[PATCH] D54720: [PPC64] toc-indirect to toc-relative relaxation.

[Sean Fertile via Phabricator via llvm-commits]

sfertile created this revision.
sfertile added reviewers: syzaara, ruiu, MaskRay, rdhindsa.
Herald added subscribers: jsji, kbarton, arichardson, nemanjai, emaste.
Herald added a reviewer: espindola.

When accessing a global variable which is not defined in the translation-unit being compiled the compilers on ppc64 will generate a toc-entry for the global and a got-indirect access using that toc-entry (as if it were a .got entry)

For example the following C code

  extern int aGlobal;
  
  int foo(void) {
    return aGlobal;
  } 

would get translated to:

          addis 3, 2, .LC0 at toc@ha 
          ld 3, .LC0 at toc@l(3)
          lwa 3, 0(3)
  
  ...
  
          .section        .toc,"aw", at progbits
  .LC0:
          .tc aGlobal[TC],aGlobal

Where `.LC0 at toc` is the offset from the TOC base-pointer to the label .LC0, and ` .tc aGlobal[TC],aGlobal` creates an entry in the .toc section to store the address of `aGlobal`.
The first 2 instructions build the address of the toc-entry and load the address of aGlobal into r3, while the following `lwa` instruction loads the value of the global into r3.

If the global being accessed is a non-preemptable definition and the offset from the TOC pointer to the definition can be materialized with 2 instructions then we can relax the first 2 instructions to add that offset to the TOC base pointer rather then loading the address out of the .toc.


Repository:
  rLLD LLVM Linker

https://reviews.llvm.org/D54720

Files:
  ELF/Arch/PPC64.cpp
  ELF/Arch/X86_64.cpp
  ELF/InputSection.cpp
  ELF/Relocations.cpp
  ELF/Relocations.h
  ELF/Target.cpp
  ELF/Target.h
  test/ELF/Inputs/ppc64-global.s
  test/ELF/Inputs/ppc64-local.s
  test/ELF/ppc64-func-entry-points.s
  test/ELF/ppc64-got-indirect.s
  test/ELF/ppc64-relocs.s
  test/ELF/ppc64-toc-relax-jumptable.s
  test/ELF/ppc64-toc-relax.s

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