linux-kernel - Re: [PATCH -tip 3/3] x86/asm/32: Modernize

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Message-ID: <CAFULd4Z6Np=_zUUFH+Jys1VxyCAp6omgnMH9OL7iCKM1Si4q0A@mail.gmail.com>
Date: Tue, 6 May 2025 19:34:34 +0200
From: Uros Bizjak <ubizjak@...il.com>
To: x86@...nel.org, linux-kernel@...r.kernel.org
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH -tip 3/3] x86/asm/32: Modernize _memcpy()

On Tue, May 6, 2025 at 6:52 PM Uros Bizjak <ubizjak@...il.com> wrote:
>
> Use inout "+" constraint modifier where appropriate, declare
> temporary variables as unsigned long and rewrite parts of assembly
> in plain C. The memcpy() function shrinks by 10 bytes, from:
>
> 00e778d0 <memcpy>:
>   e778d0:       55                      push   %ebp
>   e778d1:       89 e5                   mov    %esp,%ebp
>   e778d3:       83 ec 0c                sub    $0xc,%esp
>   e778d6:       89 5d f4                mov    %ebx,-0xc(%ebp)
>   e778d9:       89 c3                   mov    %eax,%ebx
>   e778db:       89 c8                   mov    %ecx,%eax
>   e778dd:       89 75 f8                mov    %esi,-0x8(%ebp)
>   e778e0:       c1 e9 02                shr    $0x2,%ecx
>   e778e3:       89 d6                   mov    %edx,%esi
>   e778e5:       89 7d fc                mov    %edi,-0x4(%ebp)
>   e778e8:       89 df                   mov    %ebx,%edi
>   e778ea:       f3 a5                   rep movsl %ds:(%esi),%es:(%edi)
>   e778ec:       89 c1                   mov    %eax,%ecx
>   e778ee:       83 e1 03                and    $0x3,%ecx
>   e778f1:       74 02                   je     e778f5 <memcpy+0x25>
>   e778f3:       f3 a4                   rep movsb %ds:(%esi),%es:(%edi)
>   e778f5:       8b 75 f8                mov    -0x8(%ebp),%esi
>   e778f8:       89 d8                   mov    %ebx,%eax
>   e778fa:       8b 5d f4                mov    -0xc(%ebp),%ebx
>   e778fd:       8b 7d fc                mov    -0x4(%ebp),%edi
>   e77900:       89 ec                   mov    %ebp,%esp
>   e77902:       5d                      pop    %ebp
>   e77903:       c3                      ret
>
> to:
>
> 00e778b0 <memcpy>:
>   e778b0:       55                      push   %ebp
>   e778b1:       89 e5                   mov    %esp,%ebp
>   e778b3:       83 ec 08                sub    $0x8,%esp
>   e778b6:       89 75 f8                mov    %esi,-0x8(%ebp)
>   e778b9:       89 d6                   mov    %edx,%esi
>   e778bb:       89 ca                   mov    %ecx,%edx
>   e778bd:       89 7d fc                mov    %edi,-0x4(%ebp)
>   e778c0:       c1 e9 02                shr    $0x2,%ecx
>   e778c3:       89 c7                   mov    %eax,%edi
>   e778c5:       f3 a5                   rep movsl %ds:(%esi),%es:(%edi)
>   e778c7:       83 e2 03                and    $0x3,%edx
>   e778ca:       74 04                   je     e778d0 <memcpy+0x20>
>   e778cc:       89 d1                   mov    %edx,%ecx
>   e778ce:       f3 a4                   rep movsb %ds:(%esi),%es:(%edi)
>   e778d0:       8b 75 f8                mov    -0x8(%ebp),%esi
>   e778d3:       8b 7d fc                mov    -0x4(%ebp),%edi
>   e778d6:       89 ec                   mov    %ebp,%esp
>   e778d8:       5d                      pop    %ebp
>   e778d9:       c3                      ret
>
> due to a better register allocation, avoiding the call-saved

Oops, this should have been written as "... avoiding the callee-saved ..."

> %ebx register.

Uros.