Re: Build system adjustments for subarchs

[Rich Felker <dalias@aerifal.cx>]

On Wed, Aug 14, 2013 at 07:55:25PM -0500, Strake wrote:
> On 13/08/2013, Rich Felker <dalias@aerifal.cx> wrote:
> > The current system searches for arch-specific asm in this order:
> >
> > 1. $(ARCH)$(ASMSUBARCH)/%.s, where ASMSUBARCH for the default subarch
> >    is not blank but rather a unique suffix (for plain arm, it's "el").
> >    This allows having asm that applies only to the default subarch but
> >    not others.
> >
> > 2. $(ARCH)/%.s, for shared asm used by all subarchs.
> >
> > 3. %.c, the C fallback (which is empty for code that cannot be
> >    implemented at all in C).
> 
> Another option:
> 
> Each arch has properties, which each take a value in a set; these sets
> are mutually exclusive, and all the values in each set have the same
> length. The properties have a well-defined order. The build system
> parses each directory name as an arch name and a list of properties,
> and chooses the most specific match; if no match, it uses the C code.

This would be nice, but as far as I can tell there's no simple way of
doing it without hard-coding the number of possible properties and
including a rule for each one in the makefile.

> > Unfortunately, this still provides no way to include asm that's used
> > by both soft and hardfloat little-endian, or both little- and
> > big-endian hardfloat, without having, for example:
> >
> > - armel/%.s and armhf/%.s as duplicate files or symlinked
> > - armhf/%.s and armebhf/%.s as duplicate files or symlinked
> 
> Thus arm would have 2 properties:
> byte order ∈ { "eb", "el" }
> float hardness ∈ { "hf", "sf" }
> 
> asm used by both soft- and hard-float little-endian: armel/%.s
> asm used by both little- and big-endian hard-float: armhf/%.s
> asm used by little-endian hard-float: armelhf/%.s

The system I ended up committing is not quite this logical, but it
admits a simple implementation and it's very flexible, allowing
arbitrary sharing and substitutions between subarchs.

Rich