Header file inclusion speed again

[Markus Hitter]

Hi all again,

last time I tried to measure the speed increase of precompiled headers, 
the result was: Either it's on and fast or, it's off and slow. No 
difference how much and which style headers are included.

As you perhaps remember, in most of the files, I set headers up like:

#ifdef HAS_PRECOMPILED_HEADERS
#import <Foundation/Foundation.h>
#else
#import <Foundation/NSObject.h>
...
@class NSDictionary;
...
#endif
#import <EOControl/EOWhatever.h>
...


The day after, I dicovered that one of my config header always included 
the full <Foundation/Foundation.h>, independant of wether I set 
HAS_PRECOMPILED_HEADERS to on or to off.

Today, I removed this fault and got new measurements (for a now bigger 
set of sources). This time I'm quite sure I did it right as I had to 
fix some missing header inclusions. Here are the timings:

HAS_PRECOMPILED_HEADERS undefined: 2 Minutes, 15 Seconds
HAS_PRECOMPILED_HEADERS defined: 2 Minutes, 7 Seconds
HAS_PRECOMPILED_HEADERS undefined, gcc with -no-cpp-precomp(*): 7 
Minutes, 24 Seconds
HAS_PRECOMPILED_HEADERS defined, gcc with -no-cpp-precomp: 10 Minutes, 
0 Seconds
HAS_PRECOMPILED_HEADERS undefined, early umbrella header inclusion: 2 
Minutes, 8 Seconds
HAS_PRECOMPILED_HEADERS defined, early umbrella header inclusion: 2 
Minutes, 8 Seconds
HAS_PRECOMPILED_HEADERS undefined, early umbrella header inclusion, gcc 
with -no-cpp-precomp: 9 Minutes, 58 Seconds

hmm ...

So, here's my interpretation of the results:

- using umbrella headers doesn't give as much speed increase as one 
might think. At least, with Apple's set of Foundation headers. Perhaps 
they include the umbrella header in the single-class header files.

- Using a well defined set of single headers inclusions gives a 
significant speed increase without a precompiler but a marginal speed 
drop with precompilation enabled. Thats an argument against the 
umbrella header inclusion in single-class header files.

- Any complex mechanism for differing wether a precompiled header is to 
be used is obsolete. Simply include the umbrella header in a common, 
early included file (config.h or similar).


Have fun,
Markus


* This flag switches the precompiler off which is enabled by default.
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Dipl. Ing. Markus Hitter
http://www.jump-ing.de/