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

Thu May 3 14:44:05 PDT 2018

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.

Thanks,

-- 
Mehdi

>
> Thanks,
> Teresa
>
>
>
>> Cheers,
>>
>> --
>> Mehdi
>>
>>
>>
>>
>> Le mar. 1 mai 2018 à 11:10, Peter Collingbourne <peter at pcc.me.uk> a
>> écrit :
>>
>>>
>>>
>>> On Tue, May 1, 2018 at 9:00 AM, Teresa Johnson <tejohnson at google.com>
>>> wrote:
>>>
>>>>
>>>>
>>>> On Mon, Apr 30, 2018 at 10:21 AM Peter Collingbourne <peter at pcc.me.uk>
>>>> wrote:
>>>>
>>>>> On Mon, Apr 30, 2018 at 8:32 AM, Teresa Johnson <tejohnson at google.com>
>>>>> wrote:
>>>>>
>>>>>> Hi Peter,
>>>>>> Thanks for your comments, replies below.
>>>>>> Teresa
>>>>>>
>>>>>> On Wed, Apr 25, 2018 at 2:08 PM Peter Collingbourne <peter at pcc.me.uk>
>>>>>> wrote:
>>>>>>
>>>>>>> Hi Teresa,
>>>>>>>
>>>>>>> Thanks for sending this proposal out.
>>>>>>>
>>>>>>> I would again like to register my disagreement with the whole idea
>>>>>>> of writing summaries in LLVM assembly format. In my view it is clear that
>>>>>>> this is not the right direction, as it only invites additional complexity
>>>>>>> and more ways for things to go wrong for no real benefit. However, I don't
>>>>>>> have the energy to argue that point any further, so I won't stand in the
>>>>>>> way here.
>>>>>>>
>>>>>>
>>>>>> I assume you are most concerned with the re-assembly/deserialization
>>>>>> of the summary. My main goal is to get this dumped into a text format, and
>>>>>> I went this route since the last dumper RFC was blocked with the LLVM
>>>>>> assembly direction pushed.
>>>>>>
>>>>>
>>>>> Yes, my main concern is with the deserialization. My view is that it
>>>>> should only be allowed for combined summaries -- allowing it for per-module
>>>>> is unnecessary as it creates the possibility of things gettting out of
>>>>> sync. Given that, we don't actually need an assembly representation and we
>>>>> can use whichever format is most convenient. But given the opposition to
>>>>> this viewpoint I am willing to concede getting the format (IMHO) right in
>>>>> favour of something that is acceptable to others, just so that we can test
>>>>> things in a more reasonable way.
>>>>>
>>>>>>
>>>>>>
>>>>>>> On Tue, Apr 24, 2018 at 7:43 AM, Teresa Johnson <
>>>>>>> tejohnson at google.com> wrote:
>>>>>>>
>>>>>>>> Hi everyone,
>>>>>>>>
>>>>>>>> I started working on a long-standing request to have the summary
>>>>>>>> dumped in a readable format to text, and specifically to emit to LLVM
>>>>>>>> assembly. Proposal below, please let me know your thoughts.
>>>>>>>>
>>>>>>>> Thanks,
>>>>>>>> Teresa
>>>>>>>>
>>>>>>>>
>>>>>>>>
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>>>>>>>>
>>>>>>>>
>>>>>>>> *RFC: LLVM Assembly format for ThinLTO
>>>>>>>> Summary========================================Background-----------------ThinLTO
>>>>>>>> operates on small summaries computed during the compile step (i.e. with “-c
>>>>>>>> -flto=thin”), which are then analyzed and updated during the Thin Link
>>>>>>>> stage, and utilized to perform IR updates during the post-link ThinLTO
>>>>>>>> backends. The summaries are emitted as LLVM Bitcode, however, not currently
>>>>>>>> in the LLVM assembly.There are two ways to generate a bitcode file
>>>>>>>> containing summary records for a module: 1. Compile with “clang -c
>>>>>>>> -flto=thin”2. Build from LLVM assembly using “opt -module-summary”Either of
>>>>>>>> these will result in the ModuleSummaryIndex analysis pass (which builds the
>>>>>>>> summary index in memory for a module) to be added to the pipeline just
>>>>>>>> before bitcode emission.Additionally, a combined index is created by
>>>>>>>> merging all the per-module indexes during the Thin Link, which is
>>>>>>>> optionally emitted as a bitcode file.Currently, the only way to view these
>>>>>>>> records is via “llvm-bcanalyzer -dump”, then manually decoding the raw
>>>>>>>> bitcode dumps.Relatedly, there is YAML reader/writer support for CFI
>>>>>>>> related summary fields (-wholeprogramdevirt-read-summary and
>>>>>>>> -wholeprogramdevirt-write-summary). Last summer, GSOC student Charles
>>>>>>>> Saternos implemented support to dump the summary in YAML from llvm-lto2
>>>>>>>> (D34080), including the rest of the summary fields (D34063), however, there
>>>>>>>> was pushback on the related RFC for dumping via YAML or another format
>>>>>>>> rather than emitting as LLVM assembly.Goals: 1. Define LLVM assembly format
>>>>>>>> for summary index2. Define interaction between parsing of summary from LLVM
>>>>>>>> assembly and synthesis of new summary index from IR.3. Implement printing
>>>>>>>> and parsing of summary index LLVM assemblyProposed LLVM Assembly
>>>>>>>> Format----------------------------------------------There are several top
>>>>>>>> level data structures within the ModuleSummaryIndex: 1.
>>>>>>>> ModulePathStringTable: Holds the paths to the modules summarized in the
>>>>>>>> index (only one entry for per-module indexes and multiple in the combined
>>>>>>>> index), along with their hashes (for incremental builds and global
>>>>>>>> promotion).2. GlobalValueMap: A map from global value GUIDs to the
>>>>>>>> corresponding function/variable/alias summary (or summaries for the
>>>>>>>> combined index and weak linkage).3. CFI-related data structures (TypeIdMap,
>>>>>>>> CfiFunctionDefs, and CfiFunctionDecls)I have a WIP patch to AsmWriter.cpp
>>>>>>>> to print the ModuleSummaryIndex that I was using to play with the format.
>>>>>>>> It currently prints 1 and 2 above. I’ve left the CFI related summary data
>>>>>>>> structures as a TODO for now, until the format is at least conceptually
>>>>>>>> agreed, but from looking at those I don’t see an issue with using the same
>>>>>>>> format (with a note/question for Peter on CFI type test representation
>>>>>>>> below).I modeled the proposed format on metadata, with a few key
>>>>>>>> differences noted below. Like metadata, I propose enumerating the entries
>>>>>>>> with the SlotTracker, and prefixing them with a special character. Avoiding
>>>>>>>> characters already used in some fashion (i.e. “!” for metadata and “#” for
>>>>>>>> attributes), I initially have chosen “^”. Open to suggestions
>>>>>>>> though.Consider the following example:extern void foo();int X;int bar() {
>>>>>>>>  foo();  return X;}void barAlias() __attribute__ ((alias ("bar")));int
>>>>>>>> main() {  barAlias();  return bar();}The proposed format has one entry per
>>>>>>>> ModulePathStringTable entry and one per GlobalValueMap/GUID, and looks
>>>>>>>> like:^0 = module: {path: testA.o, hash: 5487197307045666224}^1 = gv: {guid:
>>>>>>>> 1881667236089500162, name: X, summaries: {variable: {module: ^0, flags:
>>>>>>>> {linkage: common, notEligibleToImport: 0, live: 0, dsoLocal: 1}}}}^2 = gv:
>>>>>>>> {guid: 6699318081062747564, name: foo}^3 = gv: {guid: 15822663052811949562,
>>>>>>>> name: main, summaries: {function: {module: ^0, flags: {linkage: extern,
>>>>>>>> notEligibleToImport: 1, live: 0, dsoLocal: 1}, insts: 5, funcFlags:
>>>>>>>> {readNone: 0, readOnly: 0, noRecurse: 0, returnDoesNotAlias: 0}, calls:
>>>>>>>> {{callee: ^5, hotness: unknown}, {callee: ^4, hotness: unknown}}}}}^4 = gv:
>>>>>>>> {guid: 16434608426314478903, name: bar, summaries: {function: {module: ^0,
>>>>>>>> flags: {linkage: extern, notEligibleToImport: 1, live: 0, dsoLocal: 1},
>>>>>>>> insts: 3, funcFlags: {readNone: 0, readOnly: 0, noRecurse: 0,
>>>>>>>> returnDoesNotAlias: 0}, calls: {{callee: ^2, hotness: unknown}}, refs:
>>>>>>>> {^1}}}}^5 = gv: {guid: 18040127437030252312, name: barAlias, summaries:
>>>>>>>> {alias: {module: ^0, flags: {linkage: extern, notEligibleToImport: 0, live:
>>>>>>>> 0, dsoLocal: 1}, aliasee: ^4}}}Like metadata, the fields are tagged
>>>>>>>> (currently using lower camel case, maybe upper camel case would be
>>>>>>>> preferable).The proposed format has a structure that reflects the data
>>>>>>>> structures in the summary index. For example, consider the entry “^4”. This
>>>>>>>> corresponds to the function “bar”. The entry for that GUID in the
>>>>>>>> GlobalValueMap contains a list of summaries. For per-module summaries such
>>>>>>>> as this, there will be at most one summary (with no summary list for an
>>>>>>>> external function like “foo”). In the combined summary there may be
>>>>>>>> multiple, e.g. in the case of linkonce_odr functions which have definitions
>>>>>>>> in multiple modules. The summary list for bar (“^4”) contains a
>>>>>>>> FunctionSummary, so the summary is tagged “function:”. The FunctionSummary
>>>>>>>> contains both a flags structure (inherited from the base GlobalValueSummary
>>>>>>>> class), and a funcFlags structure (specific to FunctionSummary). It
>>>>>>>> therefore contains a brace-enclosed list of flag tags/values for each.Where
>>>>>>>> a global value summary references another global value summary (e.g. via a
>>>>>>>> call list, reference list, or aliasee), the entry is referenced by its
>>>>>>>> slot. E.g. the alias “barAlias” (“^5”) references its aliasee “bar” as
>>>>>>>> “^4”.Note that in comparison metadata assembly entries tend to be much more
>>>>>>>> decomposed since many metadata fields are themselves metadata (so then
>>>>>>>> entries tend to be shorter with references to other metadata
>>>>>>>> nodes).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):define  dso_local i32 @main() #0 !dbg !17 ^3 {For CFI data
>>>>>>>> structures, the format would be similar. It appears that TypeIds are
>>>>>>>> referred to by string name in the top level TypeIdMap (std::map indexed by
>>>>>>>> std::string type identifier), whereas they are referenced by GUID within
>>>>>>>> the FunctionSummary class (i.e. the TypeTests vector and the VFuncId
>>>>>>>> structure). For the LLVM assembly I think there should be a top level entry
>>>>>>>> for each TypeIdMap, which lists both the type identifier string and its
>>>>>>>> GUID (followed by its associated information stored in the map), and the
>>>>>>>> TypeTests/VFuncId references on the FunctionSummary entries can reference
>>>>>>>> it by summary slot number. I.e. something like:^1 = typeid: {guid: 12345,
>>>>>>>> identifier: name_of_type, …^2 = gv: {... {function: {.... typeTests: {^1,
>>>>>>>> …Peter - is that correct and does that sound ok?*
>>>>>>>>
>>>>>>>
>>>>>>> I don't think that would work because the purpose of the top-level
>>>>>>> TypeIdMap is to contain resolutions for each type identifier, and
>>>>>>> per-module summaries do not contain resolutions (only the combined summary
>>>>>>> does). What that means in practice is that we would not be able to recover
>>>>>>> and write out a type identifier name for per-module summaries as part of ^1
>>>>>>> in your example (well, we could in principle, because the name is stored
>>>>>>> somewhere in the function's IR, but that could get complicated).
>>>>>>>
>>>>>>
>>>>>> Ah ok. I guess the top-level map then is generated by the regular LTO
>>>>>> portion of the link (since it presumably requires IR during the Thin Link
>>>>>> to get into the combined summary)?
>>>>>>
>>>>>
>>>>> Yes, we fill in the map during the LowerTypeTests and
>>>>> WholeProgramDevirt passes in the regular LTO part of the link, e.g. here:
>>>>>  http://llvm-cs.pcc.me.uk/lib/Transforms/IPO/LowerTypeTests.cpp#823
>>>>>
>>>>>
>>>>>> Probably the easiest thing to do is to keep the type identifiers as
>>>>>>> GUIDs in the function summaries and write out the mapping of type
>>>>>>> identifiers as a top-level entity.
>>>>>>>
>>>>>>
>>>>>> To confirm, you mean during the compile step create a top-level
>>>>>> entity that maps GUID -> identifier?
>>>>>>
>>>>>
>>>>> I mean that you could represent this with something like:
>>>>>
>>>>> ^typeids = {^1, ^2, ^3}
>>>>> ^1 = typeid: {identifier: typeid1, ...}
>>>>> ^2 = typeid: {identifier: typeid2, ...}
>>>>> ^3 = typeid: {identifier: typeid3, ...}
>>>>>
>>>>> There's no need to store the GUIDs here because they can be computed
>>>>> from the type identifiers. The GUIDs would only be stored in the typeTests
>>>>> (etc.) fields in each function summary.
>>>>>
>>>>
>>>> I suppose we don't need to store the GUIDs at the top level in the
>>>> in-memory summary. But I think it would be good to emit the GUIDs in the
>>>> typeid assembly entries because it makes the association in the assembly
>>>> much more obvious. I.e. going back to my original example:
>>>>
>>>> ^1 = typeid: {guid: 12345, identifier: name_of_type, …
>>>> ^2 = gv: {... {function: {.... typeTests: {^1, …
>>>>
>>>> If we didn't include the GUID in the typeid entry, but rather just the
>>>> identifier, and put the GUID in the typeTest list in the GV's entry, it
>>>> wouldn't be obvious at all from the assembly listing which typeid goes with
>>>> which typeTest. It's also less compact to include the GUID in each
>>>> typeTests list.
>>>>
>>>
>>> I get that, but my point was that in a per-module summary the TypeIdMap
>>> is empty, so there will be no names, only GUIDs.
>>>
>>> For "making the association more obvious" we might just want to have the
>>> assembly writer emit the GUID of a name as a comment.
>>>
>>>
>>>> Or perhaps we are saying the same thing - I can't tell from your above
>>>> example if the GUID is also emitted in the "typeid:" entries.
>>>>
>>>
>>> No, it wouldn't be.
>>>
>>> I'm not sure there is a need for the:
>>>> ^typeids = {^1, ^2, ^3}
>>>> We can just build the typeids list on the fly as " = typeid: " entries
>>>> are read in.
>>>>
>>>
>>> That's true. Given that nothing actually needs to refer to them, we can
>>> just represent the typeids as something like
>>> typeid: {identifier: typeid1, ...} ; guid = 123
>>> typeid: {identifier: typeid2, ...} ; guid = 456
>>> typeid: {identifier: typeid3, ...} ; guid = 789
>>> without an associated number.
>>>
>>> Peter
>>>
>>>
>>>
>>>>
>>>>
>>>> Teresa
>>>>
>>>>
>>>>
>>>>> Peter
>>>>>
>>>>>>
>>>>>> Thanks,
>>>>>> Teresa
>>>>>>
>>>>>>
>>>>>>>
>>>>>>> Peter
>>>>>>>
>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> *Issues when Parsing of Summaries from
>>>>>>>> Assembly--------------------------------------------------------------------When
>>>>>>>> reading an LLVM assembly file containing module summary entries, a
>>>>>>>> ModuleSummaryIndex will be created from the entries.Things to consider are
>>>>>>>> the behavior when: - Invoked with “opt -module-summary” (which currently
>>>>>>>> builds a new summary index from the IR). Options:1. recompute summary and
>>>>>>>> throw away summary in the assembly file2. ignore -module-summary and build
>>>>>>>> the summary from the LLVM assembly3. give an error4. compare the two
>>>>>>>> summaries (one created from the assembly and the new one created by the
>>>>>>>> analysis phase from the IR), and error if they are different.My opinion is
>>>>>>>> to do a),  so that the behavior using -module-summary doesn’t change. We
>>>>>>>> also need a way to force building of a fresh module summary for cases where
>>>>>>>> the user has modified the LLVM assembly of the IR (see below). - How to
>>>>>>>> handle older LLVM assembly files that don’t contain new summary fields.
>>>>>>>> Options:1. Force the LLVM assembly file to be recreated with a new summary.
>>>>>>>> I.e. “opt -module-summary -o - | llvm-dis”.2. Auto-upgrade, by silently
>>>>>>>> creating conservative values for the new summary entries.I lean towards b)
>>>>>>>> (when possible) for user-friendliness and to reduce required churn on test
>>>>>>>> inputs. - How to handle partial or incorrect LLVM assembly summary entries.
>>>>>>>> How to handle partial summaries depends in part on how we answer the prior
>>>>>>>> question about auto-upgrading. I think the best option like there is to
>>>>>>>> handle it automatically when possible. However, I do think we should error
>>>>>>>> on glaring errors like obviously missing information. For example, when
>>>>>>>> there is summary data in the LLVM assembly, but summary entries are missing
>>>>>>>> for some global values. E.g. if the user modified the assembly to add a
>>>>>>>> function but forgot to add a corresponding summary entry. We could still
>>>>>>>> have subtle issues (e.g. user adds a new call but forgets to update the
>>>>>>>> caller’s summary call list), but it will be harder to detect those.*
>>>>>>>>
>>>>>>>> --
>>>>>>>> Teresa Johnson |  Software Engineer |  tejohnson at google.com |
>>>>>>>>  408-460-2413
>>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> --
>>>>>>> --
>>>>>>> Peter
>>>>>>>
>>>>>>
>>>>>>
>>>>>> --
>>>>>> Teresa Johnson |  Software Engineer |  tejohnson at google.com |
>>>>>>  408-460-2413
>>>>>>
>>>>>
>>>>>
>>>>>
>>>>> --
>>>>> --
>>>>> Peter
>>>>>
>>>>
>>>>
>>>> --
>>>> Teresa Johnson |  Software Engineer |  tejohnson at google.com |
>>>>  408-460-2413
>>>>
>>>
>>>
>>>
>>> --
>>> --
>>> Peter
>>>
>>
>
>
> --
> Teresa Johnson |  Software Engineer |  tejohnson at google.com |
>  408-460-2413
>
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