linux-kernel - Re: [PATCH] Little memset

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Message-ID: <CA+XhMqz9igfiZqRgwvfQ0c=FVyWf=ZUD0xjypEgzO27qwmBNeA@mail.gmail.com>
Date:   Mon, 26 Nov 2018 19:36:19 +0000
From:   David CARLIER <devnexen@...il.com>
To:     joeypabalinas@...il.com, linux-kernel@...r.kernel.org,
        alexander.shishkin@...ux.intel.com, gregkh@...uxfoundation.org,
        andriy.shevchenko@...ux.intel.com, aryabinin@...tuozzo.com
Subject: Re: [PATCH] Little memset_explicit optimisation

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@...il.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@...il.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