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Date: Sat, 10 Oct 2020 20:25:14 -0400
From: Rich Felker <dalias@...c.org>
To: Denys Vlasenko <vda.linux@...glemail.com>
Cc: Denys Vlasenko <dvlasenk@...hat.com>, musl@...ts.openwall.com
Subject: Re: [PATCH] x86/memset: avoid performing final store twice
On Sun, Oct 04, 2020 at 12:32:09AM +0200, Denys Vlasenko wrote:
> From: Denys Vlasenko <dvlasenk@...hat.com>
>
> For not very short NBYTES case:
>
> To handle the tail alignment, the code performs a potentially
> misaligned word store to fill the final 8 bytes of the buffer.
> This is done even if the buffer's end is aligned.
>
> Eventually code fills the rest of the buffer, which is a multiple
> of 8 bytes now, with NBYTES / 8 aligned word stores.
>
> However, this means that if NBYTES *was* divisible by 8,
> we store last word too, again.
>
> This patch decrements byte count before dividing it by 8,
> making one less store in "NBYTES is divisible by 8" case,
> and not changing anything in all other cases.
>
> CC: Rich Felker <dalias@...c.org>
> CC: musl@...ts.openwall.com
> Signed-off-by: Denys Vlasenko <dvlasenk@...hat.com>
> ---
> src/string/i386/memset.s | 7 ++++---
> src/string/x86_64/memset.s | 2 +-
> 2 files changed, 5 insertions(+), 4 deletions(-)
>
> diff --git a/src/string/i386/memset.s b/src/string/i386/memset.s
> index d00422c4..b1c5c2f8 100644
> --- a/src/string/i386/memset.s
> +++ b/src/string/i386/memset.s
> @@ -47,7 +47,7 @@ memset:
> mov %edx,(-1-2-4-8-8)(%eax,%ecx)
> mov %edx,(-1-2-4-8-4)(%eax,%ecx)
>
> -1: ret
> +1: ret
>
> 2: movzbl 8(%esp),%eax
> mov %edi,12(%esp)
> @@ -57,13 +57,14 @@ memset:
> mov %eax,-4(%edi,%ecx)
> jnz 2f
>
> -1: shr $2, %ecx
> +1: dec %ecx
> + shr $2, %ecx
> rep
> stosl
> mov 4(%esp),%eax
> mov 12(%esp),%edi
> ret
> -
> +
> 2: xor %edx,%edx
> sub %edi,%edx
> and $15,%edx
> diff --git a/src/string/x86_64/memset.s b/src/string/x86_64/memset.s
> index 2d3f5e52..85bb686c 100644
> --- a/src/string/x86_64/memset.s
> +++ b/src/string/x86_64/memset.s
> @@ -53,7 +53,7 @@ memset:
> 2: test $15,%edi
> mov %rdi,%r8
> mov %rax,-8(%rdi,%rdx)
> - mov %rdx,%rcx
> + lea -1(%rdx),%rcx
> jnz 2f
>
> 1: shr $3,%rcx
> --
> 2.25.0
Does this have measurably better performance on a system you've tested
it on? Indeed it nominally stores less, but it also performs an extra
arithmetic operation. I doubt it makes anything worse but I'd like to
avoid making random changes to such functions unless there's a good
reason to believe it actually helps.
Rich
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