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: Thu, 18 Jan 2024 13:30:52 +0300 [thread overview]
Message-ID: <af0bfe8a-8b29-43df-b385-a23f0da54a7b@moroto.mountain> (raw)
In-Reply-To: <d05f91a308174cf0b3e5707625a212bc@AcuMS.aculab.com>
On Fri, Jan 12, 2024 at 02:26:33PM +0000, David Laight wrote:
> From: Dan Carpenter
> > Sent: 12 January 2024 14:03
> >
> > 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.
>
> umin() is only defined for non-negative values.
I'm so confused by this. To me the big selling point of min_t() was
that it clamps things to between zero and the max.
> So that example is really outside the domain of the function.
>
> Consider:
> int x = some_positive_value;
> unsigned long long y;
> then:
> min_t(unsigned long long, x, y);
> Does (unsigned long long)x which is (unsigned long long)(long long)x
> and requires that x be sign extended to 64bits.
> On 32bit that is quite horrid.
I wasn't saying jump straight to ull. I was suggesting jump to ul then
ull, but skip uint. On a 32bit system, you can't allocate ULONG_MAX
bytes, so it still ends up being quite a safe number.
regards,
dan carpenter
next prev parent reply other threads:[~2024-01-18 10:30 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
2024-01-12 14:26 ` David Laight
2024-01-18 10:30 ` Dan Carpenter [this message]
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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