Yaron Keren
2015-Apr-19 14:40 UTC
[LLVMdev] SmallString + raw_svector_ostream combination should be more efficient
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. 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? -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150419/7bdc7a73/attachment.html>
Rafael Espíndola
2015-Apr-20 13:26 UTC
[LLVMdev] SmallString + raw_svector_ostream combination should be more efficient
> 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?It can be improved. I am not sure templating the stream itself is the best option. I think it is possible to implement a derived class that duplicates nothing and is not templated. * The constructor gets passed a buffer (pointer, size) and can handle (null, 0). * It sets up the base class to write past the existing data, like raw_svector_ostream does. * When more data is needed, allocate a new buffer, copy data from the old one, and update the base class pointers. A user can then pass a stack allocated buffer to get something like a raw_svector_ostream. It could pass a null to get a convenient class that handles all allocations. Cheers, Rafael
David Blaikie
2015-Apr-20 15:19 UTC
[LLVMdev] SmallString + raw_svector_ostream combination should be more efficient
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, *OutBufCurAny reason to believe this inefficiency is significant/important? Given that these are never in long-lived containers, but generally just on the stack, it doesn't seem like the extra 3 pointers would be very costly in terms of overall performance.> > 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. > > 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? > > > _______________________________________________ > LLVM Developers mailing list > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev >
Yaron Keren
2015-Apr-20 15:43 UTC
[LLVMdev] SmallString + raw_svector_ostream combination should be more efficient
Even if the memory overhead is small, they do introduce code complexity in coordinating the SmallString and raw_svector_ostream and much runtime cost: raw_ostream::write() calls raw_svector_ostream::write_impl() and raw_ostream::copy_to_buffer(). raw_svector_ostream::write_impl() calls OS.reserve and SetBuffer. SetBuffer calls SetBufferAndMode(). testing the BufferMode for every time and writing these three pointers. Every function has several code paths, adding to complexity and runtime cost. Due to this complexity, there are *additional* tests in raw_ostream::write() callers trying to take shortcuts to avoid calling write_impl()... these tests also take time. Essentially, what we are trying to achieve with SmallString SmallString + raw_svector_ostream is be much, much simpler than what we have now, along the lines Rafael suggested: if there is enough space write the string else reallocate, copy and write the string. That's simpler and shorter code by order of magnitude than the combination have now, and a bonus - with a smaller memory footprint. What more can we ask for? 2015-04-20 18:19 GMT+03:00 David Blaikie <dblaikie 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 > > Any reason to believe this inefficiency is significant/important? > Given that these are never in long-lived containers, but generally > just on the stack, it doesn't seem like the extra 3 pointers would be > very costly in terms of overall performance. > > > > > 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. > > > > 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? > > > > > > _______________________________________________ > > LLVM Developers mailing list > > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev > > >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150420/1f8928ee/attachment.html>
Mehdi Amini
2015-Apr-20 16:03 UTC
[LLVMdev] SmallString + raw_svector_ostream combination should be more efficient
> On 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.FWIW I’ve been bitten exactly by this last week while updating an out-of-tree compiler to adapt API changes around streams (raw_pwrite_stream). On the positive aspect, I had to learn about the internal of the various streams ;) The API for raw_svector_ostream is not nice on this aspect, but what is really missing is that there is no assertion to catch it. I feel it is a bit off from the principle “easy to use, hard to misuse”.> 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. > > 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?Not sure about the exact solution, but “something” could certainly be done to be more “user-friendly”. — Mehdi -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150420/a4dd865a/attachment.html>
Sean Silva
2015-Apr-20 19:17 UTC
[LLVMdev] SmallString + raw_svector_ostream combination should be more efficient
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? > >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150420/56cf968b/attachment.html>
Yaron Keren
2015-Apr-20 21:10 UTC
[LLVMdev] SmallString + raw_svector_ostream combination should be more efficient
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? >> >> >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150421/7b4a745d/attachment.html>
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