[llvm-dev] Building an array in a section from multiple object files

[Justin Bogner via llvm-dev]

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
>>>
>>
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