From: Richard Sandiford <richard.sandiford@arm.com>
To: Segher Boessenkool <segher@kernel.crashing.org>
Cc: Tejas Joshi <tejasjoshi9673@gmail.com>,
gcc@gcc.gnu.org, Martin Jambor <mjambor@suse.cz>,
hubicka@ucw.cz, joseph@codesourcery.com
Subject: Re: Expansion of narrowing math built-ins into power instructions
Date: Tue, 20 Aug 2019 15:12:00 -0000 [thread overview]
Message-ID: <mptzhk33m6h.fsf@arm.com> (raw)
In-Reply-To: <mpt8srn523k.fsf@arm.com> (Richard Sandiford's message of "Tue, 20 Aug 2019 15:43:43 +0100")
Richard Sandiford <richard.sandiford@arm.com> writes:
>> And yes, it is icky. But it is sound, as far as I can see.
>
> I really disagree that it's sound, but no point me saying why again :-)
>
> (It could certainly be made to work with sufficient hacks of course,
> like pretty much anything could, but I don't think that's the same thing.)
For an example, we have:
/* Maybe simplify x + 0 to x. The two expressions are equivalent
when x is NaN, infinite, or finite and nonzero. They aren't
when x is -0 and the rounding mode is not towards -infinity,
since (-0) + 0 is then 0. */
if (!HONOR_SIGNED_ZEROS (mode) && trueop1 == CONST0_RTX (mode))
return op0;
I think it's plausible that people will care about accurate rounding
but not signed zeroes. In that mode we could have:
(set (reg:DF r3) (plus:DF (reg:DF r1) (reg:DF r2)))
(set (reg:DF r4) (const_double:DF 0.0))
(set (reg:SF r5) (float_narrow:SF (plus:DF (reg:DF r3) (reg:DF r4))))
Then combine through normal structural simplification could (with the
rule above) fold all this down to:
(set (reg:SF r5) (float_narrow:SF (plus:DF (reg:DF r1) (reg:DF r2))))
where the truncation is now fused with r1+r2 instead of r3+r4. We would
have to have to add specific checks to avoid this happening, it wouldn't
fall out naturally from structural PoV.
Thanks,
Richard
next prev parent reply other threads:[~2019-08-20 15:12 UTC|newest]
Thread overview: 63+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-29 17:37 Martin Jambor
2019-07-29 18:40 ` Segher Boessenkool
2019-07-30 19:47 ` Joseph Myers
2019-07-30 9:20 ` Florian Weimer
2019-07-30 19:49 ` Joseph Myers
2019-07-31 6:47 ` Tejas Joshi
2019-07-31 14:47 ` Segher Boessenkool
2019-08-08 18:39 ` Tejas Joshi
2019-08-08 20:05 ` Segher Boessenkool
2019-08-08 23:09 ` Joseph Myers
2019-08-10 10:24 ` Tejas Joshi
2019-08-10 16:46 ` Segher Boessenkool
2019-08-11 4:58 ` Tejas Joshi
2019-08-11 7:20 ` Segher Boessenkool
2019-08-11 12:46 ` Tejas Joshi
2019-08-11 16:59 ` Segher Boessenkool
2019-08-12 17:25 ` Tejas Joshi
2019-08-12 17:55 ` Segher Boessenkool
2019-08-12 21:20 ` Joseph Myers
2019-08-12 21:52 ` Segher Boessenkool
2019-08-14 6:15 ` Tejas Joshi
2019-08-14 7:21 ` Segher Boessenkool
2019-08-14 16:11 ` Joseph Myers
2019-08-14 20:21 ` Segher Boessenkool
2019-08-14 20:23 ` Joseph Myers
2019-08-14 21:00 ` Segher Boessenkool
2019-08-15 9:52 ` Tejas Joshi
2019-08-15 12:47 ` Richard Sandiford
2019-08-15 13:55 ` Tejas Joshi
2019-08-15 18:45 ` Segher Boessenkool
2019-08-16 10:23 ` Richard Sandiford
2019-08-17 5:40 ` Tejas Joshi
2019-08-17 8:21 ` Richard Sandiford
2019-08-19 10:46 ` Tejas Joshi
2019-08-19 13:07 ` Segher Boessenkool
2019-08-20 7:41 ` Richard Sandiford
2019-08-20 12:11 ` Segher Boessenkool
2019-08-20 12:59 ` Richard Sandiford
2019-08-20 13:46 ` Segher Boessenkool
2019-08-20 14:43 ` Richard Sandiford
2019-08-20 15:12 ` Richard Sandiford [this message]
2019-08-20 19:42 ` Segher Boessenkool
2019-08-21 17:20 ` Tejas Joshi
2019-08-21 18:28 ` Segher Boessenkool
2019-08-21 19:17 ` Segher Boessenkool
2019-08-22 3:33 ` Tejas Joshi
2019-08-22 6:25 ` Segher Boessenkool
2019-08-22 7:57 ` Tejas Joshi
2019-08-22 9:56 ` Segher Boessenkool
2019-08-23 17:17 ` Martin Jambor
2019-08-23 19:13 ` Segher Boessenkool
2019-08-24 9:53 ` Richard Sandiford
2019-08-25 13:55 ` Tejas Joshi
2019-08-25 16:47 ` Segher Boessenkool
2019-08-26 7:07 ` Tejas Joshi
2019-08-26 7:42 ` Segher Boessenkool
2019-08-30 19:12 ` Tejas Joshi
2019-08-30 20:35 ` Segher Boessenkool
2019-09-02 3:19 ` Tejas Joshi
2019-09-02 11:30 ` Segher Boessenkool
2019-08-26 13:23 ` Martin Jambor
2019-08-20 16:04 ` Joseph Myers
2019-08-15 18:54 ` Segher Boessenkool
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