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From: Ammar Faizi <[email protected]>
To: Willy Tarreau <[email protected]>
Cc: "Thomas Weißschuh" <[email protected]>,
	"Nicholas Rosenberg" <[email protected]>,
	"Alviro Iskandar Setiawan" <[email protected]>,
	"Michael William Jonathan" <[email protected]>,
	"GNU/Weeb Mailing List" <[email protected]>,
	"Linux Kernel Mailing List" <[email protected]>
Subject: Re: [RFC PATCH v1 3/5] tools/nolibc: x86-64: Use `rep cmpsb` for `memcmp()`
Date: Fri, 1 Sep 2023 10:24:42 +0700	[thread overview]
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>

On Wed, Aug 30, 2023 at 11:26:57PM +0200, Willy Tarreau wrote:
> Out of curiosity, given that you implemented the 3 other ones directly
> in an asm statement, is there a particular reason this one mixes a bit
> of C and asm ?

Because this one maybe unused. The other are explicitly exported.

> It would probably be something around this, in the same vein:
> 
>   memcmp:
>     xchg  %esi,%eax   // source1
>     mov   %rdx,%rcx   // count
>     rep   cmpsb       // source2 in rdi; sets ZF on equal, CF if src1<src2
>     seta  %al         // 0 if src2 <= src1, 1 if src2 > src1
>     sbb   $0, %al     // 0 if src2 == src1, -1 if src2 < src1, 1 if src2 > src1
>     movsx %al, %eax   // sign extend to %eax
>     ret
> 
> Note that the output logic could have to be revisited, I'm not certain but
> at first glance it looks valid.

After thinking about this more, I think I'll drop the memcmp() patch
because it will prevent optimization when comparing a small value.

For example, without __asm__:

    memcmp(var, "abcd", 4);

may compile to:

    cmpl $0x64636261, %reg
    ...something...

But with __asm__, the compiler can't do that. Thus, it's not worth
optimizing the memcmp() in this case.

-- 
Ammar Faizi


  reply	other threads:[~2023-09-01  3:24 UTC|newest]

Thread overview: 29+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-08-30 13:57 [RFC PATCH v1 0/5] nolibc x86-64 string functions Ammar Faizi
2023-08-30 13:57 ` [RFC PATCH v1 1/5] tools/nolibc: x86-64: Use `rep movsb` for `memcpy()` and `memmove()` Ammar Faizi
2023-08-30 13:57 ` [RFC PATCH v1 2/5] tools/nolibc: x86-64: Use `rep stosb` for `memset()` Ammar Faizi
2023-08-30 14:08   ` Alviro Iskandar Setiawan
2023-08-30 14:13     ` Ammar Faizi
2023-08-30 14:24   ` Alviro Iskandar Setiawan
2023-08-30 15:09     ` Ammar Faizi
2023-08-30 15:23       ` Willy Tarreau
2023-08-30 15:44         ` Ammar Faizi
2023-08-30 15:51           ` Willy Tarreau
2023-08-30 16:08             ` Ammar Faizi
2023-08-30 16:11               ` Alviro Iskandar Setiawan
2023-08-30 13:57 ` [RFC PATCH v1 3/5] tools/nolibc: x86-64: Use `rep cmpsb` for `memcmp()` Ammar Faizi
2023-08-30 21:26   ` Willy Tarreau
2023-09-01  3:24     ` Ammar Faizi [this message]
2023-09-01  3:35       ` Willy Tarreau
2023-09-01  7:27         ` Ammar Faizi
2023-09-04  8:26     ` David Laight
2023-08-30 13:57 ` [RFC PATCH v1 4/5] tools/nolibc: string: Remove the `_nolibc_memcpy_down()` function Ammar Faizi
2023-08-30 21:27   ` Willy Tarreau
2023-08-30 13:57 ` [RFC PATCH v1 5/5] tools/nolibc: string: Remove the `_nolibc_memcpy_up()` function Ammar Faizi
2023-08-30 21:32 ` [RFC PATCH v1 0/5] nolibc x86-64 string functions Willy Tarreau
2023-09-01 11:34 ` David Laight
2023-09-01 11:46   ` Willy Tarreau
2023-09-01 13:06     ` Ammar Faizi
2023-09-01 14:23       ` David Laight
2023-09-01 14:41         ` Ammar Faizi
2023-09-01 14:54           ` David Laight
2023-09-01 15:20             ` Ammar Faizi

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