Re: [avr-gcc-list] -fpacked-struct compiler option

[Ben L. Titzer]

On Jan 6, 2005, at 12:58 PM, E. Weddington wrote:

> Anton Erasmus wrote:
>
> > Hi,
> >
> > The makefile example that comes with WinAVR uses the -fpacked-struct 
> > compiler option.
> >
> > The gcc documentation has the following warning on this option:
> > Warning: the -fpack-struct switch causes GCC to generate code that is 
> > not binary compatible with code generated without that switch. 
> > Additionally, it makes the code suboptimial. Use it to conform to a 
> > non-default application binary interface.
> 1. We don't have to worry much about "binary compatibility" as AVR 
> projects are compiled into one image, i.e. no loading of external, 3rd 
> party binary images.
> 2. AFAIK it is only suboptimal on targets that have alignments > 1.
>
> > So why is this option used in the example makefile. I would have 
> > thought that with the
> > avr being an 8-bit architecture that structures would automatically 
> > be packed, as there
> > is no advantage in accessing non-byte alligned data compared to 
> > accessing byte alligned data.
> Um, perhaps just being paranoid? Seemed like a good thing to do at the 
> time? It shouldn't hurt anything to have it in?
> Anybody have any other reasoning they'd like to throw around? :-)

Many compilers choose to align on the size of the data type, rather on 
the size of the underlying architecture. This is done so that a data 
item that might be larger than a word does not span a cache line 
boundary, which can hurt performance measurably. So for an 8-byte 
quantity like a double precision floating point or long long (on most 
architectures), it is aligned on an 8-byte boundary. Therefore on AVR, 
a 16-bit integer is likely to be aligned on a 2-byte boundary, 
regardless of the fact that the memory is byte addressable and the 
registers are 8 bits (and there is no cache). There are many 
desktop-class assumptions in GCC. They work fairly well for 
desktop-class processors, but really suck when it comes to compiling 
for an 8-bit mcu.

-B

> Eric
>
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