From: Richard Biener <richard.guenther@gmail.com>
To: Richard Sandiford <richard.sandiford@arm.com>
Cc: "Martin Liška" <mliska@suse.cz>, "GCC Patches" <gcc-patches@gcc.gnu.org>
Subject: Re: [PATCH][RFC] Come up with VEC_COND_OP_EXPRs.
Date: Tue, 24 Sep 2019 11:29:00 -0000 [thread overview]
Message-ID: <CAFiYyc0GK7z5LFjgFZOXLgV1cj4qnrtm3nw9nN0u0ypye53sdQ@mail.gmail.com> (raw)
In-Reply-To: <mptimpilziv.fsf@arm.com>
On Tue, Sep 24, 2019 at 1:11 PM Richard Sandiford
<richard.sandiford@arm.com> wrote:
>
> Martin Liška <mliska@suse.cz> writes:
> > Hi.
> >
> > The patch introduces couple of new TREE_CODEs that will help us to have
> > a proper GIMPLE representation of current VECT_COND_EXPR. Right now,
> > the first argument is typically a GENERIC tcc_expression tree with 2 operands
> > that are visited at various places in GIMPLE code. That said, based on the discussion
> > with Richi, I'm suggesting to come up with e.g.
> > VECT_COND_LT_EXPR<COND_LHS, COND_RHS, IF_CLAUSE, ELSE_CLAUSE>. Such a change logically
> > introduces new GIMPLE_QUATERNARY_RHS gassignments. For now, the VEC_COND_EXPR remains
> > and is only valid in GENERIC and gimplifier will take care of the corresponding transition.
> >
> > The patch is a prototype and missing bits are:
> > - folding support addition for GIMPLE_QUATERNARY_RHS is missing
> > - fancy tcc_comparison expressions like LTGT_EXPR, UNORDERED_EXPR, ORDERED_EXPR,
> > UNLT_EXPR and others are not supported right now
> > - comments are missing for various functions added
> >
> > Apart from that I was able to bootstrap and run tests with a quite small fallout.
> > Thoughts?
> > Martin
>
> I think this is going in the wrong direction. There are some targets
> that can only handle VEC_COND_EXPRs well if we know the associated
> condition, and others where a compare-and-VEC_COND_EXPR will always be
> two operations. In that situation, it seems like the native gimple
> representation should be the simpler representation rather than the
> more complex one. That way the comparisons can be optimised
> independently of any VEC_COND_EXPRs on targets that benefit from that.
>
> So IMO it would be better to use three-operand VEC_COND_EXPRs with
> no embedded conditions as the preferred gimple representation and
> have internal functions for the fused operations that some targets
> prefer. This means that using fused operations is "just" an instruction
> selection decision rather than hard-coded throughout gimple. (And that
> fits in well with the idea of doing more instruction selection in gimple.)
So I've been doing that before, but more generally also for COND_EXPR.
We cannot rely on TER and the existing RTL expansion "magic" for the
instruction selection issue you mention because TER isn't reliable. With
IFNs for optabs we could do actual [vector] condition instruction selection
before RTL expansion, ignoring "single-use" issues - is that what you are
hinting at? How should the vectorizer deal with this? Should it directly
use the optab IFNs then when facing "split" COND_EXPRs? IIRC the
most fallout of a simple patch (adjusting is_gimple_condexpr) is in the
vectorizer.
Note I'm specifically looking for a solution that applies to both COND_EXPR
and VEC_COND_EXPR since both suffer from the same issues.
There was also recent work in putting back possibly trapping comparisons
into [VEC_]COND_EXPR because it doesn't interfere with EH and allows
better code. Also you SVE people had VN issues with cond-exprs and
VN runs into the exact same issue (but would handle separate comparisons
better - with the caveat of breaking TER).
Richard.
>
> Thanks,
> Richard
next prev parent reply other threads:[~2019-09-24 11:29 UTC|newest]
Thread overview: 65+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-09-24 10:25 Martin Liška
2019-09-24 11:11 ` Richard Sandiford
2019-09-24 11:29 ` Richard Biener [this message]
2019-09-24 11:57 ` Richard Sandiford
2019-09-24 12:18 ` Richard Biener
2019-09-24 14:51 ` Richard Sandiford
2020-04-01 10:19 ` [stage1][PATCH] Lower VEC_COND_EXPR into internal functions Martin Liška
2020-04-06 9:17 ` Richard Sandiford
2020-04-06 12:30 ` Richard Biener
2020-05-21 12:51 ` Martin Liška
2020-05-21 13:29 ` Martin Liška
2020-05-21 20:16 ` Segher Boessenkool
2020-05-22 11:14 ` Richard Biener
2020-05-26 10:15 ` Richard Sandiford
2020-05-27 14:04 ` Martin Liška
2020-05-27 16:13 ` Richard Sandiford
2020-05-27 16:32 ` Richard Biener
2020-05-28 14:46 ` Martin Liška
2020-05-28 15:28 ` Richard Sandiford
2020-05-29 12:17 ` Richard Biener
2020-05-29 12:43 ` Richard Biener
2020-05-29 16:47 ` Segher Boessenkool
2020-05-29 17:05 ` Richard Sandiford
2020-05-29 17:30 ` Segher Boessenkool
2020-05-29 15:39 ` Segher Boessenkool
2020-05-29 16:57 ` Richard Sandiford
2020-05-29 17:09 ` Segher Boessenkool
2020-05-29 17:26 ` Richard Sandiford
2020-05-29 17:37 ` Segher Boessenkool
2020-05-30 7:15 ` Richard Sandiford
2020-05-30 13:08 ` Segher Boessenkool
2020-06-02 11:09 ` Richard Biener
2020-06-02 15:00 ` Martin Liška
2020-06-03 7:38 ` Richard Biener
2020-06-03 13:41 ` Richard Sandiford
2020-06-03 14:17 ` David Edelsohn
2020-06-03 14:46 ` Richard Biener
2020-06-03 17:01 ` Segher Boessenkool
2020-06-03 17:23 ` Richard Biener
2020-06-03 18:23 ` Segher Boessenkool
2020-06-03 18:38 ` Richard Biener
2020-06-03 18:46 ` David Edelsohn
2020-06-03 19:09 ` Segher Boessenkool
2020-06-03 19:13 ` Jakub Jelinek
2020-06-03 18:27 ` Segher Boessenkool
2020-06-08 11:04 ` Martin Liška
2020-06-09 13:42 ` Richard Biener
2020-06-10 8:51 ` Martin Liška
2020-06-10 10:50 ` Richard Biener
2020-06-10 12:27 ` Martin Liška
2020-06-10 13:01 ` Martin Liška
2020-06-11 8:52 ` Martin Liška
2020-06-12 9:43 ` Richard Biener
2020-06-12 13:24 ` Martin Liška
2020-06-15 7:14 ` Richard Biener
2020-06-15 11:19 ` Martin Liška
2020-06-15 11:59 ` Richard Biener
2020-06-15 12:20 ` Martin Liška
2020-06-17 8:50 ` Richard Biener
2020-06-17 13:15 ` Richard Biener
2020-06-18 8:10 ` Martin Liška
2020-06-18 8:52 ` Richard Biener
2020-06-18 9:02 ` Martin Liška
2020-06-18 9:29 ` Martin Liška
2020-04-06 12:33 ` Richard Biener
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