[LLVMdev] SmallString + raw_svector_ostream combination should be more efficient

[Yaron Keren]

Sean, thanks for reminding this, Alp did commit a class derived from
raw_svector_ostream conatining an internal SmallString he called
small_string_ostream.
The commit was reverted after a day due to a disagreement about the commit
approval and apparently abandoned.

http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20140623/223393.html
http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20140623/223557.html
http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20140630/223986.html

The class Alp comitted did solve the possible mismatch between the
SmallString and the stream by making the SmallString private to the class.
This however did not treat the root problem, the duplication of the
information about the buffer between SmallString and the stream.

I can make performance measurements but even if the performance difference
is neglible (and looking at all the code paths and conditionals of
non-inlined functions at raw_ostream.cpp that's hard to believe), the
current SmallString-raw_svector_ostream is simply wrong.

Personally, after the previous "Alp" discussion I found and fixed several
such issues in my out-of-tree code only to make new similar new error
earlier this year, which I caught only months after, when Rafael committed
the pwrite which reminded me the issue. Due ot the information duplication
Rafael commit also had a bug and Mehdi reports similar experience. Back
then Alp reported similar problems he found in LLVM code which were
hopefully fixed otherwise.

In addition to the information duplication, the design is quite confusing
to use

- Should one use the stream.str() function or the SmallString itself?
- flush or str?
- How do you properly clear the combination for reuse (that was my mistake,
I forgot to resync after OS.clear())?

It's safe to say this design pattern is very easy to get wrong one way or
another, we got burned by it multiple times and it should be replaced.

Alp suggested a derived class containing its own SmallString. That's a
simple and effective approach to avoid the bugs mentioned but does not fix
the memory or runtime issues. Small as they may be, why have them at a
fundemental data structure?

I was thinking about going further avoiding all duplication with a
templated class derived with its own internal buffer storage.

Rafael suggested a more modular approach, a derived adpater class adapter
to a *simple* buffer (or nullptr for fully-dynamic operation) which also
won't duplicate any information the buffer is dumb and has no state.

That solution sounds simple, efficient and safe to use. The implementation
would be actually simpler then raw_svector_ostream since all the
coordination logic is not required anymore.


2015-04-20 22:17 GMT+03:00 Sean Silva <chisophugis at gmail.com>:

>
>
> On Sun, Apr 19, 2015 at 7:40 AM, Yaron Keren <yaron.keren at gmail.com>
> wrote:
>
>> A very common code pattern in LLVM is
>>
>>  SmallString<128> S;
>>  raw_svector_ostream OS(S);
>>  OS<< ...
>>  Use OS.str()
>>
>> While raw_svector_ostream is smart to share the text buffer itself, it's
>> inefficient keeping two sets of pointers to the same buffer:
>>
>>  In SmallString: void *BeginX, *EndX, *CapacityX
>>  In raw_ostream: char *OutBufStart, *OutBufEnd, *OutBufCur
>>
>> Moreover, at runtime the two sets of pointers need to be coordinated
>> between the SmallString and raw_svector_ostream using
>> raw_svector_ostream::init, raw_svector_ostream::pwrite, raw_svector_ostream::resync
>> and raw_svector_ostream::write_impl.
>> All these functions have non-inlined implementations in raw_ostream.cpp.
>>
>> Finally, this may cause subtle bugs if S is modified without calling
>> OS::resync(). This is too easy to do by mistake.
>>
>> In this frequent case usage the client does not really care about S being
>> a SmallString with its many useful string helper function. It's just
>> boilerplate code for raw_svector_ostream. But it does cost three extra
>> pointers, some runtime performance and possible bugs.
>>
>
> I agree the bugs are real (Alp proposed something a while back regarding
> this?), but you will need to provide measurements to justify the cost in
> runtime performance. One technique I have used in the past to measure these
> sorts of things I call "stuffing": take the operation that you want to
> measure, then essentially change the logic so that you pay the cost 2
> times, 3 times, etc. You can then look at the trend in performance as N
> varies and extrapolate back to the case where N = 0 (i.e. you don't pay the
> cost).
>
> For example, in one situation where I used this method it was to measure
> the cost of stat'ing files (sys::fs::status) across a holistic build, using
> only "time" on the command line (it was on Windows and I didn't have any
> tools like DTrace available that can directly measure this). In order to do
> this, I changed sys::fs::status to call stat N times instead of 1, and
> measured with N=1 N=2 N=3 etc. The result was that the difference between
> the N and N+1 versions was about 1-2% across N=1..10 (or whatever I
> measured). In order to negate caching and other confounding effects, it is
> important to try different distributions of stats; e.g. the extra stats are
> on the same file as the "real" stat vs. the extra stats are on nonexistent
> files in the same directory as the "real" file vs. parent directories of
> the "real" file; if these match up fairly well (they did), then you have
> some indication that the "stuffing" is measuring what you want to measure.
>
> So e.g. if you think the cost of 3 extra pointers is significant, then
> "stuff" the struct with 3, 6, 9, ... extra pointers and measure the
> difference in performance (e.g. measure the time building a real project).
>
> -- Sean Silva
>
>
>>
>> To solve all three issues, would it make sense to have
>> raw_ostream-derived container with a its own SmallString like
>> templated-size built-in buffer?
>>
>>
>
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