[LLVMdev] Why is CLANG++ so freaking slow -- with example.

[Nick Lewycky]

On 12/30/2011 01:34 PM, Benjamin Kramer wrote:
>
> On 30.12.2011, at 22:12, kent williams wrote:
>
>> http://cornwarning.com/xfer/test.cxx.gz
>>
>> This is a preprocessed version of a file from from the VV image viewer
>> package.  I stripped out all the lines beginning in '#' and multiple
>> empty lines.
>>
>> It's about 75k lines of code.
>>
>> On a Mac Pro, the unix 'time' command says it's using 249 seconds of
>> user time.  Wall time on my machine somewhere around 5 minutes?
>>
>> 249.035u 1.883s 4:12.44 99.3%	0+0k 0+2io 0pf+0w
>>
>> CLang version:
>>
>> clang version 3.1 (trunk 146659)
>> Target: x86_64-apple-darwin10.8.0
>> Thread model: posix
>>
>> I built this CLang myself from the svn source a few weeks back -- I
>> compiled it with gcc, and then re-compiled it with itself.
>>
>> This is just one example. GCC compiles all of ITK and VTK in around an
>> hour. CLang++ takes 4 or 5 hours.
>>
>> Does LLVM's speed advantage go away when it has to deal with heavily
>> templated C++ code?
>
> Hi Kent,
>
> are you using a Debug or Release build of clang? Debug builds are MUCH slower.
> Your file takes less than a minute on my mac book pro (Release build).

It takes 1 minute 12 seconds with my Debug+Asserts build. A profile 
would still be interesting though.

In general Clang's speed advantage increases with heavily templated C++ 
code. It'd be great to understand what's going on in this case. Perhaps 
there was a nasty regression at the svn revision you picked?

Nick

>
> - Ben
>
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