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

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?
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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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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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