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)
Date: Fri, 12 Jan 2024 17:03:29 +0300 [thread overview]
Message-ID: <02701430-65cf-44ab-8a8b-752c5d973d21@moroto.mountain> (raw)
In-Reply-To: <8e45b321c49b4c27a61b2db076ed5383@AcuMS.aculab.com>
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
>
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)
next prev parent reply other threads:[~2024-01-12 14:03 UTC|newest]
Thread overview: 35+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-09-18 8:14 [PATCH next v4 0/5] minmax: Relax type checks in min() and max() David Laight
2023-09-18 8:16 ` [PATCH next v4 1/5] minmax: Add umin(a, b) and umax(a, b) David Laight
2024-01-12 12:49 ` Dan Carpenter
2024-01-12 13:40 ` David Laight
2024-01-12 14:03 ` Dan Carpenter [this message]
2024-01-12 14:26 ` David Laight
2024-01-18 10:30 ` Dan Carpenter
2023-09-18 8:17 ` [PATCH next v4 2/5] minmax: Allow min()/max()/clamp() if the arguments have the same signedness David Laight
2023-09-18 8:17 ` [PATCH next v4 3/5] minmax: Fix indentation of __cmp_once() and __clamp_once() David Laight
2023-09-18 8:18 ` [PATCH next v4 4/5] minmax: Allow comparisons of 'int' against 'unsigned char/short' David Laight
2023-09-18 8:19 ` [PATCH next v4 5/5] minmax: Relax check to allow comparison between unsigned arguments and signed constants David Laight
2023-09-27 17:30 ` [PATCH next v4 0/5] minmax: Relax type checks in min() and max() Andrew Morton
2023-09-28 8:10 ` David Laight
2024-01-08 11:46 ` Jiri Slaby
2024-01-08 13:34 ` David Laight
2024-01-08 18:19 ` Linus Torvalds
2024-01-08 20:04 ` Linus Torvalds
2024-01-08 21:11 ` Linus Torvalds
2024-01-09 0:39 ` [PATCH next v4 0/5] minmax: Relax type checks in min() and max().^[[C John Stoffel
2024-01-09 6:54 ` [PATCH next v4 0/5] minmax: Relax type checks in min() and max() Jiri Slaby
2024-01-10 6:17 ` Stephen Rothwell
2024-01-10 9:03 ` David Laight
2024-01-10 19:35 ` Linus Torvalds
2024-01-10 22:58 ` David Laight
2024-01-20 21:33 ` Linus Torvalds
2024-01-21 22:18 ` David Laight
2024-01-09 9:35 ` David Laight
2024-01-09 9:41 ` David Laight
2024-01-09 12:09 ` Kalle Valo
2024-01-19 7:14 ` Jiri Slaby
2024-01-19 8:23 ` Hans Verkuil
2024-01-19 9:14 ` David Laight
2024-01-12 9:13 ` Dan Carpenter
2024-01-12 12:16 ` David Laight
2024-01-12 12:40 ` Dan Carpenter
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