[AARCH64] Renumber relocation codes R_AARCH64_TLS_TPREL64 and DTPMOD64

[peter.smith]

Summary

The ELF for the Arm 64-bit Architecture document originally specified

R_AARCH64_TLS_DTPREL64 = 0x404    // 1028
R_AARCH64_TLS_DTPMOD64 = 0x405 // 1029

LLVM correctly followed the document. Unfortunately in binutils(1) these two
codes were reversed:

R_AARCH64_TLS_DTPMOD64 = 0x404 // 1028
R_AARCH64_TLS_DTPREL64 = 0x405    // 1029

Given that binaries had shipped this change has become the defacto standard interpretation of these relocation codes for any toolchain that wanted to remain compatible with GNU.

To recognize this the latest version of the ABI document (2) has renamed the relocations to R_AARCH64_TLS_IMPDEF1 and R_AARCH64_TLS_IMPDEF2 permitting a toolchain to choose between the two relocation types, and recommending that toolchains follow the GNU interpretation for maximum compatibility.

Given that upstream llvm has never implemented the standard ELF TLS model for AArch64, we have no binary legacy. Therefore synchronize with GCC so that we don't create incompatible objects in the future. So far the only visible change is in llvm-readobj as it can decode these relocations so this change will mean that llvm-readobj decodes the same way as GNU readelf.

(1) https://chromium.googlesource.com/native_client/nacl-binutils/+/binutils-2_25_1/elfcpp/aarch64.h#174
(2) https://developer.arm.com/docs/ihi0056/latest/elf-for-the-arm-64-bit-architecture-aarch64-abi-2019q2-documentation

fixes https://bugs.llvm.org/show_bug.cgi?id=40507

[rprichard]

Looks good to me.

[MaskRay]

Renumber relocation codes R_AARCH64_TLS_TPREL64 and DTPMOD64

A small typo: s/R_AARCH64_TLS_TPREL64/R_AARCH64_TLS_DTPREL64/

[peter.smith]

Thanks for the review, committed r367437. Forgot to put differential revision so it won't automatically get closed, apologies.

include/llvm/BinaryFormat/ELFRelocs/AArch64.def

 ELF_RELOC(R_AARCH64_GLOB_DAT,                        0x401)
 ELF_RELOC(R_AARCH64_JUMP_SLOT,                       0x402)
 ELF_RELOC(R_AARCH64_RELATIVE,                        0x403)
-ELF_RELOC(R_AARCH64_TLS_DTPREL64,                    0x404)
-ELF_RELOC(R_AARCH64_TLS_DTPMOD64,                    0x405)
+// 0x404 and 0x405 are now R_AARCH64_TLS_IMPDEF1 and R_AARCH64_TLS_IMPDEF2
+// We follow GNU and define TLS_IMPDEF1 as TLS_DTPMOD64 and TLS_IMPDEF2 as
+// TLS_DTPREL64
+ELF_RELOC(R_AARCH64_TLS_DTPMOD64,                    0x404)
+ELF_RELOC(R_AARCH64_TLS_DTPREL64,                    0x405)
 ELF_RELOC(R_AARCH64_TLS_TPREL64,                     0x406)
 ELF_RELOC(R_AARCH64_TLSDESC,                         0x407)
 ELF_RELOC(R_AARCH64_IRELATIVE,                       0x408)

test/tools/llvm-readobj/Inputs/relocs.py

   R_AARCH64_GLOB_DAT                    = 0x401
   R_AARCH64_JUMP_SLOT                   = 0x402
   R_AARCH64_RELATIVE                    = 0x403
-  R_AARCH64_TLS_DTPREL64                = 0x404
-  R_AARCH64_TLS_DTPMOD64                = 0x405
+  R_AARCH64_TLS_DTPMOD64                = 0x404
+  R_AARCH64_TLS_DTPREL64                = 0x405
   R_AARCH64_TLS_TPREL64                 = 0x406
   R_AARCH64_TLSDESC                     = 0x407
   R_AARCH64_IRELATIVE                   = 0x408

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

 # CHECK: Type: R_AARCH64_GLOB_DAT (1025)
 # CHECK: Type: R_AARCH64_JUMP_SLOT (1026)
 # CHECK: Type: R_AARCH64_RELATIVE (1027)
-# CHECK: Type: R_AARCH64_TLS_DTPREL64 (1028)
-# CHECK: Type: R_AARCH64_TLS_DTPMOD64 (1029)
+# CHECK: Type: R_AARCH64_TLS_DTPMOD64 (1028)
+# CHECK: Type: R_AARCH64_TLS_DTPREL64 (1029)
 # CHECK: Type: R_AARCH64_TLS_TPREL64 (1030)
 # CHECK: Type: R_AARCH64_TLSDESC (1031)
 # CHECK: Type: R_AARCH64_IRELATIVE (1032)

       - Offset: 0x00000000000001D4
         Type:   R_AARCH64_RELATIVE
       - Offset: 0x00000000000001D8
-        Type:   R_AARCH64_TLS_DTPREL64
-      - Offset: 0x00000000000001DC
         Type:   R_AARCH64_TLS_DTPMOD64
+      - Offset: 0x00000000000001DC
+        Type:   R_AARCH64_TLS_DTPREL64
       - Offset: 0x00000000000001E0
         Type:   R_AARCH64_TLS_TPREL64
       - Offset: 0x00000000000001E4