linux-kernel - Re: [PATCH next 1/1] lib: Optimise hex_dump_to

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Message-ID: <Z7RxGlCkQG9M4AeM@smile.fi.intel.com>
Date: Tue, 18 Feb 2025 13:38:02 +0200
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: David Laight <david.laight.linux@...il.com>
Cc: linux-kernel@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Randy Dunlap <rdunlap@...radead.org>
Subject: Re: [PATCH next 1/1] lib: Optimise hex_dump_to_buffer()

On Sun, Feb 16, 2025 at 10:37:15PM +0000, David Laight wrote:
> On Sun, 16 Feb 2025 22:47:49 +0200
> Andy Shevchenko <andriy.shevchenko@...ux.intel.com> wrote:
> > On Sun, Feb 16, 2025 at 08:19:01PM +0000, David Laight wrote:

...

> > > +extern size_t hex_dump_to_buffer(const void *buf, size_t len, size_t rowsize,
> > > +			      size_t groupsize, char *linebuf,
> > > +			      size_t linebuflen, bool ascii);  
> > 
> > Looking at another thread where upper layer function wants to have unsigned
> > long flags instead of bool ascii, I would also do the new API, that takes flags
> > and leave the old one as a simple wrapper with all restrictions being applied.
> 
> I can't imagine any code relying on the rowsize being converted to 16.
> And (elsewhere) I've definitely needed to do hexdumps with strange numbers
> of bytes/line.

I didn't get how this is related to my comment.

...

> > > +			dst[0] = hex_asc_hi(ch);
> > > +			dst[1] = hex_asc_lo(ch);  
> > 
> > We have hex_pack_byte() or so
> 
> At least some versions of gcc have generated better code if you don't
> use *ptr++ but do the increment afterwards.
> It is also what the old version used.

Do we really care? What versions, btw, are you talk about?

> Not to mention being another wrapper you need to look up to work out
> what the code is doing.

That's not an issue. We use a lot of wrappers in the Linux kernel.
Without a skill to quickly find this information it would be very
hard to develop the kernel code without reinventing a wheel.

...

> > > -		linebuf[lx++] = (isascii(ch) && isprint(ch)) ? ch : '.';
> > > +		*dst++ = ch >= ' ' && ch < 0x7f ? ch : '.';  
> > 
> > Please also add a test case for this to make sure it has no changes.
> 
> Well isascii() usually checks for the 0x80 bit being clear and isprint()
> rejects control characters and 'del' (0x7f).
> I'm not sure what isascii() does for EBCDIC type charsets - but I don't
> expect Linux runs on any of those so who cares.

Still, it's good to have a test cases for this change.

> Oh, and isprint() seems to be based on a memory lookup in an _ctype[] array.
> Very 1970s.

With enough CPU (data) caches it's quite effective.

-- 
With Best Regards,
Andy Shevchenko