[RISCV] Support Parsing Nonstandard Relocations

This allows nonstandard relocation names to be used in `.reloc` assembly
directives (giving the correct relocation number).

No translation is done by the assembler into `R_RISCV_CUSTOM<n>` names,
and the assembler does not automatically add the relevant
`R_RISCV_VENDOR` relocation with the vendor symbol. If we want, we can
have a different directive that does this later.

The first batch of relocations to be added are from Qualcomm's RISC-V
psABI extensions.

Author: lenary

llvm/include/llvm/BinaryFormat/ELFRelocs/RISCV_nonstandard.def

@@ -0,0 +1,30 @@
+//===--- RISC-V Nonstandard Relocation List ---------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef ELF_RISCV_NONSTANDARD_RELOC
+#error "ELF_RISCV_NONSTANDARD_RELOC must be defined"
+#endif
+
+/*
+  ELF_RISCV_NONSTANDARD_RELOC(VENDOR, NAME, ID) defines information about
+  nonstandard relocation codes. This can be used when parsing relocations, or
+  when printing them, to provide better information.
+
+  VENDOR should be the symbol name expected in the associated `R_RISCV_VENDOR`
+  relocation. NAME and ID work like `ELF_RELOC` but the mapping is not expected
+  to be 1:1.
+
+  The mapping in RISCV.def is 1:1, and should be used when the only information
+  available is the relocation enum value.
+*/
+
+/* Qualcomm Nonstandard Relocations */
+ELF_RISCV_NONSTANDARD_RELOC(QUALCOMM, R_RISCV_QC_LO20_U,     192)
+ELF_RISCV_NONSTANDARD_RELOC(QUALCOMM, R_RISCV_QC_E_BRANCH,   193)
+ELF_RISCV_NONSTANDARD_RELOC(QUALCOMM, R_RISCV_QC_E_32,       194)
+ELF_RISCV_NONSTANDARD_RELOC(QUALCOMM, R_RISCV_QC_E_JUMP_PLT, 195)

llvm/lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.cpp

@@ -38,9 +38,12 @@ std::optional<MCFixupKind> RISCVAsmBackend::getFixupKind(StringRef Name) const {
   if (STI.getTargetTriple().isOSBinFormatELF()) {
     unsigned Type;
     Type = llvm::StringSwitch<unsigned>(Name)
-#define ELF_RELOC(X, Y) .Case(#X, Y)
+#define ELF_RELOC(NAME, ID) .Case(#NAME, ID)
 #include "llvm/BinaryFormat/ELFRelocs/RISCV.def"
 #undef ELF_RELOC
+#define ELF_RISCV_NONSTANDARD_RELOC(_VENDOR, NAME, ID) .Case(#NAME, ID)
+#include "llvm/BinaryFormat/ELFRelocs/RISCV_nonstandard.def"
+#undef ELF_RISCV_NONSTANDARD_RELOC
                .Case("BFD_RELOC_NONE", ELF::R_RISCV_NONE)
                .Case("BFD_RELOC_32", ELF::R_RISCV_32)
                .Case("BFD_RELOC_64", ELF::R_RISCV_64)

llvm/test/MC/RISCV/custom_reloc.s

@@ -33,3 +33,20 @@
   nop
   # CHECK-ASM: nop
   # CHECK-OBJ: addi zero, zero, 0x0
+
+  .reloc ., R_RISCV_VENDOR,    QUALCOMM
+  .reloc ., R_RISCV_QC_LO20_U, my_bar + 2
+  addi a1, a1, 0
+  # CHECK-ASM: [[L3:.L[^:]+]]:
+  # CHECK-ASM-NEXT: .reloc [[L3]], R_RISCV_VENDOR, QUALCOMM
+  # CHECK-ASM-NEXT: [[L4:.L[^:]+]]:
+  # CHECK-ASM-NEXT: .reloc [[L4]], R_RISCV_QC_LO20_U, my_bar+2
+  # CHECK-ASM-NEXT: mv a1, a1
+
+  # CHECK-OBJ: addi a1, a1, 0
+  # CHECK-OBJ-NEXT: R_RISCV_VENDOR    QUALCOMM
+  # CHECK-OBJ-NEXT: R_RISCV_CUSTOM192 my_bar+0x2
+
+  nop
+  # CHECK-ASM: nop
+  # CHECK-OBJ: addi zero, zero, 0x0