llvm dev - Nov 2016 - [LLD] Writing thunks before the corresponding section

Hi,

Sorry for delay with reply.

It looks like now thunks can be implemented as a synthetic sections.
In that case we give flexible solution and will be able to put thunks
before/after related sections, using different alignment etc. As far
as I know BFD linker uses the same approach at least for MIPS thunks.
I will try to implement this idea.

On Thu, Sep 8, 2016 at 1:44 AM, Rui Ueyama <ruiu at google.com>
wrote:
> This seems to be a reasonable optimization, and I don't have any
particular
> concern about implementing it.
>
> On Wed, Sep 7, 2016 at 6:58 AM, Simon Atanasyan <simon at
atanasyan.com> wrote:
>>
>> Hi,
>>
>> MIPS LA25 thunk is used to call PIC function from non-PIC code.
>> Usually it contains three instructions:
>>
>> lui   $25, %hi(func)
>> addiu $25, $25, %lo(func)
>> j     func
>>
>> We can write such thunk in an arbitrary place of the generated file.
>> But if a PIC function requires the thunk is the first routine in a
>> section, we can optimize the code and escape jump instruction. To do
>> so we just write the following thunk right before the PIC routine.
>>
>> lui   $25, %hi(func)
>> addiu $25, $25, %lo(func)
>>
>> In fact GNU bfd/gold linkers write all MIPS LA25 thunks required for
>> the section "A" into a separate input section "S"
and put section "S"
>> before "A". The last thunk in the section "S" might
have an optimized
>> two-instructions form.
>>
>> I would like to implement such optimization in LLD. My question is
>> about ARM thunks - is it okay to write them before corresponding input
>> section not after like LLD does now?
-- 
Simon Atanasyan

Sure. One thing I want to remind you is that there is a place in Writer.cpp
where we assume all synthetic sections were appended to end of Sections
list. Look for `llvm::reverse`. If you add synthetic section thunks right
before/after non-synthetic sections, you also want to change that.

On Tue, Nov 29, 2016 at 1:18 PM, Simon Atanasyan <simon at atanasyan.com>
wrote:
> Hi,
>
> Sorry for delay with reply.
>
> It looks like now thunks can be implemented as a synthetic sections.
> In that case we give flexible solution and will be able to put thunks
> before/after related sections, using different alignment etc. As far
> as I know BFD linker uses the same approach at least for MIPS thunks.
> I will try to implement this idea.
>
> On Thu, Sep 8, 2016 at 1:44 AM, Rui Ueyama <ruiu at google.com>
wrote:
> > This seems to be a reasonable optimization, and I don't have any
> particular
> > concern about implementing it.
> >
> > On Wed, Sep 7, 2016 at 6:58 AM, Simon Atanasyan <simon at
atanasyan.com>
> wrote:
> >>
> >> Hi,
> >>
> >> MIPS LA25 thunk is used to call PIC function from non-PIC code.
> >> Usually it contains three instructions:
> >>
> >> lui   $25, %hi(func)
> >> addiu $25, $25, %lo(func)
> >> j     func
> >>
> >> We can write such thunk in an arbitrary place of the generated
file.
> >> But if a PIC function requires the thunk is the first routine in a
> >> section, we can optimize the code and escape jump instruction. To
do
> >> so we just write the following thunk right before the PIC routine.
> >>
> >> lui   $25, %hi(func)
> >> addiu $25, $25, %lo(func)
> >>
> >> In fact GNU bfd/gold linkers write all MIPS LA25 thunks required
for
> >> the section "A" into a separate input section
"S" and put section "S"
> >> before "A". The last thunk in the section "S"
might have an optimized
> >> two-instructions form.
> >>
> >> I would like to implement such optimization in LLD. My question is
> >> about ARM thunks - is it okay to write them before corresponding
input
> >> section not after like LLD does now?
>
> --
> Simon Atanasyan
>
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Thanks to point that.

On Thu, Dec 1, 2016 at 12:39 AM, Rui Ueyama <ruiu at google.com>
wrote:
> Sure. One thing I want to remind you is that there is a place in Writer.cpp
> where we assume all synthetic sections were appended to end of Sections
> list. Look for `llvm::reverse`. If you add synthetic section thunks right
> before/after non-synthetic sections, you also want to change that.
>
> On Tue, Nov 29, 2016 at 1:18 PM, Simon Atanasyan <simon at
atanasyan.com>
> wrote:
>>
>> Hi,
>>
>> Sorry for delay with reply.
>>
>> It looks like now thunks can be implemented as a synthetic sections.
>> In that case we give flexible solution and will be able to put thunks
>> before/after related sections, using different alignment etc. As far
>> as I know BFD linker uses the same approach at least for MIPS thunks.
>> I will try to implement this idea.
>>
>> On Thu, Sep 8, 2016 at 1:44 AM, Rui Ueyama <ruiu at google.com>
wrote:
>> > This seems to be a reasonable optimization, and I don't have
any
>> > particular
>> > concern about implementing it.
>> >
>> > On Wed, Sep 7, 2016 at 6:58 AM, Simon Atanasyan <simon at
atanasyan.com>
>> > wrote:
>> >>
>> >> Hi,
>> >>
>> >> MIPS LA25 thunk is used to call PIC function from non-PIC
code.
>> >> Usually it contains three instructions:
>> >>
>> >> lui   $25, %hi(func)
>> >> addiu $25, $25, %lo(func)
>> >> j     func
>> >>
>> >> We can write such thunk in an arbitrary place of the generated
file.
>> >> But if a PIC function requires the thunk is the first routine
in a
>> >> section, we can optimize the code and escape jump instruction.
To do
>> >> so we just write the following thunk right before the PIC
routine.
>> >>
>> >> lui   $25, %hi(func)
>> >> addiu $25, $25, %lo(func)
>> >>
>> >> In fact GNU bfd/gold linkers write all MIPS LA25 thunks
required for
>> >> the section "A" into a separate input section
"S" and put section "S"
>> >> before "A". The last thunk in the section
"S" might have an optimized
>> >> two-instructions form.
>> >>
>> >> I would like to implement such optimization in LLD. My
question is
>> >> about ARM thunks - is it okay to write them before
corresponding input
>> >> section not after like LLD does now?
>>
>> --
>> Simon Atanasyan
>
>


-- 
Simon Atanasyan