From: Segher Boessenkool <segher@kernel.crashing.org>
To: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: nathanl@linux.ibm.com, linux-arch@vger.kernel.org,
vincenzo.frascino@arm.com, arnd@arndb.de,
linux-kernel@vger.kernel.org, Paul Mackerras <paulus@samba.org>,
luto@kernel.org, tglx@linutronix.de,
linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH v10 2/5] powerpc/vdso: Prepare for switching VDSO to generic C implementation.
Date: Wed, 5 Aug 2020 13:40:54 -0500 [thread overview]
Message-ID: <20200805184054.GQ6753@gate.crashing.org> (raw)
In-Reply-To: <3db2a590-b842-83db-ed2b-f3ee62595f18@csgroup.eu>
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?)
> >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.
Segher
WARNING: multiple messages have this Message-ID (diff)
From: Segher Boessenkool <segher@kernel.crashing.org>
To: Christophe Leroy <christophe.leroy@csgroup.eu>
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: Wed, 5 Aug 2020 13:40:54 -0500 [thread overview]
Message-ID: <20200805184054.GQ6753@gate.crashing.org> (raw)
Message-ID: <20200805184054.xOnIXdPE6ot-_QFyA13htJ6b4oUiNYXUAi4YTAjG07w@z> (raw)
In-Reply-To: <3db2a590-b842-83db-ed2b-f3ee62595f18@csgroup.eu>
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?)
> >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.
Segher
next prev parent reply other threads:[~2020-08-05 18:40 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 [this message]
2020-08-05 18:40 ` Segher Boessenkool
2020-08-06 5:46 ` Christophe Leroy
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=20200805184054.GQ6753@gate.crashing.org \
--to=segher@kernel.crashing.org \
--cc=arnd@arndb.de \
--cc=christophe.leroy@csgroup.eu \
--cc=linux-arch@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linuxppc-dev@lists.ozlabs.org \
--cc=luto@kernel.org \
--cc=nathanl@linux.ibm.com \
--cc=paulus@samba.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).