linux-kernel - RE: [PATCH next resend 1/5] minmax: Add min_unsigned(a, b) and max

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Date:   Tue, 25 Jul 2023 13:20:01 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Matthew Wilcox' <willy@...radead.org>
CC:     "'linux-kernel@...r.kernel.org'" <linux-kernel@...r.kernel.org>,
        "'Andy Shevchenko'" <andriy.shevchenko@...ux.intel.com>,
        'Andrew Morton' <akpm@...ux-foundation.org>,
        'Christoph Hellwig' <hch@...radead.org>,
        "'Jason A. Donenfeld'" <Jason@...c4.com>
Subject: RE: [PATCH next resend 1/5] minmax: Add min_unsigned(a, b) and
 max_unsigned(a, b)

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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