[PATCH] D31528: [ELF][MIPS] Multi-GOT implementation

[Simon Atanasyan via Phabricator via llvm-commits]

atanasyan created this revision.
atanasyan added a project: lld.
Herald added a subscriber: sdardis.

Almost all entries inside MIPS GOT are referenced by signed 16-bit index. Zero entry lies approximately in the middle of the GOT. So the total number of GOT entries cannot exceed ~16384 for 32-bit architecture and ~8192 for 64-bit architecture. This limitation makes impossible to link rather large application like for example LLVM+Clang. There are two workaround for this problem. The first one is using the `-mxgot` compiler's flag. It enables using a 32-bit index to access GOT entries. But each access requires two assembly instructions two load GOT entry index to a register. Another workaround is multi-GOT. This patch implements it.

Here is a brief description of multi-GOT for detailed one see the following link https://dmz-portal.mips.com/wiki/MIPS_Multi_GOT.

If the sum of local, global and tls entries is less than 64K only single got is enough. Otherwise, multi-got is created. Series of primary and multiple secondary GOTs have the following layout:

  - Primary GOT
      Header
      Local entries
      Global entries
      Relocation only entries
      TLS entries
  
  - Secondary GOT
      Local entries
      Global entries
      TLS entries
  ...

All GOT entries required by relocations from a single input file entirely belong to either primary or one of secondary GOTs. To reference GOT entries each GOT has its own _gp value points to the "middle" of the GOT. In the code this value loaded to the register which is used for GOT access.

MIPS 32 function's prologue:

  lui     v0,0x0
  0: R_MIPS_HI16  _gp_disp
  addiu   v0,v0,0
  4: R_MIPS_LO16  _gp_disp

MIPS 64 function's prologue:

  lui     at,0x0
  14: R_MIPS_GPREL16  main

Dynamic linker does not know anything about secondary GOTs and cannot use a regular MIPS mechanism for GOT entries initialization. So we have to use an approach accepted by other architectures and create dynamic relocations R_MIPS_REL32 to initialize global entries (and local in case of PIC code) in secondary GOTs. But ironically MIPS dynamic linker requires GOT entries and correspondingly ordered dynamic symbol table entries to deal with dynamic relocations. To handle this problem relocation-only section in the primary GOT contains entries for all symbols referenced in global parts of secondary GOTs. Although the sum of local and normal global entries of the primary got should be less than 64K, the size of the primary got (including relocation-only entries can be greater than 64K, because parts of the primary got that overflow the 64K limit are used only by the dynamic linker at dynamic link-time and not by 16-bit gp-relative addressing at run-time.

The patch affects common LLD code in the following places:

- Added new hidden `-mips-got-size` flag. This flag required to set low maximum size of a single GOT to be able to test the implementation using small test cases.
- Added `InputFile` argument to the `getRelocTargetVA` function. The same symbol referenced by GOT relocation from different input file might be allocated in different GOT. So result of relocation depends on the file.
- Added new ctor to the `DynamicReloc` class. This constructor records settings of dynamic relocation which used to adjust address of 64kb page lies inside a specific output section.

With the patch LLD is able to link all LLVM+Clang+LLD applications and libraries for MIPS 32/64 targets.


Repository:
  rL LLVM

https://reviews.llvm.org/D31528

Files:
  ELF/Config.h
  ELF/Driver.cpp
  ELF/DriverUtils.cpp
  ELF/InputFiles.h
  ELF/InputSection.cpp
  ELF/Options.td
  ELF/Relocations.cpp
  ELF/SyntheticSections.cpp
  ELF/SyntheticSections.h
  ELF/Writer.cpp
  test/ELF/Inputs/mips-mgot-1.s
  test/ELF/Inputs/mips-mgot-2.s
  test/ELF/mips-dynamic.s
  test/ELF/mips-got-weak.s
  test/ELF/mips-mgot.s
  test/ELF/mips-plt-copy.s
  test/ELF/mips-sto-plt.s
  test/ELF/mips-tls-64.s
  test/ELF/mips-tls-static.s
  test/ELF/mips-tls.s

-------------- next part --------------
A non-text attachment was scrubbed...
Name: D31528.93610.patch
Type: text/x-patch
Size: 69191 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20170331/ae280bc9/attachment.bin>