[cfe-dev] Weird clang++ slowness

[Óscar Fuentes]

Hello Ben,

Benjamin Kramer <benny.kra at gmail.com>
writes:

>> On 25.10.2013, at 13:13, "Óscar Fuentes" <ofv at wanadoo.es> wrote:
>>
>> I found certain source files which are taking a lot of time to compile.
>> Upon investigation I measured this times:
>>
>>
>> 9.8 seconds for command: clang++ -c -std=c++11 file.cpp
>>
>> 0.2 seconds for command: clang++ -E -std=c++11 file.cpp > f.CC
>>    (creates a 2.9 MB file)
>>
>> 3.4 seconds for command: clang++ -c -std=c++11 f.CC
>>
>>
>> So explicitly preprocessing and then compiling takes less than half the
>> time of compiling the original file.
>>
>> I guess this is due to macro expansion tracking for diagnostic purposes.
>> The C++ file contains some errors (it belongs to a test suite) although
>> I see no references to macros on the output of the compiler.
>>
>> So how can I speed up the compilation? Can this be considered a bug?
>
> Could be a bug or just macro tracking with very deep instantiations.
> Impossible to say without a test case.

I think I hit a huge pessimization on Clang's diagnostic machinery. See
this test case:



#include "boost/asio.hpp"

double mainf();

int mainf() {
  foo(sdafo_2, 0);
  return 0;
}



Some timings:

Time (seconds)  Command
6.8             clang -c -std=c++11 test.cpp
4.8             clang -c test.cpp

    (after replacing `sdafo_2' with `x')
1.0             clang -c test.cpp

    (after replacing `sdafo_2' with `o_2')
2.8             clang -c test.cpp



Also please note how Clang quickly reports the first error about the
ilegal overload of `mainf' and then there is a long lapse of time until
it reports the error concerning `sdafo_2'.