[LLVMdev] Explicit template instantiations in libc++

[Justin Bogner]

Sean Silva <chisophugis at gmail.com> writes:
> Given the dates, this seems unrelated, but there have been
> compile-time issues with libc++ for some time now:
> http://llvm.org/bugs/show_bug.cgi?id=14587

Restoring these explicit instantiations will certainly help, but it
sounds like that PR is a bit more involved.

I ran some LNT numbers for posterity, and there are *many* compile time
improvements of 5-10% in our test suite, and no compile regressions.

           Performance Improvements - Compile Time                Δ    Previous Current   σ   
SingleSource/Benchmarks/Shootout-C++/strcat                    -11.50% 0.6436   0.5696  0.0005
SingleSource/Benchmarks/Misc-C++/bigfib                        -10.70% 1.1776   1.0516  0.0021
SingleSource/UnitTests/Vectorizer/gcc-loops                    -8.79%  0.9554   0.8714  0.0032
SingleSource/Benchmarks/Shootout-C++/matrix                    -8.08%  0.6670   0.6131  0.0010
SingleSource/Benchmarks/Shootout-C++/EH/except                 -7.94%  0.6474   0.5960  0.0027
SingleSource/Benchmarks/Shootout-C++/ackermann                 -7.26%  0.6377   0.5914  0.0012
MultiSource/Benchmarks/Prolangs-C++/employ/employ              -7.07%  0.7394   0.6871  0.0026
SingleSource/Benchmarks/Shootout-C++/methcall                  -6.97%  0.6444   0.5995  0.0040
SingleSource/Benchmarks/Shootout-C++/lists1                    -6.94%  0.7980   0.7426  0.0032
SingleSource/Benchmarks/Shootout-C++/ary3                      -6.93%  0.7299   0.6793  0.0018
SingleSource/Benchmarks/Shootout-C++/objinst                   -6.91%  0.6517   0.6067  0.0021
SingleSource/Benchmarks/Misc-C++/Large/sphereflake             -6.56%  0.7693   0.7188  0.0045
MultiSource/Benchmarks/Prolangs-C++/city/city                  -6.18%  4.5336   4.2535  0.0053
SingleSource/Benchmarks/Shootout-C++/sieve                     -6.06%  0.8268   0.7767  0.0014
SingleSource/Benchmarks/Adobe-C++/functionobjects              -5.90%  0.4964   0.4671  0.0005
SingleSource/Benchmarks/Shootout-C++/ary2                      -5.86%  0.6929   0.6523  0.0013
SingleSource/Benchmarks/Shootout-C++/nestedloop                -5.81%  0.5985   0.5637  0.0009
SingleSource/Benchmarks/Shootout-C++/fibo                      -5.75%  0.5913   0.5573  0.0017
SingleSource/Benchmarks/Shootout-C++/random                    -5.63%  0.6007   0.5669  0.0005
SingleSource/Benchmarks/Shootout-C++/hash2                     -5.52%  0.9050   0.8550  0.0013
SingleSource/Benchmarks/Misc-C++/Large/ray                     -5.40%  0.8162   0.7721  0.0007
SingleSource/Benchmarks/CoyoteBench/fftbench                   -5.28%  0.8547   0.8096  0.0008
External/SPEC/CFP2006/450_soplex/450_soplex                    -4.91%  48.1568  45.7941 0.0032
SingleSource/Benchmarks/Shootout-C++/lists                     -4.64%  0.7248   0.6912  0.0024
SingleSource/Benchmarks/Shootout-C++/ary                       -4.63%  0.6829   0.6513  0.0004
MultiSource/Applications/hexxagon/hexxagon                     -3.80%  2.1581   2.0760  0.0049
SingleSource/Benchmarks/Shootout-C++/hash                      -3.71%  0.8454   0.8140  0.0012
MultiSource/Applications/kimwitu++/kc                          -3.03%  18.1397  17.5900 0.0186
SingleSource/Benchmarks/Misc-C++/stepanov_container            -2.49%  1.4438   1.4079  0.0010
MultiSource/Benchmarks/tramp3d-v4/tramp3d-v4                   -1.67%  24.2177  23.8125 0.0248
External/SPEC/CINT2006/471_omnetpp/471_omnetpp                 -1.33%  54.8197  54.0908 0.0226


>
> -- Sean Silva
>
> On Tue, Aug 12, 2014 at 2:17 PM, Justin Bogner <mail at justinbogner.com> wrote:
>
>     Most of libc++ doesn't have explicit template instantiations, which
>     leads to a pretty significant build time and code size cost when using
>     libc++, since a large number of common templates will be emitted by the
>     compiler and coalesced by the linker. Notably, in include/__config, we
>     have:
>    
>         #ifndef _LIBCPP_EXTERN_TEMPLATE
>         #define _LIBCPP_EXTERN_TEMPLATE(...)
>         #endif
>    
>     whereas before r189601 this was:
>    
>         #define _LIBCPP_EXTERN_TEMPLATE(...) extern template __VA_ARGS__;
>    
>     This was apparently done to fix http://llvm.org/bugs/show_bug.cgi?id=17027,
>     but disabling explicit instantiations seems like a pretty big hammer
>     considering the drawbacks.
>    
>     I'd like to restore these instantiations. Any thoughts on how to handle
>     things like pr17027 in a less heavy handed way?
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