[all-commits] [llvm/llvm-project] b257d3: [ELF][PPC64] Suppress toc-indirect to toc-relative...

[Fangrui Song via All-commits]

  Branch: refs/heads/master
  Home:   https://github.com/llvm/llvm-project
  Commit: b257d3c8a897b9999b9bdb660fc8eaad772a0085
      https://github.com/llvm/llvm-project/commit/b257d3c8a897b9999b9bdb660fc8eaad772a0085
  Author: Fangrui Song <maskray at google.com>
  Date:   2020-04-30 (Thu, 30 Apr 2020)

  Changed paths:
    M lld/ELF/InputSection.cpp
    M lld/ELF/InputSection.h
    M lld/ELF/Relocations.cpp
    M lld/ELF/Writer.cpp
    A lld/test/ELF/ppc64-toc-relax2.s

  Log Message:
  -----------
  [ELF][PPC64] Suppress toc-indirect to toc-relative relaxation if R_PPC64_TOC16_LO is seen

The current implementation assumes that R_PPC64_TOC16_HA is always followed
by R_PPC64_TOC16_LO_DS. This can break with R_PPC64_TOC16_LO:

  // Load the address of the TOC entry, instead of the value stored at that address
  addis 3, 2, .LC0 at tloc@ha  # R_PPC64_TOC16_HA
  addi  3, 3, .LC0 at tloc@l   # R_PPC64_TOC16_LO
  blr

which is used by boringssl's util/fipstools/delocate/delocate.go
https://github.com/google/boringssl/blob/master/crypto/fipsmodule/FIPS.md has some documentation.
In short, this tool converts an assembly file to avoid any potential relocations.
The distance to an input .toc is not a constant after linking, so it cannot use an `addis;ld` pair.
Instead, it jumps to a stub which loads the TOC entry address with `addis;addi`.

This patch checks the presence of R_PPC64_TOC16_LO and suppresses
toc-indirect to toc-relative relaxation if R_PPC64_TOC16_LO is seen.
This approach is conservative and loses some relaxation opportunities but is easy to implement.

  addis 3, 2, .LC0 at toc@ha  # no relaxation
  addi  3, 3, .LC0 at toc@l   # no relaxation
  li    9, 0
  addis 4, 2, .LC0 at toc@ha  # can relax but suppressed
  ld    4, .LC0 at toc@l(4)   # can relax but suppressed

Also note that interleaved R_PPC64_TOC16_HA and R_PPC64_TOC16_LO_DS is
possible and this patch accounts for that.

  addis 3, 2, .LC1 at toc@ha  # can relax
  addis 4, 2, .LC2 at toc@ha  # can relax
  ld    3, .LC1 at toc@l(3)   # can relax
  ld    4, .LC2 at toc@l(4)   # can relax

Reviewed By: #powerpc, sfertile

Differential Revision: https://reviews.llvm.org/D78431