On Tue, 28 Nov 2023, Jeff Law wrote:
>
>
> On 11/28/23 00:50, Richard Biener wrote:
>
> >
> > There's no way to distinguish a partial vs. non-partial MEM on RTL and
> > while without the bogus MEM_ATTR the alias oracle pieces that
> > miscompiled the original case are fended off we still see the load/store
> > as full given they have a mode with a size - that for example means
> > that DSE can elide a previous store to a masked part. Eventually
> > that's fended off by using an UNSPEC, but whether the RTL IL has
> > the correct semantics is questionable.
> >
> > That said, I did propose scrapping the MEM_EXPR which I think is
> > the correct thing to do unless we want to put a CALL_EXPR into it
> > (nothing would use that at the moment) or re-do MEM_EXPR and instead
> > have an ao_ref (or sth slightly more complete) instead of the current
> > MEM_ATTRs - but that would be a lot of work.
> >
> > This leaves the question wrt. semantics of for example x86 mask_store:
> >
> > (insn 23 22 24 5 (set (mem:V4DF (plus:DI (reg/v/f:DI 106 [ x ])
> > (reg:DI 101 [ ivtmp.15 ])) [2 MEM <vector(4) double>
> > [(double *)x_11(D) + ivtmp.15_33 * 1]+0 S32 A64])
> > (unspec:V4DF [
> > (reg:V4DI 104 [ mask__16.8 ])
> > (reg:V4DF 105 [ vect_cst__42 ])
> > (mem:V4DF (plus:DI (reg/v/f:DI 106 [ x ])
> > (reg:DI 101 [ ivtmp.15 ])) [2 MEM <vector(4)
> > double> [(double *)x_11(D) + ivtmp.15_33 * 1]+0 S32 A64])
> > ] UNSPEC_MASKMOV)) "t.c":5:12 8523 {avx_maskstorepd256}
> > (nil))
> >
> > it uses a read-modify-write which makes it safe for DSE.
> Agreed.
>
>
> mask_load
> > looks like
> >
> > (insn 28 27 29 6 (set (reg:V4DF 115 [ vect__7.11 ])
> > (unspec:V4DF [
> > (reg:V4DI 114 [ mask__8.8 ])
> > (mem:V4DF (plus:DI (reg/v/f:DI 118 [ val ])
> > (reg:DI 103 [ ivtmp.29 ])) [2 MEM <vector(4)
> > double> [(double *)val_13(D) + ivtmp.29_22 * 1]+0 S32 A64])
> > ] UNSPEC_MASKMOV)) "t.c":5:17 8515 {avx_maskloadpd256}
> > (nil))
> So with the mem:V4DF inside the unspec, ISTM we must treat that as a potential
> full read, but we can't rely on it being a full read. I don't think UNSPEC
> semantics are that it must read/consume all its operands in full, just that it
> might. That might be worth a documentation clarification.
>
>
> >
> > both have (as operand of the UNSPEC) a MEM with V4DFmode (and a
> > MEM_EXPR with a similarly bougs MEM_EXPR) indicating the loads
> > are _not_ partial. That means the disambiguation against a store
> > to an object that's smaller than V4DF is still possible.
> > Setting MEM_SIZE to UNKNOWN doesn't help - that just asks to look
> > at the mode. As discussed using a BLKmode MEM _might_ be a way
> > out but I didn't try what will happen then (patterns would need to
> > be adjusted I guess).
> >
> > That said, I'm happy to commit the partial fix, scrapping the
> > bogus MEM_EXPRs.
> >
> > OK for that?
> Works for me.
I'm re-testing the change and will push. If the UNSPEC uses are really
OK I think we're set. We can incrementally try to restore missing
alias info.
Richard.
> jeff
>
--
Richard Biener <rguent...@suse.de>
SUSE Software Solutions Germany GmbH,
Frankenstrasse 146, 90461 Nuernberg, Germany;
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