[llvm-dev] Profiling data structure

[Roger Pau Monné via llvm-dev]

Adding who I think are the maintainers of the profile related llvm
bits, sorry should have done that earlier.

On Wed, Oct 25, 2017 at 08:26:18AM +0100, Roger Pau Monné via llvm-dev wrote:
> Hello,
> 
> I've been working on implementing some basic functionality in order to
> use the llvm profiling functionality inside of a kernel (the Xen
> hypervisor). The only functionality I'm interested in is being able to
> reset the counters, get the size of the data, and dump the data into a
> memory buffer.
> 
> I have to admit I haven't been able to find a lot of documentation
> about how this data is stored inside of the different sections, but
> with the code in compiler_rt/libs/profile I've been able to craft
> something that seems to do something sensible.
> 
> I have however a couple of questions:
> 
>  - The "Values" field in __llvm_profile_data always seems to be NULL.
>    Is this expected? What/why could cause this?
> 
>  - The fields in the NumValueSites array inside of __llvm_profile_data
>    also seem to always be 0.
> 
>  - Since what I'm coding is a decoupled replacement for the profiling
>    runtime inside of compiler_rt, is there anyway that at compile or
>    run time I can fetch the version of the profiling data?
>    I'm mostly worried that at some point llvm will bump the version
>    and change the layout, and then I will have to update my runtime
>    accordingly, but without llvm reporting the version used by the
>    compiler it's going to be very hard to keep backwards
>    compatibility or to detect version bumps.
> 
> Thanks, Roger.
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