public inbox for libc-alpha@sourceware.org
 help / color / mirror / Atom feed
From: "Tulio Magno Quites Machado Filho" <tuliom@linux.vnet.ibm.com>
To: Rajalakshmi Srinivasaraghavan <raji@linux.vnet.ibm.com>,
	libc-alpha@sourceware.org
Subject: Re: [PATCH] powerpc: Use aligned stores in memset
Date: Mon, 21 Aug 2017 02:20:00 -0000	[thread overview]
Message-ID: <87y3qdslu3.fsf@linux.vnet.ibm.com> (raw)
In-Reply-To: <1503033107-20047-1-git-send-email-raji@linux.vnet.ibm.com>

Rajalakshmi Srinivasaraghavan <raji@linux.vnet.ibm.com> writes:

> The powerpc hardware does not allow unaligned accesses on non cacheable
> memory.  This patch avoids misaligned stores for sizes less than 8 in
> memset to avoid such cases.  Tested on powerpc64 and powerpc64le.

This commit message is misleading.  I think it's necessary to improve with:

 1. Remove the first line.
 2. Mention the performance impact and what causes it.
 3. Reference the section "3.1.4.2 Alignment Interrupts" of the "POWER8
    Processor User's Manual for the Single-Chip Module", which describes
    this behavior.
 4. Mention which kind of programs are affected by the old behavior.

> 2017-08-17  Rajalakshmi Srinivasaraghavan  <raji@linux.vnet.ibm.com>
>
> 	* sysdeps/powerpc/powerpc64/power8/memset.S: Store byte by byte
> 	for unaligned inputs if size is less than 8.
> ---
>  sysdeps/powerpc/powerpc64/power8/memset.S | 68 ++++++++++++++++++++++++++++++-
>  1 file changed, 66 insertions(+), 2 deletions(-)
>
> diff --git a/sysdeps/powerpc/powerpc64/power8/memset.S b/sysdeps/powerpc/powerpc64/power8/memset.S
> index 7ad3bb1b00..504bab0841 100644
> --- a/sysdeps/powerpc/powerpc64/power8/memset.S
> +++ b/sysdeps/powerpc/powerpc64/power8/memset.S
> @@ -377,7 +377,8 @@ L(write_LT_32):
>  	subf	r5,r0,r5
>
>  2:	bf	30,1f
> -	sth	r4,0(r10)
> +	stb	r4,0(r10)
> +	stb	r4,1(r10)

Needs a comment to prevent future mistakes in the future.

> @@ -437,11 +438,74 @@ L(tail5):
>  	/* Handles copies of 0~8 bytes.  */
>  	.align	4
>  L(write_LE_8):
> -	bne	cr6,L(tail4)
> +	/* Use stb instead of sth which is safe for
> +	   both aligned and unaligned inputs.  */

I don't think "safe" is the correct term.  What about this?

    Use stb instead of sth because it doesn't generate alignment interrupts
    on cache-inhibited storage.

> +	bne	cr6,L(LE7_tail4)
> +	/* If input is word aligned, use stw, Else use stb.  */

s/Else/else/


-- 
Tulio Magno

      parent reply	other threads:[~2017-08-21  2:20 UTC|newest]

Thread overview: 31+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-08-18  5:13 Rajalakshmi Srinivasaraghavan
2017-08-18  6:21 ` Florian Weimer
2017-08-18  6:51   ` Rajalakshmi Srinivasaraghavan
2017-08-18  9:10     ` Florian Weimer
2017-08-18 12:13       ` Adhemerval Zanella
2017-09-12 10:30       ` Florian Weimer
2017-09-12 12:18         ` Zack Weinberg
2017-09-12 13:57           ` Steven Munroe
2017-09-12 14:37           ` Joseph Myers
2017-09-12 15:06             ` Zack Weinberg
2017-09-12 17:09           ` Florian Weimer
2017-09-12 13:38         ` Steven Munroe
2017-09-12 14:08           ` Florian Weimer
2017-09-12 14:16             ` Steven Munroe
2017-09-12 17:04               ` Florian Weimer
2017-09-12 19:21                 ` Steven Munroe
2017-09-12 19:45                   ` Florian Weimer
2017-09-12 20:25                     ` Steven Munroe
2017-09-13 13:12         ` Tulio Magno Quites Machado Filho
2017-09-18 13:54           ` Florian Weimer
2017-10-03 18:29             ` Adhemerval Zanella
2017-10-05 12:13               ` Rajalakshmi Srinivasaraghavan
2017-11-08 18:52               ` Tulio Magno Quites Machado Filho
2017-12-08 19:52                 ` [PATCHv2] powerpc: POWER8 memcpy optimization for cached memory Tulio Magno Quites Machado Filho
2017-12-08 20:06                   ` Florian Weimer
2017-12-11 12:44                     ` Tulio Magno Quites Machado Filho
2017-12-11 20:09                       ` Adhemerval Zanella
2017-12-10  7:11                   ` Rajalakshmi Srinivasaraghavan
2017-12-11 19:48                     ` Tulio Magno Quites Machado Filho
2017-08-18  6:25 ` [PATCH] powerpc: Use aligned stores in memset Andrew Pinski
2017-08-21  2:20 ` Tulio Magno Quites Machado Filho [this message]

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=87y3qdslu3.fsf@linux.vnet.ibm.com \
    --to=tuliom@linux.vnet.ibm.com \
    --cc=libc-alpha@sourceware.org \
    --cc=raji@linux.vnet.ibm.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 read-only IMAP folder(s) and NNTP newsgroup(s).