[llvm-dev] RFC: LLVM Assembly format for ThinLTO Summary

Thu May 10 08:43:27 PDT 2018

Initial, mostly writer-side, patch sent for review (D46699). PTAL.

A few misc changes from original format sent here:
- When a GV name is available (i.e. when printing the per-module summaries
  to assembly after the summary is built from the IR), instead of printing
both
  the name and the GUID, which are redundant, print only the name and put
the
  GUID in a comment after the summary entry. This should make it easier to
write
  tests using summary assembly as input also, since the name can be used
and the
  parser can compute the GUID on the fly.
- Support for printing of the type test related summary info, as discussed
with
  Peter.
- Use "(" and ")" instead of curly braces, since the latter made it harder
to
  test (FileCheck got confused when there were two curly braces in a row
thinkig
  it was a regex).
- Suppression of printing of some less common flags/fields, when they hold
their
  default 0 values.

Teresa

On Fri, May 4, 2018 at 7:11 AM Teresa Johnson <tejohnson at google.com> wrote:

>
>
> On Thu, May 3, 2018 at 10:13 PM David Blaikie via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>>
>>
>> On Thu, May 3, 2018 at 6:03 PM Mehdi AMINI <joker.eph at gmail.com> wrote:
>>
>>> Le jeu. 3 mai 2018 à 15:52, Peter Collingbourne <peter at pcc.me.uk> a
>>> écrit :
>>>
>>>>
>>>>
>>>> On Thu, May 3, 2018 at 3:29 PM, David Blaikie <dblaikie at gmail.com>
>>>> wrote:
>>>>
>>>>>
>>>>>
>>>>> On Thu, May 3, 2018 at 3:08 PM Peter Collingbourne via llvm-dev <
>>>>> llvm-dev at lists.llvm.org> wrote:
>>>>>
>>>>>> On Thu, May 3, 2018 at 2:44 PM, Mehdi AMINI <joker.eph at gmail.com>
>>>>>> wrote:
>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> Le mar. 1 mai 2018 à 16:50, Teresa Johnson <tejohnson at google.com> a
>>>>>>> écrit :
>>>>>>>
>>>>>>>> Hi Mehdi, thanks for the comments, responses and a tweaked proposal
>>>>>>>> below. Teresa
>>>>>>>>
>>>>>>>> On Tue, May 1, 2018 at 11:37 AM, Mehdi AMINI <joker.eph at gmail.com>
>>>>>>>> wrote:
>>>>>>>>
>>>>>>>>> Hi,
>>>>>>>>>
>>>>>>>>> My main concern is this one:
>>>>>>>>>
>>>>>>>>> > Currently, I am emitting the summary entries at the end, after
>>>>>>>>> the metadata nodes. Note that the ModuleSummaryIndex is not currently
>>>>>>>>> referenced from the Module, and isn’t currently created when parsing the
>>>>>>>>> Module IR bitcode (there is a separate derived class for reading the
>>>>>>>>> ModuleSummaryIndex from bitcode). This is because they are not currently
>>>>>>>>> used at the same time. However, in the future there is no reason why we
>>>>>>>>> couldn’t tag the global values in the Module’s LLVM assembly with the
>>>>>>>>> corresponding summary entry if the ModuleSummaryIndex is available when
>>>>>>>>> printing the Module in the assembly writer. I.e. we could do the following
>>>>>>>>> for “main” from the above example when printing the IR definition (note the
>>>>>>>>> “^3” at the end):
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> I believe the reason that the ModuleSummaryIndex is not attached
>>>>>>>>> to the Module is that it is fundamentally not a piece of IR, but it is
>>>>>>>>> conceptually really an Analysis result.
>>>>>>>>> Usually other analyses don't serialize their result, we happen to
>>>>>>>>> serialize this one for an optimization purpose (reloading it and making the
>>>>>>>>> thin-link faster).
>>>>>>>>>
>>>>>>>>
>>>>>>>> True. My understanding is that the push for having it serialized
>>>>>>>> via assembly is due to the fact that it is emitted into the bitcode. I know
>>>>>>>> there is disagreement on this reasoning, I am hoping to have a proposal
>>>>>>>> that is acceptable to everyone. =)
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>> The fundamental problem is that an analysis result has to be able
>>>>>>>>> to be invalidated with IR changes, attaching this directly to the module
>>>>>>>>> wouldn't achieve this. The risk is that when the IR and the summary get
>>>>>>>>> out-of-sync (`clang -O2 my_module_with_summaries.ll -emit-llvm -o
>>>>>>>>> my_optimized module_with_summaries.ll`) the summaries would be badly wrong.
>>>>>>>>>
>>>>>>>>> Have you looked into what it'd take to make it a "real" analysis
>>>>>>>>> in the pass manager?
>>>>>>>>>
>>>>>>>>
>>>>>>>> Thanks for raising this issue specifically, I hadn't addressed it
>>>>>>>> in my proposal and it is a big one. I am not proposing that we attempt to
>>>>>>>> maintain the summary through optimization passes, and definitely don't
>>>>>>>> think we should do that. IMO deserializing it should be for testing the
>>>>>>>> thin link and the combined summaries in the backends only. To that end, I
>>>>>>>> have an idea (below some background first).
>>>>>>>>
>>>>>>>> Note that in some cases the module summary analysis is an analysis
>>>>>>>> pass. I.e. when invoked by "opt -module-summary=". However, some time ago
>>>>>>>> when Peter added the support for splitting the bitcode (for CFI purposes)
>>>>>>>> and therefore needed to generate a summary in each partition (Thin and
>>>>>>>> Regular), he added the ThinLTOBitcodeWriterPass, which invokes the module
>>>>>>>> summary builder directly (twice). This writer is what gets invoked now when
>>>>>>>> building via "clang -flto=thin", and with "opt -thinlto-bc". So there it is
>>>>>>>> not invoked/maintained as an analysis pass/result. It would be tricky to
>>>>>>>> figure out how to even split rather than recompute the module summary index
>>>>>>>> in that case. Even in the case where we are still invoking as an analysis
>>>>>>>> pass (opt -module-summary), we would need to figure out how to read in the
>>>>>>>> module summary to use as the analysis result when available (so that it
>>>>>>>> could be invalidated and recomputed when stale).
>>>>>>>>
>>>>>>>> Rather than add this plumbing, and just have it discarded if opt
>>>>>>>> does any optimization, I think we should focus at least for the time being
>>>>>>>> on supporting reading the summary from assembly exactly where we currently
>>>>>>>> read in the summary from bitcode:
>>>>>>>> 1) For the thin link (e.g. tools such as llvm-lto2 or llvm-lto,
>>>>>>>> which currently have to be preceded by "opt -module-summary/-thinlto-bc" to
>>>>>>>> generate an index, but could just build it from assembly instead).
>>>>>>>> 2) For the LTO backends (e.g. tools such as llvm-lto which can
>>>>>>>> consume a combined index and invoke the backends, or "clang
>>>>>>>> -fthinlto-index=" for distributed ThinLTO backend testing), where we could
>>>>>>>> build the combined summary index from assembly instead.
>>>>>>>>
>>>>>>>> This greatly simplifies the reading side, as there are no
>>>>>>>> optimizations performed on the IR after the index is read in these cases
>>>>>>>> that would require invalidation. It also simplifies adding the parsing
>>>>>>>> support, since it gets invoked exactly where we expect to build an index
>>>>>>>> currently (i.e. since we don't currently build or store the
>>>>>>>> ModuleSummaryIndex when parsing the Module from bitcode). It doesn't
>>>>>>>> preclude someone from figuring out how to compute the module summary
>>>>>>>> analysis result from the assembly, and invalidating it after optimization,
>>>>>>>> when reading the Module IR via 'opt' in the future.
>>>>>>>>
>>>>>>>> Does this seem like a reasonable proposal to everyone?
>>>>>>>>
>>>>>>>
>>>>>>> Sounds good to me.
>>>>>>>
>>>>>>> That would make .ll files quite convenient during debugging I think?
>>>>>>> We could disassemble, manually change summaries, and re-assemble a bitcode
>>>>>>> file before running the (thin-)link again.
>>>>>>>
>>>>>>
>>>>>> Yes, that seems more reasonable than what I thought you had in mind.
>>>>>> If the only consumer of this information is llvm-as, then the purpose of
>>>>>> the asm summary format is just to provide a way to create a .bc file for
>>>>>> testing purposes, which is certainly a useful capability.
>>>>>>
>>>>>
>>>>> That feels a bit surprising if ".ll -> llvm-as -> .bc -> <some other
>>>>> tool - opt, etc>" is different from ".ll -> <opt, etc>". Is that what we're
>>>>> talking about here? Any chance that can be avoided & feeding a .ll file
>>>>> works (in the sense of does the same thing/tests the same behavior) in all
>>>>> the same places that feeding a .bc file does? (as is the case with
>>>>> non-summary-based IR, to the best of my knowledge)
>>>>>
>>>>
>>>> I may be mistaken, but I don't think we have a lot of tools that can
>>>> read both .ll and .bc and end up using the summary if it is a .bc file. LTO
>>>> can't read .ll, for example. The only one that I can think of is clang and
>>>> presumably we could make that use whichever API we would use in llvm-as for
>>>> reading the summary from .ll. So the behaviour of most tools would be that
>>>> ".ll -> llvm-as -> .bc -> <some tool>" vs ".ll -> <some tool>" would end up
>>>> being the same in both cases: the summary gets discarded.
>>>>
>>>
>>> There is still a discrepancy in that:
>>>
>>>  .ll -> llvm-as -> .bc
>>>
>>> would be different from:
>>>
>>>  .ll -> opt -> .bc
>>>
>>> The latter would drop the summary.
>>>
>>
>> Though by the sounds of it .bc -> opt -> .bc drops the summary already?
>> So there's some consistency there.
>>
>> But, yeah, it's not clear to me if it'd be better if that did something
>> else. This is mostly/purely for testing/development purposes anyway.
>>
>> view these records is via “llvm-bcanalyzer -dump”, then manually decoding
>> the raw bitcode dumps.
>>
>
> Yep, I sent a similar response - we're all crossing wires here because the
> long emails keep getting held for moderator approval. =)  Trimmed a bunch
> of stuff so this one should go through.
>
> It sounds like we have a pretty good consensus at this point. Unless there
> is an objection, I'll start preparing patches.
>
> Thanks,
> Teresa
>
> --
> Teresa Johnson |  Software Engineer |  tejohnson at google.com |
>  408-460-2413
>

-- 
Teresa Johnson |  Software Engineer |  tejohnson at google.com |  408-460-2413
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