From: David Laight <David.Laight@ACULAB.COM>
To: 'Matthew Wilcox' <willy@infradead.org>
Cc: "'linux-kernel@vger.kernel.org'" <linux-kernel@vger.kernel.org>,
"'Andy Shevchenko'" <andriy.shevchenko@linux.intel.com>,
'Andrew Morton' <akpm@linux-foundation.org>,
'Christoph Hellwig' <hch@infradead.org>,
"'Jason A. Donenfeld'" <Jason@zx2c4.com>
Subject: RE: [PATCH next resend 1/5] minmax: Add min_unsigned(a, b) and max_unsigned(a, b)
Date: Tue, 25 Jul 2023 13:20:01 +0000 [thread overview]
Message-ID: <66e58cd2cffa462a979ac5415874a570@AcuMS.aculab.com> (raw)
In-Reply-To: <ZL/CUuIhi6qmTK5+@casper.infradead.org>
From: Matthew Wilcox
> Sent: 25 July 2023 13:39
>
> On Tue, Jul 25, 2023 at 11:48:14AM +0000, David Laight wrote:
> > +#define min_unsigned(x, y) \
> > + __careful_cmp((x) + 0u + 0ul + 0ull, (y) + 0u + 0ul + 0ull, <)
>
> What is the point of "+ 0u + 0ul + 0ull"? How is that any different
> from "+ 0ull"? And why force the compiler to do a 64-bit comparison
> when it could do a 32-bit comparison?
The "+ 0u + 0ul + 0ull" causes a signed 32bit value to be zero extended
to 64bit. This is significantly cheaper than the sign extension.
(Adding 0ull first converts a signed 32bit value to a signed
64bit one - the same as a cast.)
The compiler also then knows that the high 32bit are zero and
optimises away any associated compares.
So you get a 32bit compare (on both 32bit and 64bit) if both
arguments are 32bit.
This happens even at -O0.
It also has no effect on pointer types.
David
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Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)
next prev parent reply other threads:[~2023-07-25 13:20 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-07-25 10:00 [PATCH next 0/5] minmax: Relax type checks in min() and max() David Laight
2023-07-25 10:38 ` 'Andy Shevchenko'
2023-07-25 11:17 ` David Laight
2023-07-25 11:48 ` [PATCH next resend 1/5] minmax: Add min_unsigned(a, b) and max_unsigned(a, b) David Laight
2023-07-25 12:38 ` Matthew Wilcox
2023-07-25 13:20 ` David Laight [this message]
2023-07-25 11:51 ` [PATCH next resend 2/5] minmax: Allow min()/max()/clamp() if the arguments have the same signedness David Laight
2023-07-25 18:02 ` kernel test robot
2023-07-25 18:33 ` kernel test robot
2023-07-26 9:19 ` David Laight
[not found] ` <86ila7t30q.wl-maz@kernel.org>
2023-07-26 10:25 ` David Laight
2023-07-25 11:52 ` [PATCH next resend 3/5] minmax: Fix indentation of __cmp_once() and __clamp_once() David Laight
2023-07-25 11:53 ` [PATCH next resend 4/5] minmax: Allow comparisons of 'int' against 'unsigned char/short' David Laight
2023-07-25 11:54 ` [PATCH next resend 5/5] minmax: Relax check to allow comparison between int and small unsigned constants David Laight
2023-07-25 19:36 ` kernel test robot
2023-07-26 9:29 ` David Laight
2023-07-27 5:20 ` kernel test robot
2023-07-28 7:15 ` kernel test robot
2023-07-28 8:08 ` kernel test robot
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