Re: [boost] Proposal for #pragma once support

Subject: Re: [boost] Proposal for #pragma once support
From: Christopher Jefferson (chris_at_[hidden])
Date: 2009-06-09 14:03:17

On 9 Jun 2009, at 16:59, Zachary Turner wrote:

> I would like to propose conditional support for #pragma once be  
> added to all
> header files in boost.  This would involve, at the beginning of  
> every header
> file, a preprocessor check to determine whether or not the particular
> compiler is on a whitelist of known compilers to optimized #pragma  
> once.  If
> so, #pragma once is used.  so, something like this:
>
>
>
> Here are the build timings on my machine (Visual Studio 2008 Service  
> Pack 2)
>
> RedundantGuard - 1 second compile + link time
> PragmaOnce - 1 second compile + link time
> Nothing - 15 seconds compile + link time
>

Here are my build-timings on g++, Mac OS X.

I had to remove 3 of the header files (I emptied the files Header197,  
198 and 199) due to gcc complaining about include depth. I hope this  
won't effect things too much. I also had to change the return type of  
main to int.

I compiled by doing "g++ main.cpp"

Note that modern g++ does support '#pragma once' fine.

I get the following results:

RedundantGuard: 0.130s
Nothing: 0.170s
PragmaOnce: 0.710s

So PragmaOnce is over 7 times slower. That time is almost entirely in  
the 'sys' column, so I suspect it querying files to check if they are  
identical or not.

Chris