[cfe-dev] clang performance when building Linux

[Benjamin Kramer]

On 13.04.2011, at 23:50, Török Edwin wrote:

> Hi,
> 
> I did a performance profile of clang while building Linux kernel.
> The difference is not much compared to gcc, clang with asserts off is a
> bit faster than gcc, and with asserts on a bit slower.
> 
> Here is the 'perf report' for clang with asserts off:
> https://gist.github.com/918497

>     3.65%     clang  clang                              [.] llvm::StringMapImpl::LookupBucketFor(llvm::StringRef)

I'm a bit surprised that StringMap is the most expensive entry here, maybe microoptimizing
the hash function (which is a byte-wise djb hash at the moment) can help a bit. If someone is
really bored it would also be useful to test if other string hash functions like murmurhash or google's
new city hash give better performance.

>     2.87%     clang  clang                              [.] clang::Lexer::LexTokenInternal(clang::Token&)

This is expected, the lexer usually dominates any C compiler's run time (well, except for crazy stuff like C++ ;),
clang's lexer is heavily optimized, I doubt we can improve much here.

>     2.74%     clang  [kernel.kallsyms]                  [k] clear_page_c

This is probably zero filling pages for malloc.

>     2.07%  genksyms  genksyms                           [.] yylex
>     1.88%     clang  clang                              [.] clang::SourceManager::getFileIDSlow(unsigned int) const

Despite that the name contains the word slow, this function is optimized well and known to be hot.

>     1.66%     clang  libc-2.13.so                       [.] _int_malloc

Reducing mallocs is an ongoing process, clang already pool allocates a lot of its data,
LLVM's types, constants and other stuff aren't allocated in pools at the moment but there are plans to fix that.

>     1.65%     clang  clang                              [.] clang::Lexer::LexIdentifier(clang::Token&, char const*)
>     1.08%     clang  libstdc++.so.6.0.15                [.] 0xa5750         
>     1.08%     clang  clang                              [.] llvm::SmallPtrSetImpl::insert_imp(void const*)
>     1.07%     clang  clang                              [.] clang::TokenLexer::Lex(clang::Token&)