[llvm-dev] [RFC] Introduce Dump Accumulator

[Zachary Turner via llvm-dev]

Could this be emitted as debug info?  CodeView debug info (used with
COFF/Windows) already supports an (almost) equivalent mechanism called
annotations where the programmer can emit arbitrary data directly into the
debug info.  If you could piggyback off of this same infrastructure, it
could avoid reinventing many parts of the same wheel.  I can’t recall if
clang-cl ever added support for this, but I’m pretty sure it did.

On Wed, Aug 5, 2020 at 4:17 PM Mircea Trofin via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

>
>
> On Wed, Aug 5, 2020 at 3:51 PM Eli Friedman <efriedma at quicinc.com> wrote:
>
>> I’m not a fan of keeping important data outside the IR in an analysis.
>> If we’re planning to emit it, it should be represented directly in the IR.
>> Is there some reason we can’t just stick the data in a global variable?
>>
>
> The analysis in the scenarios here is external to LLVM - ML training, for
> example. It's really a way to do printf, but where the data could be large
> (challenging in a distributed build env, where IO may be throttled), or
> non-textual (for instance, capture IR right before a pass). An alternative
> would be to produce a side-file, but then (again, distributed build), you
> have to collect those files and concatenate them, and modify the build
> system to be aware of all that.
>
>
>>
>> I’m not sure it’s helpful to have a generic mechanism for this; it’s not
>> clear how this would work if multiple different features were trying to
>> emit data into the llvm_dump section at the same time.
>>
>
> You could layer the approach: the one llvm_dump section has a pluggable
> reader.
>
>
>>
>>
>> -Eli
>>
>>
>>
>> *From:* llvm-dev <llvm-dev-bounces at lists.llvm.org> *On Behalf Of *Kazu
>> Hirata via llvm-dev
>> *Sent:* Wednesday, August 5, 2020 3:36 PM
>> *To:* llvm-dev at lists.llvm.org; Mircea Trofin <mtrofin at google.com>; Wei
>> Mi <wmi at google.com>; james.henderson at sony.com
>> *Subject:* [EXT] [llvm-dev] [RFC] Introduce Dump Accumulator
>>
>>
>>
>> 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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