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

[Simon Atanasyan via llvm-commits]

Hi,

On Sat, Apr 1, 2017 at 4:20 AM, Rui Ueyama via Phabricator
<reviews at reviews.llvm.org> wrote:
>
> This is not your fault, but I have to say that this MIPS GOT layout is very odd,
> too different from other architectures, and too complicated. I want to avoid supporting
> this unless I'm convinced that it is absolutely necessary. It seems to me that MIPS
> needs a clean, common new ABI. Only the MIPS ABI imposes a lot of restrictions
> on the size of GOT sections and the order of GOT section members, even though MIPS
> as a processor is an ordinary RISC ISA. This change would really hurt maintainability
> of LLD which I already found some MIPS-specific behavior is hard to keep it right
> when editing code for all the other architectures.

MIPS will not always use old, obsoleted ABIs. It will switch to new
one. But it does not
happen this year or so. Besides other obstacles, there is a hardware
problem prevents from
fast switching and common acceptance of the new ABI. Historically many
MIPS instructions
are partitioned as 16 bit for opcode and 16 bit bit for address/index.
That is one of
the source of GOT size limitation and reason of multi-GOT invention.

The biggest part of the patch isolated in the MipsGotSection class. It
adds some new
MIPS specific code like new constructor of the DynamicReloc class. But
at the same
time it removes some `if (Config->EMachine == EM_MIPS)` statements and
MIPS specific
fields from the `SymbolBody` class.

> I wonder what is the performance penalty you would have to pay when you use the -mxgot
> option. With the option, you'll need three instructions as opposed to a single instruction
> to access an GOT entry. Does that actually make observable difference in performance?

Regular (without -mxgot) access to GOT requires a single instruction:

lw  t9,0(gp)

I was wrong when say about two instructions. With -mxgot option we get
three instructions.

lui     at,0x0
addu    at,at,gp
lw      t9,0(at)

In case of MIPS global offset table is used not only to call external
functions / access
external data but for local calls / access under some conditions. So
using -mxgot we can
easily grow the code size and reduce performance.

> ================
> Comment at: ELF/DriverUtils.cpp:31
>  using namespace llvm;
> +using namespace llvm::opt;
>  using namespace llvm::sys;
> ----------------
> Do you need this?

It's necessary to use the `HelpHidden` flag in the `mips_got_size`
option definition. That allows to hide this option from the list shown
by the `-help`.

-- 
Simon Atanasyan