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

Dean Michael Berris via llvm-dev llvm-dev at lists.llvm.org
Thu Jun 23 11:23:14 PDT 2016

Awesome, thanks Peter!


On Thu, Jun 23, 2016 at 10:35 AM Peter Collingbourne <peter at pcc.me.uk>

> 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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