Dean Michael Berris via llvm-dev
2016-Jun-23 17:26 UTC
[llvm-dev] 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? -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20160623/34a1a493/attachment.html>
Peter Collingbourne via llvm-dev
2016-Jun-23 17:35 UTC
[llvm-dev] Building an array in a section from multiple object files
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 -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20160623/6485cbe9/attachment.html>
Dean Michael Berris via llvm-dev
2016-Jun-23 18:23 UTC
[llvm-dev] 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 >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20160623/67c8bb3f/attachment.html>
Possibly Parallel Threads
- Building an array in a section from multiple object files
- Building an array in a section from multiple object files
- Building an array in a section from multiple object files
- [XRay] Alternatives to relocations in .text section
- [XRay] RFC: LLVM-side Changes for nop-sleds