llvm dev - Jun 2016 - Building an array in a section from multiple object files

Awesome, thanks Peter!

Cheers

On Thu, Jun 23, 2016 at 10:35 AM Peter Collingbourne <peter at pcc.me.uk>
wrote:
> If you give your section a valid C identifier name, i.e. something like
> "xray_instr_map" (no period), the linker will synthesize symbols
named
> "__start_xray_instr_map" and "__stop_xray_instr_map",
which will point to
> the start and end of the combined section.
>
> Peter
>
> On Thu, Jun 23, 2016 at 10:26 AM, Dean Michael Berris via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Hi,
>>
>> As part of working on XRay, I'm trying to accomplish the following:
>> create a section that contains an array of entries pertaining to the
>> instrumentation map (nop sleds) in an object file, and have those
merged
>> into a single section in the final binary where the contents are
>> concatenated. The trick though is I'd like to be able to reference
the
>> whole array with a single symbol (or two, one to mark the start and the
>> other to mark the end). I'm trying to get this to work initially on
x86 and
>> Linux (ELF).
>>
>> What I've got currently in http://reviews.llvm.org/D19904 creates
this
>> section (.xray_instr_map) and defines two globals in that section named
>> "__xray_instr_map" and "__xray_instr_map_end". The
problem inevitably with
>> this approach is that having multiple object files have these
definitions
>> cause issues with multiple definition violations at link time.
>>
>> I *think* the solution involves having COMDAT sections per-function
that
>> has the instrumentation points, then have those concatenated into a
single
>> .xray_instr_map section.
>>
>> What I'm not sure about is how to define the
"__xray_instr_map" and
>> "__xray_instr_map" symbols such that they refer to the
contents of the
>> section in the final binary.
>>
>> Thoughts?
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>>
>
>
> --
> --
> Peter
>
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Just to close this out, I've updated http://reviews.llvm.org/D19904 to use
named ELF groups per-function, and have the runtime library use
__start_xray_instr_map and __stop_xray_instr_map as weak symbols from the
C++ side. I've sent a patch to make creating these COMDAT/Group sections
easier when lowering through the MCStreamer interface (
http://reviews.llvm.org/D21743).

Cheers

On Fri, Jun 24, 2016 at 4:23 AM Dean Michael Berris <dberris at
google.com>
wrote:
> Awesome, thanks Peter!
>
> Cheers
>
> On Thu, Jun 23, 2016 at 10:35 AM Peter Collingbourne <peter at
pcc.me.uk>
> wrote:
>
>> If you give your section a valid C identifier name, i.e. something like
>> "xray_instr_map" (no period), the linker will synthesize
symbols named
>> "__start_xray_instr_map" and
"__stop_xray_instr_map", which will point to
>> the start and end of the combined section.
>>
>> Peter
>>
>> On Thu, Jun 23, 2016 at 10:26 AM, Dean Michael Berris via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>>> Hi,
>>>
>>> As part of working on XRay, I'm trying to accomplish the
following:
>>> create a section that contains an array of entries pertaining to
the
>>> instrumentation map (nop sleds) in an object file, and have those
merged
>>> into a single section in the final binary where the contents are
>>> concatenated. The trick though is I'd like to be able to
reference the
>>> whole array with a single symbol (or two, one to mark the start and
the
>>> other to mark the end). I'm trying to get this to work
initially on x86 and
>>> Linux (ELF).
>>>
>>> What I've got currently in http://reviews.llvm.org/D19904
creates this
>>> section (.xray_instr_map) and defines two globals in that section
named
>>> "__xray_instr_map" and "__xray_instr_map_end".
The problem inevitably with
>>> this approach is that having multiple object files have these
definitions
>>> cause issues with multiple definition violations at link time.
>>>
>>> I *think* the solution involves having COMDAT sections per-function
that
>>> has the instrumentation points, then have those concatenated into a
single
>>> .xray_instr_map section.
>>>
>>> What I'm not sure about is how to define the
"__xray_instr_map" and
>>> "__xray_instr_map" symbols such that they refer to the
contents of the
>>> section in the final binary.
>>>
>>> Thoughts?
>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>>>
>>
>>
>> --
>> --
>> Peter
>>
>
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Dean Michael Berris via llvm-dev <llvm-dev at lists.llvm.org>
writes:
> Just to close this out, I've updated http://reviews.llvm.org/D19904 to
use
> named ELF groups per-function, and have the runtime library use
> __start_xray_instr_map and __stop_xray_instr_map as weak symbols from the
> C++ side.
In case you're not aware, the __start_/__stop_ trick isn't portable.
You may want to look at compiler-rt/lib/profile/InstrProfilingPlatform*.c.
There, we do the __start_ trick for linux and freebsd, a similar trick
for darwin, and fall back to a static registration scheme elsewhere.
> I've sent a patch to make creating these COMDAT/Group sections easier
> when lowering through the MCStreamer interface (
> http://reviews.llvm.org/D21743).
>
> Cheers
>
> On Fri, Jun 24, 2016 at 4:23 AM Dean Michael Berris <dberris at
google.com>
> wrote:
>
>> Awesome, thanks Peter!
>>
>> Cheers
>>
>> On Thu, Jun 23, 2016 at 10:35 AM Peter Collingbourne <peter at
pcc.me.uk>
>> wrote:
>>
>>> If you give your section a valid C identifier name, i.e. something
like
>>> "xray_instr_map" (no period), the linker will synthesize
symbols named
>>> "__start_xray_instr_map" and
"__stop_xray_instr_map", which will point to
>>> the start and end of the combined section.
>>>
>>> Peter
>>>
>>> On Thu, Jun 23, 2016 at 10:26 AM, Dean Michael Berris via llvm-dev
<
>>> llvm-dev at lists.llvm.org> wrote:
>>>
>>>> Hi,
>>>>
>>>> As part of working on XRay, I'm trying to accomplish the
following:
>>>> create a section that contains an array of entries pertaining
to the
>>>> instrumentation map (nop sleds) in an object file, and have
those merged
>>>> into a single section in the final binary where the contents
are
>>>> concatenated. The trick though is I'd like to be able to
reference the
>>>> whole array with a single symbol (or two, one to mark the start
and the
>>>> other to mark the end). I'm trying to get this to work
initially on x86 and
>>>> Linux (ELF).
>>>>
>>>> What I've got currently in http://reviews.llvm.org/D19904
creates this
>>>> section (.xray_instr_map) and defines two globals in that
section named
>>>> "__xray_instr_map" and
"__xray_instr_map_end". The problem inevitably with
>>>> this approach is that having multiple object files have these
definitions
>>>> cause issues with multiple definition violations at link time.
>>>>
>>>> I *think* the solution involves having COMDAT sections
per-function that
>>>> has the instrumentation points, then have those concatenated
into a single
>>>> .xray_instr_map section.
>>>>
>>>> What I'm not sure about is how to define the
"__xray_instr_map" and
>>>> "__xray_instr_map" symbols such that they refer to
the contents of the
>>>> section in the final binary.
>>>>
>>>> Thoughts?
>>>>
>>>> _______________________________________________
>>>> LLVM Developers mailing list
>>>> llvm-dev at lists.llvm.org
>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>
>>>>
>>>
>>>
>>> --
>>> --
>>> Peter
>>>
>>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev