On Tue, Jul 3, 2018 at 12:57 PM Andrew Stubbs <andrew_stu...@mentor.com> wrote:
>
> On 03/07/18 11:33, Andrew Stubbs wrote:
> > On 03/07/18 11:15, Richard Biener wrote:
> >> AVX ones are all UNSPECs I believe - how do your patterns look like?
> >
> > AVX has both unspec and vec_merge variants (at least for define_expand,
> > in GCC8), but in any case, AFAICT dse.c only cares about the destination
> > MEM, and all the AVX and SVE patterns appear to use nothing special there.
>
> Sorry, my patterns look something like this:
>
> (set (mem:V64DI (reg:DI)
> (vec_merge:V64DI (reg:V64DI) (unspec ...) (reg:DI)))
>
> Where the unspec just means that the destination remains unchanged. We
> could also use (match_dup 0) there, but we don't, so probably there was
> an issue with that at some point.
I believe that the AVX variants like
(define_expand "maskstore<mode><avx512fmaskmodelower>"
[(set (match_operand:V48_AVX512VL 0 "memory_operand")
(vec_merge:V48_AVX512VL
(match_operand:V48_AVX512VL 1 "register_operand")
(match_dup 0)
(match_operand:<avx512fmaskmode> 2 "register_operand")))]
"TARGET_AVX512F")
are OK since they represent a use of the memory due to the match_dup 0
while your UNSPEC one doesn't so as the store doesn't actually take
place to all of 0 your insn variant doesn't represent observable behavior.
> >>> I was thinking of solving the problem by adding a target hook to query
> >>> the true length of vector types. Any thoughts on that?
> >>
> >> It isn't about the length but about the mask because there can be mask
> >> values that do not affect the length?
> >
> > The problem I have right now is that the vector write conflicts with
> > writes to distinct variables, in which case the vector length is what's
> > important, and it's probably(?) safe to assume that if the vector mask
> > is not constant then space for the whole vector has been allocated on
> > the stack.
> >
> > But yes, in general it's true that subsequent writes to the same vector
> > could well write distinct elements, in which case the value of the mask
> > is significant to DSE analysis.
> >
> > Andrew
>
