From: Nicolas Pitre <nico@fluxnic.net>
To: Wilco Dijkstra <Wilco.Dijkstra@arm.com>
Cc: "gcc-patches@gcc.gnu.org" <gcc-patches@gcc.gnu.org>,
Umesh Kalappa <umesh.kalappa0@gmail.com>,
Richard Earnshaw <Richard.Earnshaw@arm.com>, nd <nd@arm.com>,
Szabolcs Nagy <Szabolcs.Nagy@arm.com>,
Ramana Radhakrishnan <Ramana.Radhakrishnan@arm.com>
Subject: Re: [Patch-86512]: Subnormal float support in armv7(with -msoft-float) for intrinsics
Date: Fri, 27 Jul 2018 15:50:00 -0000 [thread overview]
Message-ID: <nycvar.YSQ.7.76.1807271132190.13885@knanqh.ubzr> (raw)
In-Reply-To: <DB5PR08MB1030498B09A69B37DC8D43EC832A0@DB5PR08MB1030.eurprd08.prod.outlook.com>
On Fri, 27 Jul 2018, Wilco Dijkstra wrote:
> Hi Nicolas,
>
> I think your patch doesn't quite work as expected:
>
> @@ -238,9 +238,10 @@ LSYM(Lad_a):
> movs ip, ip, lsl #1
> adcs xl, xl, xl
> adc xh, xh, xh
> - tst xh, #0x00100000
> - sub r4, r4, #1
> - bne LSYM(Lad_e)
> + subs r4, r4, #1
> + do_it hs
> + tsths xh, #0x00100000
> + bhi LSYM(Lad_e)
>
> If the exponent in r4 is zero, the carry bit will be clear, so we don't execute the tsths
> and fallthrough (the denormal will be normalized and then denormalized again, but
> that's so rare it doesn't matter really).
And that's what is intended.
> However if r4 is non-zero, the carry will be set, and the tsths will be executed. This
> clears the carry and sets the Z flag based on bit 20.
No, not at all. The carry is not affected. And that's the point of the
tst instruction here rather than a cmp: it sets the N and Z flags but
leaves C alone as there is no shifter involved.
Nicolas
next prev parent reply other threads:[~2018-07-27 15:50 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-07-20 13:38 Wilco Dijkstra
2018-07-20 16:00 ` Umesh Kalappa
2018-07-20 16:34 ` Wilco Dijkstra
2018-07-23 7:46 ` Umesh Kalappa
2018-07-23 9:58 ` Richard Earnshaw (lists)
2018-07-23 11:09 ` Umesh Kalappa
2018-07-23 11:42 ` Ramana Radhakrishnan
2018-07-23 11:54 ` Wilco Dijkstra
2018-07-24 8:39 ` Umesh Kalappa
2018-07-25 17:04 ` Umesh Kalappa
2018-07-26 16:36 ` Nicolas Pitre
2018-07-27 13:32 ` Wilco Dijkstra
2018-07-27 15:50 ` Nicolas Pitre [this message]
2018-07-27 16:01 ` Nicolas Pitre
2018-07-27 16:05 ` Wilco Dijkstra
2018-07-27 16:45 ` Nicolas Pitre
2018-08-02 16:50 ` Richard Earnshaw (lists)
-- strict thread matches above, loose matches on Subject: below --
2018-07-18 12:31 Umesh Kalappa
2018-07-20 12:05 ` Umesh Kalappa
2018-07-20 13:30 ` Szabolcs Nagy
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