MC: Introduce R_AARCH64_PATCHINST relocation type.

The R_AARCH64_PATCHINST relocation type is to support deactivation
symbols. For more information, see the RFC:
https://discourse.llvm.org/t/rfc-deactivation-symbols/85556

Part of the AArch64 psABI extension:
ARM-software/abi-aa#340

Author: pcc

llvm/include/llvm/BinaryFormat/ELFRelocs/AArch64.def

@@ -61,6 +61,7 @@ ELF_RELOC(R_AARCH64_LD64_GOT_LO12_NC,                0x138)
 ELF_RELOC(R_AARCH64_LD64_GOTPAGE_LO15,               0x139)
 ELF_RELOC(R_AARCH64_PLT32,                           0x13a)
 ELF_RELOC(R_AARCH64_GOTPCREL32,                      0x13b)
+ELF_RELOC(R_AARCH64_PATCHINST,                       0x13c)
 // General dynamic TLS relocations
 ELF_RELOC(R_AARCH64_TLSGD_ADR_PREL21,                0x200)
 ELF_RELOC(R_AARCH64_TLSGD_ADR_PAGE21,                0x201)

llvm/lib/Target/AArch64/MCTargetDesc/AArch64ELFObjectWriter.cpp

@@ -40,6 +40,7 @@ class AArch64ELFObjectWriter : public MCELFObjectTargetWriter {
                         bool IsPCRel) const override;
   bool needsRelocateWithSymbol(const MCValue &, unsigned Type) const override;
   bool isNonILP32reloc(const MCFixup &Fixup, AArch64::Specifier RefKind) const;
+  void sortRelocs(std::vector<ELFRelocationEntry> &Relocs) override;
 
   bool IsILP32;
 };
@@ -498,6 +499,17 @@ bool AArch64ELFObjectWriter::needsRelocateWithSymbol(const MCValue &Val,
                       Val.getSpecifier());
 }
 
+void AArch64ELFObjectWriter::sortRelocs(
+    std::vector<ELFRelocationEntry> &Relocs) {
+  // PATCHINST relocations should be applied last because they may overwrite the
+  // whole instruction and so should take precedence over other relocations that
+  // modify operands of the original instruction.
+  std::stable_partition(Relocs.begin(), Relocs.end(),
+                        [](const ELFRelocationEntry &R) {
+                          return R.Type != ELF::R_AARCH64_PATCHINST;
+                        });
+}
+
 std::unique_ptr<MCObjectTargetWriter>
 llvm::createAArch64ELFObjectWriter(uint8_t OSABI, bool IsILP32) {
   return std::make_unique<AArch64ELFObjectWriter>(OSABI, IsILP32);

llvm/test/MC/AArch64/patchinst.s

@@ -0,0 +1,7 @@
+// RUN: llvm-mc -triple aarch64-elf -filetype=obj %s -o - | llvm-objdump -r - | FileCheck %s
+
+// Test that PATCHINST appears after JUMP26.
+// CHECK:      R_AARCH64_JUMP26
+// CHECK-NEXT: R_AARCH64_PATCHINST
+.reloc ., R_AARCH64_PATCHINST, ds
+b f1

llvm/test/tools/llvm-readobj/ELF/reloc-types-aarch64.test

@@ -57,6 +57,7 @@
 # CHECK: Type: R_AARCH64_LD64_GOT_LO12_NC (312)
 # CHECK: Type: R_AARCH64_LD64_GOTPAGE_LO15 (313)
 # CHECK: Type: R_AARCH64_PLT32 (314)
+# CHECK: Type: R_AARCH64_PATCHINST (316)
 # CHECK: Type: R_AARCH64_TLSGD_ADR_PREL21 (512)
 # CHECK: Type: R_AARCH64_TLSGD_ADR_PAGE21 (513)
 # CHECK: Type: R_AARCH64_TLSGD_ADD_LO12_NC (514)
@@ -214,6 +215,7 @@ Sections:
       - Type: R_AARCH64_LD64_GOT_LO12_NC
       - Type: R_AARCH64_LD64_GOTPAGE_LO15
       - Type: R_AARCH64_PLT32
+      - Type: R_AARCH64_PATCHINST
       - Type: R_AARCH64_TLSGD_ADR_PREL21
       - Type: R_AARCH64_TLSGD_ADR_PAGE21
       - Type: R_AARCH64_TLSGD_ADD_LO12_NC