* [PATCH] Little memset_explicit optimisation
@ 2018-11-24 12:35 David CARLIER
2018-11-26 11:29 ` Andy Shevchenko
2018-11-26 11:37 ` Joey Pabalinas
0 siblings, 2 replies; 5+ messages in thread
From: David CARLIER @ 2018-11-24 12:35 UTC (permalink / raw)
To: linux-kernel; +Cc: alexander.shishkin, gregkh, andriy.shevchenko, aryabinin
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From c6bb758902b3b358de9a98bb4156c306685f59d7 Mon Sep 17 00:00:00 2001
From: David Carlier <devnexen@gmail.com>
Date: Sat, 17 Nov 2018 14:13:13 +0000
Subject: [PATCH] memzero_explicit, optimisation for size.
Using the return value of memset for save/load sake.
Signed-off-by: David Carlier <devnexen@gmail.com>
---
lib/string.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/lib/string.c b/lib/string.c
index 38e4ca08e757..92da04a0213b 100644
--- a/lib/string.c
+++ b/lib/string.c
@@ -720,7 +720,7 @@ EXPORT_SYMBOL(memset);
*/
void memzero_explicit(void *s, size_t count)
{
- memset(s, 0, count);
+ s = memset(s, 0, count);
barrier_data(s);
}
EXPORT_SYMBOL(memzero_explicit);
--
2.19.1
^ permalink raw reply related [flat|nested] 5+ messages in thread
* Re: [PATCH] Little memset_explicit optimisation
2018-11-24 12:35 [PATCH] Little memset_explicit optimisation David CARLIER
@ 2018-11-26 11:29 ` Andy Shevchenko
2018-11-26 11:37 ` Joey Pabalinas
1 sibling, 0 replies; 5+ messages in thread
From: Andy Shevchenko @ 2018-11-26 11:29 UTC (permalink / raw)
To: David CARLIER; +Cc: linux-kernel, alexander.shishkin, gregkh, aryabinin
On Sat, Nov 24, 2018 at 12:35:43PM +0000, David CARLIER wrote:
>
Hmm... Can we see the difference in assembly generation?
--
With Best Regards,
Andy Shevchenko
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH] Little memset_explicit optimisation
2018-11-24 12:35 [PATCH] Little memset_explicit optimisation David CARLIER
2018-11-26 11:29 ` Andy Shevchenko
@ 2018-11-26 11:37 ` Joey Pabalinas
2018-11-26 19:36 ` David CARLIER
1 sibling, 1 reply; 5+ messages in thread
From: Joey Pabalinas @ 2018-11-26 11:37 UTC (permalink / raw)
To: David CARLIER
Cc: linux-kernel, alexander.shishkin, gregkh, andriy.shevchenko,
aryabinin, Joey Pabalinas
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On Sat, Nov 24, 2018 at 12:35:43PM +0000, David CARLIER wrote:
> Using the return value of memset for save/load sake.
>
> Signed-off-by: David Carlier <devnexen@gmail.com>
> ---
> lib/string.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/lib/string.c b/lib/string.c
> index 38e4ca08e757..92da04a0213b 100644
> --- a/lib/string.c
> +++ b/lib/string.c
> @@ -720,7 +720,7 @@ EXPORT_SYMBOL(memset);
> */
> void memzero_explicit(void *s, size_t count)
> {
> - memset(s, 0, count);
> + s = memset(s, 0, count);
> barrier_data(s);
> }
> EXPORT_SYMBOL(memzero_explicit);
Could you elaborate on the optimization that this patch performs?
--
Cheers,
Joey Pabalinas
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^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH] Little memset_explicit optimisation
2018-11-26 11:37 ` Joey Pabalinas
@ 2018-11-26 19:36 ` David CARLIER
2018-11-26 21:18 ` Joey Pabalinas
0 siblings, 1 reply; 5+ messages in thread
From: David CARLIER @ 2018-11-26 19:36 UTC (permalink / raw)
To: joeypabalinas, linux-kernel, alexander.shishkin, gregkh,
andriy.shevchenko, aryabinin
Sorry I m not used yet at all to LKML rules.
So here a slight difference in assembly generated between the two
versions (amd64) :
`
.loc 1 7 7
leaq -12(%rbp), %rax
movq %rax, -8(%rbp)
- .loc 1 11 2
+ .loc 1 9 6
movq -8(%rbp), %rax
movl $4, %edx
movl $0, %esi
movq %rax, %rdi
call memset@PLT
+ movq %rax, -8(%rbp)
.loc 1 13 23
movq -8(%rbp), %rax
movl (%rax), %eax
`
On Mon, 26 Nov 2018 at 11:37, Joey Pabalinas <joeypabalinas@gmail.com> wrote:
>
> On Sat, Nov 24, 2018 at 12:35:43PM +0000, David CARLIER wrote:
> > Using the return value of memset for save/load sake.
> >
> > Signed-off-by: David Carlier <devnexen@gmail.com>
> > ---
> > lib/string.c | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/lib/string.c b/lib/string.c
> > index 38e4ca08e757..92da04a0213b 100644
> > --- a/lib/string.c
> > +++ b/lib/string.c
> > @@ -720,7 +720,7 @@ EXPORT_SYMBOL(memset);
> > */
> > void memzero_explicit(void *s, size_t count)
> > {
> > - memset(s, 0, count);
> > + s = memset(s, 0, count);
> > barrier_data(s);
> > }
> > EXPORT_SYMBOL(memzero_explicit);
>
> Could you elaborate on the optimization that this patch performs?
>
> --
> Cheers,
> Joey Pabalinas
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: Re: [PATCH] Little memset_explicit optimisation
2018-11-26 19:36 ` David CARLIER
@ 2018-11-26 21:18 ` Joey Pabalinas
0 siblings, 0 replies; 5+ messages in thread
From: Joey Pabalinas @ 2018-11-26 21:18 UTC (permalink / raw)
To: David CARLIER
Cc: joeypabalinas, linux-kernel, alexander.shishkin, gregkh,
andriy.shevchenko, aryabinin
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On Mon, Nov 26, 2018 at 07:36:19PM +0000, David CARLIER wrote:
> Sorry I m not used yet at all to LKML rules.
>
> So here a slight difference in assembly generated between the two
> versions (amd64) :
> `
> .loc 1 7 7
> leaq -12(%rbp), %rax
> movq %rax, -8(%rbp)
> - .loc 1 11 2
> + .loc 1 9 6
> movq -8(%rbp), %rax
> movl $4, %edx
> movl $0, %esi
> movq %rax, %rdi
> call memset@PLT
> + movq %rax, -8(%rbp)
> .loc 1 13 23
> movq -8(%rbp), %rax
> movl (%rax), %eax
What is the advantage of having the added `movq %rax, -8(%rbp)` here?
The next instruction is `movq -8(%rbp), %rax` and nothing afterwords
uses the value stored in `-8(%rbp)`.
Also, is this compiled without optimization? Take a looks at the
assembly in a small test case with -O1 (making sure to use the target
variable so it isn't optimized out) and compare the assembly generated
with and without that assignment.
--
Cheers,
Joey Pabalinas
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^ permalink raw reply [flat|nested] 5+ messages in thread
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2018-11-24 12:35 [PATCH] Little memset_explicit optimisation David CARLIER
2018-11-26 11:29 ` Andy Shevchenko
2018-11-26 11:37 ` Joey Pabalinas
2018-11-26 19:36 ` David CARLIER
2018-11-26 21:18 ` Joey Pabalinas
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