
   /// A 64-bit delta.
   ///
   /// Delta from the fixup to the target.
@@ -435,6 +446,15 @@ inline Error applyFixup(LinkGraph &G, Block &B, const Edge &E,
     break;
   }
 
+  case Pointer8: {
+    uint64_t Value = E.getTarget().getAddress().getValue() + E.getAddend();
+    if (LLVM_LIKELY(isUInt<8>(Value)))
+      *(uint8_t *)FixupPtr = Value;
+    else
+      return makeTargetOutOfRangeError(G, B, E);
+    break;
+  }
+
   case PCRel32:
   case BranchPCRel32:
   case BranchPCRel32ToPtrJumpStub:

llvm/lib/ExecutionEngine/JITLink/ELF_x86_64.cpp

@@ -162,6 +162,9 @@ class ELFLinkGraphBuilder_x86_64 : public ELFLinkGraphBuilder<object::ELF64LE> {
     case ELF::R_X86_64_16:
       Kind = x86_64::Pointer16;
       break;
+    case ELF::R_X86_64_8:
+      Kind = x86_64::Pointer8;
+      break;
     case ELF::R_X86_64_32S:
       Kind = x86_64::Pointer32Signed;
       break;

llvm/lib/ExecutionEngine/JITLink/x86_64.cpp

@@ -28,6 +28,8 @@ const char *getEdgeKindName(Edge::Kind K) {
     return "Pointer32Signed";
   case Pointer16:
     return "Pointer16";
+  case Pointer8:
+    return "Pointer8";
   case Delta64:
     return "Delta64";
   case Delta32:

llvm/test/ExecutionEngine/JITLink/x86-64/ELF_R_X86_64_8.s

@@ -0,0 +1,36 @@
+# RUN: llvm-mc -triple=x86_64-unknown-linux -position-independent \
+# RUN:     -filetype=obj -o %t.o %s
+# RUN: llvm-jitlink -noexec -abs X=0x12 -check=%s %t.o
+# RUN: not llvm-jitlink -noexec -abs X=0x123 %t.o 2>&1 | \
+# RUN:   FileCheck -check-prefix=CHECK-ERROR %s
+#
+# Check success and failure cases of R_X86_64_16 handling.
+
+# jitlink-check: *{8}P = X
+
+# CHECK-ERROR: relocation target "X" {{.*}} is out of range of Pointer8 fixup
+
+	.text
+	.section	.text.main,"ax",@progbits
+	.globl	main
+	.p2align	4, 0x90
+	.type	main,@function
+main:
+	xorl	%eax, %eax
+	retq
+.Lfunc_end0:
+	.size	main, .Lfunc_end0-main
+
+	.type	P,@object
+	.data
+	.globl	P
+P:
+	.byte	X    # Using byte here generates R_X86_64_8.
+	.byte   0
+	.byte   0
+	.byte   0
+	.byte   0
+	.byte   0
+	.byte   0
+	.byte   0
+	.size	P, 8