[ELF][MIPS] Multi-GOT implementation

Authored by atanasyan on Jun 11 2018, 12:24 AM.


[ELF][MIPS] Multi-GOT implementation

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
    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

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.

Differential revision: https://reviews.llvm.org/D31528

llvm-svn: 334390


atanasyanJun 11 2018, 12:24 AM
Differential Revision
D31528: [ELF][MIPS] Multi-GOT implementation
rGc48435bfe505: [X86] Explicitly mark unsupported classes in scheduling models.