[RISCV] Always emit a symbol for R_RISCV_PCREL_LO12_I.

[Bigcheese]

Summary

riscv-ld mislinks if a section relative symbol is used instead.

I've reported this as a bug
(https://github.com/riscv/riscv-binutils-gdb/issues/138), but it may be
defined as requiring a symbol.

Depends on: https://reviews.llvm.org/D43157

[Bigcheese]

ping

[asb]

Thanks for pursuing this issue on the binutils bug tracker. Might you be interested in proposing a patch to the psabi doc that clarifies the requirement here? It seems that the main reason for forcing this behaviour is for linker relaxation - I hope we can co-ordinate with binutils to allow addends when linker relaxation is disabled.

https://github.com/riscv/riscv-elf-psabi-doc/blob/master/riscv-elf.md

lib/Target/RISCV/MCTargetDesc/RISCVELFObjectWriter.cpp

 
 namespace {
 class RISCVELFObjectWriter : public MCELFObjectTargetWriter {
 public:
   RISCVELFObjectWriter(uint8_t OSABI, bool Is64Bit);
 
   ~RISCVELFObjectWriter() override;
 
+  bool needsRelocateWithSymbol(const MCSymbol &Sym,
+                               unsigned Type) const override;
+
 protected:
   unsigned getRelocType(MCContext &Ctx, const MCValue &Target,
                         const MCFixup &Fixup, bool IsPCRel) const override;
 };
 }
 
 RISCVELFObjectWriter::RISCVELFObjectWriter(uint8_t OSABI, bool Is64Bit)
     : MCELFObjectTargetWriter(Is64Bit, OSABI, ELF::EM_RISCV,
                               /*HasRelocationAddend*/ true) {}
 
 RISCVELFObjectWriter::~RISCVELFObjectWriter() {}
 
+bool RISCVELFObjectWriter::needsRelocateWithSymbol(const MCSymbol &Sym,
+                                                   unsigned Type) const {
+  // riscv-ld depends on R_RISCV_PCREL_LO12_I directly targetting a symbol.
+  // See https://github.com/riscv/riscv-binutils-gdb/issues/138
+  return Type == ELF::R_RISCV_PCREL_LO12_I;
+}
+
 unsigned RISCVELFObjectWriter::getRelocType(MCContext &Ctx,
                                             const MCValue &Target,
                                             const MCFixup &Fixup,
                                             bool IsPCRel) const {
   // Determine the type of the relocation
   switch ((unsigned)Fixup.getKind()) {
   default:
     llvm_unreachable("invalid fixup kind!");

test/MC/RISCV/pcrel-hilo.s

 auipc t1, %pcrel_hi(foo)
 # CHECK-FIXUP: fixup A - offset: 0, value: %pcrel_hi(foo), kind: fixup_riscv_pcrel_hi20
 # CHECK-INSTR: auipc t1, 0
 addi t1, t1, %pcrel_lo(1b)
 # CHECK-FIXUP: fixup A - offset: 0, value: %pcrel_lo(.Ltmp0), kind: fixup_riscv_pcrel_lo12_i
 # CHECK-INSTR: addi t1, t1, 0
 
 # CHECK-REL: 0x0 R_RISCV_PCREL_HI20 foo 0x0
-# CHECK-REL: 0x4 R_RISCV_PCREL_LO12_I .text 0x0
+# CHECK-REL: 0x4 R_RISCV_PCREL_LO12_I .Ltmp0 0x0