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Date: Fri, 12 Jan 2024 17:03:29 +0300
From: Dan Carpenter <dan.carpenter@linaro.org>
To: David Laight <David.Laight@aculab.com>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	'Andy Shevchenko' <andriy.shevchenko@linux.intel.com>,
	'Andrew Morton' <akpm@linux-foundation.org>,
	"'Matthew Wilcox (Oracle)'" <willy@infradead.org>,
	'Christoph Hellwig' <hch@infradead.org>,
	"'Jason A. Donenfeld'" <Jason@zx2c4.com>
Subject: Re: [PATCH next v4 1/5] minmax: Add umin(a, b) and umax(a, b)
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On Fri, Jan 12, 2024 at 01:40:30PM +0000, David Laight wrote:
> From: Dan Carpenter
> > Sent: 12 January 2024 12:50
> > 
> > On Mon, Sep 18, 2023 at 08:16:30AM +0000, David Laight wrote:
> > > +/**
> > > + * umin - return minimum of two non-negative values
> > > + *   Signed types are zero extended to match a larger unsigned type.
> > > + * @x: first value
> > > + * @y: second value
> > > + */
> > > +#define umin(x, y)	\
> > > +	__careful_cmp((x) + 0u + 0ul + 0ull, (y) + 0u + 0ul + 0ull, <)
> > 
> > Why do we match "a larger unsigned type" instead of ULL_MAX?  Presumably
> > it helps performance somehow...  I agree that it's probably fine but I
> > would be more comfortable if it skipped UINT_MAX and jumped directly to
> > ULONG_MAX.  These days 4 gigs is small potatoes.  The vmalloc() function
> > can allocate 4G so we've had integer overflow bugs with this before.
> 
> The '+ 0ul*' carefully zero extend signed values without changing
> unsigned values.
> The compiler detects when it has zero-extended both sides and
> uses the smaller compare.
> In essence:
> 	x + 0u converts 'int' to 'unsigned int'.
> 		Avoids the sign extension adding 0ul on 64bit.
> 	x + 0ul converts a 'long' to 'unsigned long'.
> 		Avoids the sign extension adding 0ull on 32bit
> 	x + 0ull converts a 'long long' to 'unsigned long long'.
> You need all three to avoid sign extensions and get an unsigned
> compare.

So unsigned int compares are faster than unsigned long compares?

It's just sort of weird how it works.

	min_t(unsigned long, -1, 10000000000)); => 10000000000
	umin(umin(-1, 10000000000)); => UINT_MAX

UINT_MAX is just kind of a random value.  I would have prefered
ULONG_MAX, it's equally random but it's more safe because nothing can
allocate ULONG_MAX bytes.

regards,
dan carpenter


> If the type is __int128 (signed or unsigned) then nothing happens.
> (which means you can still get a signed v unsigned error.)
> You could add in (__uint128)0 on 64bit systems that support it,
> but it is so uncommon it really isn't worth the hassle.
> 
> Unlike any kind of cast the arithmetic cannot discard high bits.
> I've found a few min_t() with dubious types.
> One was a real bug found by someone else at much the same time.
> 
> 	David
> 
> -
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