Sean Silva via llvm-dev
2017-Sep-10 08:34 UTC
[llvm-dev] Performance of large llvm::ConstantDataArrays
On Sat, Sep 9, 2017 at 10:18 PM, Chris Lattner via llvm-dev < llvm-dev at lists.llvm.org> wrote:> > On Sep 7, 2017, at 11:06 PM, Chris Lovett via llvm-dev < > llvm-dev at lists.llvm.org> wrote: > > I'm running into some pretty bad performance in llc.exe when compiling > some large neural networks into code that contains some very large llvm::ConstantDataArrays, > some are { size=102,760,448 }. There's a small about of actual code for > processing the network, but the assembly is mostly global data. > > > Yes, llvm’s representation of constant arrays is insanity for cases like > this. Your case is bad, but just imagine the cost of a large char[] > initialization: even though each byte is stored as a ConstantInt, the bloat > isn’t huge because they are uniqued. The real problem comes from each > entry in the ConstantArray being stored as an operand list. An operand in > the operand list consumes something like 3-4 words per operand to maintain > the uselist and a bunch of other nonsense that isn’t right for this. > > IMO, there is a relatively easy solution for this. Introduce a new > subclass of ConstantData which represents a blob of data that gets emitted > to the .o file, stored in a reasonable native format. I think it would be > fine to limit this to only representing arrays of primitive types (e.g. > array of float, array of bytes, etc) since this keeps the API to the type > simple (the type models an array, so it should have array element members > only), and things that want to get the elements of the array out can have > them returned as ConstantInt’s (or whatever). I’d name this something like > “ConstantArrayBlob”. >What's the relationship between ConstantDataArray and ConstantArray? The former's doxygen says "An array constant whose element type is a simple 1/2/4/8-byte integer or float/double, and whose elements are just simple data values (i.e. ConstantInt/ConstantFP). This Constant node has no operands because it stores all of the elements of the constant as densely packed data, instead of as Value*'s." so I assumed that it was a dense representation and it seemed reasonable that an i8 typed one of them would basically operate as a "ConstantArrayBlob". (but I guess if MC still creates one fragment per element that will still be a memory hog regardless of the IR's representation) -- Sean Silva> > There are cases this wouldn’t cover well, e.g. an array of small structs, > but I think that is ok, and it could be feature crept to support that over > time. The next trick is adding the corresponding special case to Clang to > not generate the ConstantArray and the ConstantFP/Int members when given a > candidate initialization. This can be done as a secondary optimization > after the basic mechanics are in place. > > -Chris > > > > I'm finding that llc.exe memory spikes up around 30 gigabytes and the job > takes 20-30 minutes compiling from bitcode. When I looked into it I found > that every single floating point number is loaded into ConstantFP object > where the float is parsed into exponent, mantissa and stored in an integer > part is stored in a heap allocated array, then these are emitted into > MCDataFragments where again more heap allocated data, the float appears to > be stored in SmallVectorImpl<char>. On top of this I see a lot of > MCFillFragments added because of long double padding. > > All up the code I'm compiling ends up with 276 million MCFragments, which > just take a super long time in each phase of compiling (loading from > bitcode, emitting, layout and writing). With a peak working set of 30 > gigabytes each float is taking around 108 bytes! > > Is there a more efficient way to do this? Or is there any plan in the > works to handle global data more efficiently in llc ? > _______________________________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev > > > > _______________________________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev > >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20170910/5b74f4ec/attachment.html>
Chris Lattner via llvm-dev
2017-Sep-12 03:24 UTC
[llvm-dev] Performance of large llvm::ConstantDataArrays
On Sep 10, 2017, at 1:34 AM, Sean Silva <chisophugis at gmail.com> wrote:> > IMO, there is a relatively easy solution for this. Introduce a new subclass of ConstantData which represents a blob of data that gets emitted to the .o file, stored in a reasonable native format. I think it would be fine to limit this to only representing arrays of primitive types (e.g. array of float, array of bytes, etc) since this keeps the API to the type simple (the type models an array, so it should have array element members only), and things that want to get the elements of the array out can have them returned as ConstantInt’s (or whatever). I’d name this something like “ConstantArrayBlob”. > > > What's the relationship between ConstantDataArray and ConstantArray?Ah, it looks like ConstantDataArray is exactly what I was advocating for. Does Clang generate it from an array of doubles? Maybe that is all that is missing.> densely packed data, instead of as Value*'s." so I assumed that it was a dense representation and it seemed reasonable that an i8 typed one of them would basically operate as a "ConstantArrayBlob". (but I guess if MC still creates one fragment per element that will still be a memory hog regardless of the IR's representation)Yeah, MC should totally be fixed. That’s crazy! -Chris -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20170911/d3c3073e/attachment.html>
Chris Lovett via llvm-dev
2017-Sep-13 19:25 UTC
[llvm-dev] Performance of large llvm::ConstantDataArrays
Well I have a great test case if someone wants to help show me where/how to fix this in MC. On Mon, Sep 11, 2017 at 8:24 PM, Chris Lattner <clattner at nondot.org> wrote:> On Sep 10, 2017, at 1:34 AM, Sean Silva <chisophugis at gmail.com> wrote: > > > IMO, there is a relatively easy solution for this. Introduce a new >> subclass of ConstantData which represents a blob of data that gets emitted >> to the .o file, stored in a reasonable native format. I think it would be >> fine to limit this to only representing arrays of primitive types (e.g. >> array of float, array of bytes, etc) since this keeps the API to the type >> simple (the type models an array, so it should have array element members >> only), and things that want to get the elements of the array out can have >> them returned as ConstantInt’s (or whatever). I’d name this something like >> “ConstantArrayBlob”. >> > > > What's the relationship between ConstantDataArray and ConstantArray? > > > Ah, it looks like ConstantDataArray is exactly what I was advocating for. > Does Clang generate it from an array of doubles? Maybe that is all that is > missing. > > densely packed data, instead of as Value*'s." so I assumed that it was a > dense representation and it seemed reasonable that an i8 typed one of them > would basically operate as a "ConstantArrayBlob". (but I guess if MC still > creates one fragment per element that will still be a memory hog regardless > of the IR's representation) > > > Yeah, MC should totally be fixed. That’s crazy! > > -Chris > > >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20170913/c3870608/attachment.html>