[cfe-dev] Parsing benchmark: LibTomMath

[Neil Booth]

Chris Lattner wrote:-

> It would be better to run the gcc test 3  times then the clang test 3  
> times and take the minimum of each.  That said, we should be  
> significantly faster... 3x is awesome!

I don't want to spoil the party :), but the comparison is not really
fair.

cc1 should be being invoked to avoid an extra fork/exec that
penalizes GCC.  GCC handles character sets to some extent, which
is a non-trivial cost, which clang does not.  Clang doesn't implement
all the language yet; the extra semantic checking will likely slow
it down slightly.  I believe GCC does lowering of some kind in
addition to semantic analysis with -fsyntax-only, but I guess that
can be viewed as an imlpementation weakness.

Of course, once complete, clang will probably be further improved
speed-wise, and having said all that I'm sure clang will end up much
faster than GCC.

As another data point, here are timings on my (slow) machine,
fastest of three runs, for compiling the preprocessor files of my
own C front end.  Like clang, cfe is stand-alone, and doesn't handle
charsets, so to that extent it is a fairer comparison.  Like clang
there hasn't really been any time spent optimizing for speed.
However unlike clang it does semantically analyze and diagnose
pretty much the whole language.  clang/llvm was compiled with gmake
MAKE_OPTIMIZED=1 -- I believe that's how to get an optimized
executable.

$ time for file in cpp/*.c; do /usr/libexec/cc1 -quiet -fsyntax-only -I.
$file ;done

real    0m1.098s
user    0m0.843s
sys     0m0.190s

$ time for file in cpp/*.c; do ~/src/nobackup/llvm/Release/bin/clang
-fsyntax-only -I. $file ;done
cpp/macro.c:686:15: error: variable has incomplete type 'char []'
  static char compile_date[] = "\"Mmm dd yyyy\"";
              ^
cpp/macro.c:687:15: error: variable has incomplete type 'char []'
  static char compile_time[] = "\"hh:mm:ss\"";
              ^
2 diagnostics generated.

real    0m0.456s
user    0m0.281s
sys     0m0.147s

$ time for file in cpp/*.c; do ~/src/cfe/cfe -I. $file ;done
 
real    0m0.205s
user    0m0.118s
sys     0m0.083s

So there is still room for improvement, curing the C++ bloat and all
that. 8)

Neil.