[PATCH] D129425: [ELF][RISCV] Relax local-exec TLS model

[Fangrui Song via Phabricator via llvm-commits]

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In -mrelax mode, GCC/Clang may generate a local-exec TLS code sequence like:

  # R_RISCV_TPREL_HI20, R_RISCV_RELAX
  lui rd, %tprel_hi(x)
  # R_RISCV_TPREL_ADD, R_RISCV_RELAX
  add rd, rd, tp, %tprel_add(x)
  # (R_RISCV_TPREL_LO12_I || R_RISCV_TPREL_LO12_S), R_RISCV_RELAX
  addi rd, rd, %tprel_lo(x) || sw rs, %tprel(x)(rd)

Note: st_value(x) for TLS should in the range [0,p_memsz(PT_TLS)).
When st_value(x) < 2048 (i.e. hi20(x) == 0), the linker can relax
the code sequence to:

  addi rd, tp, st_value(x) || sw rs, st_value(x)(rd)

R_RISCV_TPREL_I/R_RISCV_TPREL_S are relocation types for linkers' internal use.
Their is no %tprel modifier to generate them.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D129425

Files:
  lld/ELF/Arch/RISCV.cpp
  lld/test/ELF/riscv-tls-le.s
  llvm/include/llvm/BinaryFormat/ELFRelocs/RISCV.def

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