linux-kernel - Re: [PATCH] x86/boot: drop memset from copy.S

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Date:   Thu, 10 Jan 2019 16:28:38 +0800
From:   Cao jin <caoj.fnst@...fujitsu.com>
To:     <hpa@...or.com>, <x86@...nel.org>, <linux-kernel@...r.kernel.org>
CC:     <tglx@...utronix.de>, <mingo@...hat.com>, <bp@...en8.de>
Subject: Re: [PATCH] x86/boot: drop memset from copy.S

Hello HPA,

On 1/8/19 4:38 PM, hpa@...or.com wrote:
> On January 7, 2019 12:52:57 AM PST, Cao jin <caoj.fnst@...fujitsu.com> wrote:
>> Hi,
>>
>> On 1/7/19 3:59 PM, hpa@...or.com wrote:
>>> On January 6, 2019 11:40:56 PM PST, Cao jin
>> <caoj.fnst@...fujitsu.com> wrote:
>>>> According to objdump output of setup, function memset is not used in
>>>> setup code. Currently, all usage of memset in setup come from macro
>>>> definition of string.h.
>>>>
>>>> Signed-off-by: Cao jin <caoj.fnst@...fujitsu.com>
>>>> ---
>>>> Compiled and booted under x86_64; compiled under i386.
>>>>
>>>> Questions: now there is 2 definition of memcpy, one lies in copy.S,
>>>> another lies in string.h which is mapped to gcc builtin function. Do
>> we
>>>> still need 2 definition? Could we move the content of copy.S to
>>>> boot/string.c?
>>>>
>>>> At first glance, the usage of string.{c.h} of setup is kind of
>>>> confusing,
>>>> they are also used in compressed/ and purgatory/
>>>>
>>>> arch/x86/boot/copy.S | 15 ---------------
>>>> 1 file changed, 15 deletions(-)
>>>>
>>>> diff --git a/arch/x86/boot/copy.S b/arch/x86/boot/copy.S
>>>> index 15d9f74b0008..5157d08b0ff2 100644
>>>> --- a/arch/x86/boot/copy.S
>>>> +++ b/arch/x86/boot/copy.S
>>>> @@ -33,21 +33,6 @@ GLOBAL(memcpy)
>>>> 	retl
>>>> ENDPROC(memcpy)
>>>>
>>>> -GLOBAL(memset)
>>>> -	pushw	%di
>>>> -	movw	%ax, %di
>>>> -	movzbl	%dl, %eax
>>>> -	imull	$0x01010101,%eax
>>>> -	pushw	%cx
>>>> -	shrw	$2, %cx
>>>> -	rep; stosl
>>>> -	popw	%cx
>>>> -	andw	$3, %cx
>>>> -	rep; stosb
>>>> -	popw	%di
>>>> -	retl
>>>> -ENDPROC(memset)
>>>> -
>>>> GLOBAL(copy_from_fs)
>>>> 	pushw	%ds
>>>> 	pushw	%fs
>>>
>>> This is dependent on both gcc version and flags.
>>>
>>
>> Thanks for your info, but I still don't quite get your point. All files
>> who has memset reference in setup will also #include "string.h", so how
>> gcc version and flags will associate memset reference to the assembly
>> function by force?  Hope to get a little more details when you are
>> convenient:)
> 
> GCC will sometimes emit calls to certain functions including memcpy().
> 

Thanks very much. After spending some time on GCC document, I think you
are talking about a condition that, for example, __builtin_memcpy() is
not optimized as inline code, but a function call to memcpy() in copy.S.

But I failed to find out the details how would gcc version & flags make
it this way, even I checked out the .cmd file of these .c. Or is this
born to be obscure for programmers?

-- 
Sincerely,
Cao jin