[Bug target/106770] PPCLE: Unnecessary xxpermdi before mfvsrd

["segher at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106770

--- Comment #5 from Segher Boessenkool <segher at gcc dot gnu.org> ---
(In reply to Jens Seifert from comment #4)
> PPCLE with no special option means -mcpu=power8 -maltivec  (altivecle to be
> mor precise).

What?  No.

$ sh config.sub ppcle      
powerpcle-unknown-none

This is typically the old 32-bit PowerPC ELF format.  powerpcle-elf (which
non-canonically can be called ppcle-elf) for example, or ppcle-linux, but not
ppcle-aix and the like (that one doesn't even exist; at least one COFF
format has existed in the past though).

This may not matter for you, but it is awfully confusing for others.


powerpc64le-linux (and I believe all other existing ELFv2 ports) require
a p8 or later CPU, sure; but it is perfectly valid to have no AltiVec even
then, or for a port to default to some other CPU.  Currently we have no such
thing, and all default defaults are like you say, but that might change.

> vec_promote(<double value>, 1) should be a noop on ppcle.

It never is, not on powerpc64le either.  It always duplicates the selected
elt to all lanes.

> But value gets
> splatted to both left and right part of vector register. => 2 unnecesary
> xxpermdi

So why are those not optimised away?  *That* is the question!

> The rest of the operations are done on left and right part.
> 
> vec_extract(<vector double>, 1) should be noop on ppcle. But value gets
> taken from right part of register which requires a xxpermdi
> 
> Overall 3 unnecessary xxpermdi. Don't know why the right part of register
> gets "preferred".

I don't know what you mean there?  The ABIs say where parameters and return
values are stored, but you mean something else?