Re: Support for out-of-tree build

[Petr Hosek <phosek@chromium.org>]

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Absolutely, that's probably the simplest solution in the sense that it
avoids using additional .mk/.sub files or complicated build-time logic.

If that's the solution we all agree on, I'd be happy to implement this as a
separate patch and then rebase the out-of-tree build patch on top of that.

On Tue, Nov 17, 2015 at 2:01 PM Rich Felker <dalias@libc.org> wrote:

> On Tue, Nov 17, 2015 at 09:37:37PM +0000, Petr Hosek wrote:
> > How would you express the fact that e.g. armhf subarch should use armel
> > implementation of memcpy.s?
>
> The point of the *.sub files is to define a mapping from a fully
> specific subarch (with 'default'/'empty' replaced with an explicit
> default; this is why configure sets ASMSUBARCH=el for plain arm) to
> the actual file to use, which is almost always going ot be shared
> between a number of subarchs when there's more than one 'dimension'
> involved.
>
> However there's no need for this mapping to be defined per-file. In
> reality it's sufficient to have a fixed fallback sequence. In the rare
> case where there were asm files that depended on endianness and
> hard/soft float in the same file, the fallback order would look
> something like (for le-hf):
>
>         le-hf le-any any-hf any-any
>
> But for our actual usage cases it suffices just to have:
>
>         le hf any
>
> However, at this point I'm strongly considering whether we should just
> do away with the subarch dirs entirely and use preprocessed asm or C
> with inline asm in their place. The one place this is mildly difficult
> is when only some of the subarchs want the asm at all, and others want
> the generic C. This is common for fenv.s and also applies to arm
> memcpy.s where we lack a big-endian variant.
>
> One possible solution would be to have (for example)
> src/string/arm/memcpy.c that just does
>
> #ifdef __ARMEB__
> #include <../memcpy.c>
> #endif
>
> and then put memcpy.S in arch/arm/src/ with #ifndef __ARMEB__ around
> the whole file.
>
> That would fully eliminate the subarch mess from the build system and
> leave us with a fixed fallback order for it to use:
>
> $(ARCH)/%.S
> $(ARCH)/%.s
> $(ARCH)/%.c
> %.S
> %.s
> %.c
>
> Currently the unadorned %.S/%.s is not needed, but I would consider
> adding it so we can put asm files in arch/$(ARCH)/src without needing
> dummy .c files for them to pull in asm from arch/$(ARCH)/src/$(ARCH).
>
> Would this make things simpler for the build system? I think so.
>
> Rich
>

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