[fpc-devel] Class field reordering

[Craig Peterson]

On Jul 13, 2012, at 6:44 PM, Jonas Maebe <jonas.maebe at elis.ugent.be> wrote:

> 
> Hi,
> 
> I've implemented an optimization that reorders the instance fields of Delphi-style classes (and only of Delphi-style classes) to minimise memory gaps caused by alignment differences and odd sizes. The effect is the same as when you would change the order of the fields in the source code to achieve this effect.
> 
> In general, I think this should be safe since unlike records and TP-style objects, a Delphi class is normally never blockwritten to disk or so. There is a switch to disable this transformation, but I'm wondering whether anyone sees a problem with enabling it by default when -O2 or higher is used.
> 
> It works fine with at least the compiler and Lazarus without any ill effects, saving a small amount of memory for both (about 2.5MB on 87MB for the Darwin/x86-64 compiler compiling itslf, and 2.5MB on 62.7MB for a Carbon/i386 Lazarus right after startup and loading some source files).
> 
> 
> Jonas
> 
> PS: a similar reordering could be performed for local variables, although the implementation would be quite different. Additionally, different kinds of reordering, e.g. targeting cache improvements by putting fields/local variables often used together next to each other, could also be implemented in the future.
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