[llvm-dev] [RFC] Introduce Dump Accumulator

[James Henderson via llvm-dev]

 One area the proposal doesn't cover is the ELF file format properties.
What section type will this be? What flags will it have? Is the output
intended to be part of the executable loadable image or not? Does it even
need to be in the final executable output, or is it just being in the
object sufficient? The same or similar questions arise if you are
considering this for other output formats (e.g. COFF, Mach-O etc) for each
of them too.

On Wed, 5 Aug 2020 at 23:36, Kazu Hirata via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Introduction
> ============
>
> This RFC proposes a mechanism to dump arbitrary messages into object
> files during compilation and retrieve them from the final executable.
>
> Background
> ==========
>
> We often need to collect information from all object files of
> applications.  For example:
>
> - Mircea Trofin needs to collect information from the function
>   inlining pass so that he can train the machine learning model with
>   the information.
>
> - I sometimes need to dump messages from optimization passes to see
>   where and how they trigger.
>
> Now, this process becomes challenging when we build large applications
> with a build system that caches and distributes compilation jobs.  If
> we were to dump messages to stderr, we would have to be careful not to
> interleave messages from multiple object files.  If we were to modify
> a source file, we would have to flush the cache and rebuild the entire
> application to collect dump messages from all relevant object files.
>
> High Level Design
> =================
>
> - LLVM: We provide machinery for individual passes to dump arbitrary
>   messages into a special ELF section in a compressed manner.
>
> - Linker: We simply concatenate the contents of the special ELF
>   section.  No change is needed.
>
> - llvm-readobj: We add an option to retrieve the contents of the
>   special ELF section.
>
> Detailed Design
> ===============
>
> DumpAccumulator analysis pass
> -----------------------------
>
> We create a new analysis pass called DumpAccumulator.  We add the
> analysis pass right at the beginning of the pass pipeline.  The new
> analysis pass holds the dump messages throughout the pass pipeline.
>
> If you would like to dump messages from some pass, you would obtain
> the result of DumpAccumulator in the pass:
>
>   DumpAccumulator::Result *DAR = MAMProxy.getCachedResult<DumpAccumulator>(M);
>
> Then dump messages:
>
>   if (DAR) {
>     DAR->Message += "Processing ";
>     DAR->Message += F.getName();
>     DAR->Message += "\n";
>   }
>
> AsmPrinter
> ----------
>
> We dump the messages from DumpAccumulator into a section called
> ".llvm_dump" in a compressed manner.  Specifically, the section
> contains:
>
> - LEB128 encoding of the original size in bytes
> - LEB128 encoding of the compressed size in bytes
> - the message compressed by zlib::compressed
>
> in that order.
>
> llvm-readobj
> ------------
>
> We read the .llvm_dump section.  We dump each chunk of compressed data
> one after another.
>
> Existing Implementation
> =======================
> https://reviews.llvm.org/D84473
>
> Future Directions
> =================
>
> The proposal above does not support the ThinLTO build flow.  To
> support that, I am thinking about putting the message as metadata in
> the IR at the prelink stage.
>
> Thoughts?
>
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>
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