[fpc-devel] Data alignment feature

J. Gareth Moreton gareth at moreton-family.com
Fri Jan 19 21:08:07 CET 2018

Hi everyone,

So unless anyone has any objections, I would like to start experimenting to implement a feature that allows 
for per-type data alignment.  The main purpose for this is to better support x86-64 SIMD extensions, where 
aligned data is far faster to process.  While there is a compiler directive that enforces byte alignment, 
this risks causing conflicts with third party units and is a bit much of a demand for a programmer wishing 
to use an SIMD-tuned unit, say.

Bug/feature details: https://bugs.freepascal.org/view.php?id=32780

The format would be as follows:

type TypeName = TypeInfo [align ByteAmount];

The syntax for declaring a type is exactly the same, with a new optional "align" keyword before the 
terminating semicolon. "ByteAmount" has to be a power of 2, which I will probably limit to 128 currently, 
just so it doesn't cause ridiculous or malicious memory usage.  There will probably have to be a minimum 
alignment as well, although this will probably just be automatically enforced (e.g. Single is always aligned 
on a 4-byte boundary minimum).

The intention here is that this feature will be a stepping stone for properly supporting vectorisation, 
implementing some form of the "M128" and "M256" types, and the "vectorcall" compiler directive for Windows.

Gareth aka. Kit

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