linux-arch.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: Christophe Leroy <christophe.leroy@csgroup.eu>
To: Segher Boessenkool <segher@kernel.crashing.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Paul Mackerras <paulus@samba.org>,
	Michael Ellerman <mpe@ellerman.id.au>,
	nathanl@linux.ibm.com, anton@ozlabs.org,
	linux-arch@vger.kernel.org, arnd@arndb.de,
	linux-kernel@vger.kernel.org, luto@kernel.org,
	tglx@linutronix.de, vincenzo.frascino@arm.com,
	linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH v10 2/5] powerpc/vdso: Prepare for switching VDSO to generic C implementation.
Date: Thu, 6 Aug 2020 05:46:18 +0000	[thread overview]
Message-ID: <24b9a5ea-f860-3b38-aee8-4d5676453d50@csgroup.eu> (raw)
In-Reply-To: <20200805184054.GQ6753@gate.crashing.org>

Hi,

On 08/05/2020 06:40 PM, Segher Boessenkool wrote:
> Hi!
> 
> On Wed, Aug 05, 2020 at 04:40:16PM +0000, Christophe Leroy wrote:
>>> It cannot optimise it because it does not know shift < 32.  The code
>>> below is incorrect for shift equal to 32, fwiw.
>>
>> Is there a way to tell it ?
> 
> Sure, for example the &31 should work (but it doesn't, with the GCC
> version you used -- which version is that?)

GCC 10.1

> 
>>> What does the compiler do for just
>>>
>>> static __always_inline u64 vdso_shift_ns(u64 ns, unsigned long shift)
>>> 	return ns >> (shift & 31);
>>> }
>>>
>>
>> Worse:
> 
> I cannot make heads or tails of all that branch spaghetti, sorry.
> 
>>   73c:	55 8c 06 fe 	clrlwi  r12,r12,27
>>   740:	7f c8 f0 14 	addc    r30,r8,r30
>>   744:	7c c6 4a 14 	add     r6,r6,r9
>>   748:	7c c6 e1 14 	adde    r6,r6,r28
>>   74c:	34 6c ff e0 	addic.  r3,r12,-32
>>   750:	41 80 00 70 	blt     7c0 <__c_kernel_clock_gettime+0x114>
> 
> This branch is always true.  Hrm.

As a standalone function:

With your suggestion:

000006ac <vdso_shift_ns>:
  6ac:	54 a5 06 fe 	clrlwi  r5,r5,27
  6b0:	35 25 ff e0 	addic.  r9,r5,-32
  6b4:	41 80 00 10 	blt     6c4 <vdso_shift_ns+0x18>
  6b8:	7c 64 4c 30 	srw     r4,r3,r9
  6bc:	38 60 00 00 	li      r3,0
  6c0:	4e 80 00 20 	blr
  6c4:	54 69 08 3c 	rlwinm  r9,r3,1,0,30
  6c8:	21 45 00 1f 	subfic  r10,r5,31
  6cc:	7c 84 2c 30 	srw     r4,r4,r5
  6d0:	7d 29 50 30 	slw     r9,r9,r10
  6d4:	7c 63 2c 30 	srw     r3,r3,r5
  6d8:	7d 24 23 78 	or      r4,r9,r4
  6dc:	4e 80 00 20 	blr


With the version as is in my series:

000006ac <vdso_shift_ns>:
  6ac:	21 25 00 20 	subfic  r9,r5,32
  6b0:	7c 69 48 30 	slw     r9,r3,r9
  6b4:	7c 84 2c 30 	srw     r4,r4,r5
  6b8:	7d 24 23 78 	or      r4,r9,r4
  6bc:	7c 63 2c 30 	srw     r3,r3,r5
  6c0:	4e 80 00 20 	blr


Christophe

  parent reply	other threads:[~2020-08-06  5:46 UTC|newest]

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-08-05  7:09 [PATCH v10 0/5] powerpc: switch VDSO to C implementation Christophe Leroy
2020-08-05  7:09 ` [PATCH v10 1/5] powerpc/processor: Move cpu_relax() into asm/vdso/processor.h Christophe Leroy
2020-08-05  7:09 ` [PATCH v10 2/5] powerpc/vdso: Prepare for switching VDSO to generic C implementation Christophe Leroy
2020-08-05 14:03   ` Segher Boessenkool
2020-08-05 14:03     ` Segher Boessenkool
2020-08-05 16:40     ` Christophe Leroy
2020-08-05 16:40       ` Christophe Leroy
2020-08-05 18:40       ` Segher Boessenkool
2020-08-05 18:40         ` Segher Boessenkool
2020-08-06  5:46         ` Christophe Leroy [this message]
2020-08-05 16:51     ` Christophe Leroy
2020-08-05 16:51       ` Christophe Leroy
2020-08-05 20:55       ` Segher Boessenkool
2020-08-05  7:09 ` [PATCH v10 3/5] powerpc/vdso: Save and restore TOC pointer on PPC64 Christophe Leroy
2020-08-05  7:09 ` [PATCH v10 4/5] powerpc/vdso: Switch VDSO to generic C implementation Christophe Leroy
2020-08-05  7:09   ` Christophe Leroy
2020-08-05  7:09 ` [PATCH v10 5/5] powerpc/vdso: Provide __kernel_clock_gettime64() on vdso32 Christophe Leroy
2020-08-05  7:09   ` Christophe Leroy

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=24b9a5ea-f860-3b38-aee8-4d5676453d50@csgroup.eu \
    --to=christophe.leroy@csgroup.eu \
    --cc=anton@ozlabs.org \
    --cc=arnd@arndb.de \
    --cc=benh@kernel.crashing.org \
    --cc=linux-arch@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linuxppc-dev@lists.ozlabs.org \
    --cc=luto@kernel.org \
    --cc=mpe@ellerman.id.au \
    --cc=nathanl@linux.ibm.com \
    --cc=paulus@samba.org \
    --cc=segher@kernel.crashing.org \
    --cc=tglx@linutronix.de \
    --cc=vincenzo.frascino@arm.com \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body. 
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).