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

Thu May 3 16:12:34 PDT 2018

On Thu, May 3, 2018 at 4:09 PM Peter Collingbourne <peter at pcc.me.uk> wrote:

> On Thu, May 3, 2018 at 3:58 PM, David Blaikie <dblaikie at gmail.com> wrote:
>
>>
>>
>> On Thu, May 3, 2018 at 3:52 PM Peter Collingbourne <peter at pcc.me.uk>
>> wrote:
>>
>>> 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.
>>>
>>
>> Oh, I didn't know that - why doesn't it read .ll? That seems like an
>> oversight - most/all the other LLVM tools treat .ll and .bc pretty equally,
>> don't they?
>>
>
> Yes, the developer-facing tools do (clang being one of them, as it has
> some developer-facing features), but linkers (the LTO API consumers) are
> user-facing, and have no need to consume .ll files. Getting them to read
> .ll has other complications as well. For example, Unix linkers will
> interpret textual input files as linker scripts, so there would need to be
> some mechanism to distinguish .ll files from linker scripts.
>

Yeah, no worries about the linker - though I guess it could be convenient
if it did some lookahead & could detect a .ll file as being different from
a linker script - but that's a "nice to have"/heroics, not something I'd
expect.

> llvm-lto2 (the LTO API test driver) is a developer-facing tool, and we
> could conceivably make it read .ll files and pass them to the API as .bc
> files.
>

Right - this was more what I had in mind. Basically I'd find it weird if I
came across an LLVM test case that first ran a .ll file through llvm-as,
then ran it on the tool (llvm-lto2) in question. I'd likely try to remove
the indirection & be a bit annoyed if that failed.

- Dave

>
> Peter
>
>
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