[llvm-dev] llvm-profdata determinism

[David Blaikie via llvm-dev]

On Thu, Jun 29, 2017 at 7:03 PM Xinliang David Li <davidxl at google.com>
wrote:

> On Thu, Jun 29, 2017 at 6:27 PM, David Blaikie <dblaikie at gmail.com> wrote:
>
>> I haven't tested it, but it looks to me like llvm-profdata merge (well,
>> InstrProfWriter specifically) would not have deterministic output.
>>
>> Certainly the textual output iterates over FunctionData which is a
>> StringMap of SmallDenseMaps, neither of which has deterministic iteration
>>
>
> Does the iteration order of these maps depend on the order of hashes (of
> strings for StringMap)?
>

Right - that's my understanding.

(Some folks have started adding interesting things to LLVM to try to help
weed out these sort of issues (ENABLE_REVERSE_ITERATION - though it's only
been implemented in one of LLVM's containers so far))


>    Anyway, text dump is for debugging purpose only.
>

Sure, though even test cases written against this should be deterministic,
but yeah - less of a concern, for sure.


> .
>>
>> The binary writing looks like it'd have similar issues - looping through
>> these unordered maps & writing output (eg:
>> InstrProfRecordWriterTrait::EmitData loops through the data in the same
>> SmallDenseMap and writes content in that order so far as I can tell.
>>
>>
> Binary dump does not have the problem. The binary format  of the indexed
> profile data is on-disk hashtable. Before serializing into the disk, the
> instProfRecord is first inserted into the in memory hashtable and the
> hashtable is then dumped into the disk. The entry order of the hashtable
> only depends on the string hashes and in case of conflicts, the function
> content hash.
>

Ah, so you mean the loop in InstrProfRecordWriterTrait::EmitData that
inserts into the SummaryBuilder - the SummaryBuilder itself doesn't depend
on the order of 'addRecord' calls - ah, I see, addRecord -> addEntryCount
-> addCount -> CountFrequencies, a std::map (thus, ordered by value,
independent of call order of addRecord).

But inside that loop, it also starts writing directly to 'Out' based on the
order of the SmallDenseMap, doesn't it? I don't immediately see anywhere
that seeks within 'Out' to ensure that the loop there doesn't affect the
ordering of the bytes written within it.


> If you do see case of non-determinism, then we have a bug there which
> should be fixed, but there is no need to change the iteration order of the
> IntrProfWriter.
>

>
>
>>
>> Generally it's important that the compiler (& I believe related tools)
>> have deterministic output. Is there a reason that wouldn't be the case for
>> llvm-profdata? Or have I misunderstood how the output is determined?
>>
>>
> For the binary output, we definitely need deterministic behavior.  For
> debug output, we can relax that requirement.
>
>
>> Ensuring deterministic output may be expensive in terms of memory usage,
>> though perhaps not prohibitive. The usual approach is to use some of LLVM's
>> deterministic maps (like MapVector), though they're not exactly tuned for
>> memory usage. An alternative might be to take the data in each
>> SmallDenseMap and sort it by the hash as a key - it's unique after all, and
>> doing each map separately won't do crazy bad things to memory usage (a
>> small constant overhead).
>>
>> Handling the StringMap, I'm not sure about - it might be cheap enough to
>> make a separate vector of StringMapEntry*s, sorting based on the strings
>> and iterating over that instead of the StringMap itself? (I guess the same
>> approach could be taken with the SmallDenseMaps, rather than duplicating
>> anything)
>>
>> How's all that sound?
>>
>
> See above -- I don't think we have a need to change the use of StringMap.
> If we see a case where the non-determinism happens, we need to root-cause
> it first.
>
> David
>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20170630/9b8e2f8a/attachment.html>