[PATCH] D105254: [RISCV] Support machine constraint "S"

[Fangrui Song via Phabricator via cfe-commits]

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Similar to D46745 <https://reviews.llvm.org/D46745>, "S" represents an absolute symbolic operand, which
can be used to specify the access models, e.g.

  extern int var;
  void *addr_via_asm() {
    void *ret;
    asm("lui %0, %%hi(%1)\naddi %0,%0,%%lo(%1)" : "=r"(ret) : "S"(&var));
    return ret;
  }

We need 'S' to be documented on GCC: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101275


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D105254

Files:
  clang/lib/Basic/Targets/RISCV.cpp
  clang/test/CodeGen/RISCV/riscv-inline-asm.c
  llvm/lib/Target/RISCV/RISCVISelLowering.cpp
  llvm/test/CodeGen/RISCV/inline-asm-S-constraint.ll

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