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minmax.h: update some comments

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- Change three to several.
- Remove the comment about retaining constant expressions, no longer true.
- Realign to nearer 80 columns and break on major punctiation.
- Add a leading comment to the block before __signed_type() and __is_nonneg()
  Otherwise the block explaining the cast is a bit 'floating'.
  Reword the rest of that comment to improve readability.

Link: https://lkml.kernel.org/r/85b050c81c1d4076aeb91a6cded45fee@AcuMS.aculab.com
Signed-off-by: David Laight <david.laight@aculab.com>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Arnd Bergmann <arnd@kernel.org>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Dan Carpenter <dan.carpenter@linaro.org>
Cc: Jason A. Donenfeld <Jason@zx2c4.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Mateusz Guzik <mjguzik@gmail.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Pedro Falcato <pedro.falcato@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
David Laight 2024-11-18 19:12:07 +00:00 committed by Andrew Morton
parent cd577d2b0c
commit db6a793477
1 changed files with 24 additions and 29 deletions
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53
include/linux/minmax.h
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@ -8,13 +8,10 @@
#include <linux/types.h>
/*
* min()/max()/clamp() macros must accomplish three things:
* min()/max()/clamp() macros must accomplish several things:
*
* - Avoid multiple evaluations of the arguments (so side-effects like
* "x++" happen only once) when non-constant.
* - Retain result as a constant expressions when called with only
* constant expressions (to avoid tripping VLA warnings in stack
* allocation usage).
* - Perform signed v unsigned type-checking (to generate compile
* errors instead of nasty runtime surprises).
* - Unsigned char/short are always promoted to signed int and can be
@ -31,25 +28,23 @@
* bit #0 set if ok for unsigned comparisons
* bit #1 set if ok for signed comparisons
*
* In particular, statically non-negative signed integer
* expressions are ok for both.
* In particular, statically non-negative signed integer expressions
* are ok for both.
*
* NOTE! Unsigned types smaller than 'int' are implicitly
* converted to 'int' in expressions, and are accepted for
* signed conversions for now. This is debatable.
* NOTE! Unsigned types smaller than 'int' are implicitly converted to 'int'
* in expressions, and are accepted for signed conversions for now.
* This is debatable.
*
* Note that 'x' is the original expression, and 'ux' is
* the unique variable that contains the value.
* Note that 'x' is the original expression, and 'ux' is the unique variable
* that contains the value.
*
* We use 'ux' for pure type checking, and 'x' for when
* we need to look at the value (but without evaluating
* it for side effects! Careful to only ever evaluate it
* with sizeof() or __builtin_constant_p() etc).
* We use 'ux' for pure type checking, and 'x' for when we need to look at the
* value (but without evaluating it for side effects!
* Careful to only ever evaluate it with sizeof() or __builtin_constant_p() etc).
*
* Pointers end up being checked by the normal C type
* rules at the actual comparison, and these expressions
* only need to be careful to not cause warnings for
* pointer use.
* Pointers end up being checked by the normal C type rules at the actual
* comparison, and these expressions only need to be careful to not cause
* warnings for pointer use.
*/
#define __signed_type_use(x, ux) (2 + __is_nonneg(x, ux))
#define __unsigned_type_use(x, ux) (1 + 2 * (sizeof(ux) < 4))
@ -57,19 +52,19 @@
__signed_type_use(x, ux) : __unsigned_type_use(x, ux))
/*
* To avoid warnings about casting pointers to integers
* of different sizes, we need that special sign type.
* Check whether a signed value is always non-negative.
*
* On 64-bit we can just always use 'long', since any
* integer or pointer type can just be cast to that.
* A cast is needed to avoid any warnings from values that aren't signed
* integer types (in which case the result doesn't matter).
*
* This does not work for 128-bit signed integers since
* the cast would truncate them, but we do not use s128
* types in the kernel (we do use 'u128', but they will
* be handled by the !is_signed_type() case).
* On 64-bit any integer or pointer type can safely be cast to 'long'.
* But on 32-bit we need to avoid warnings about casting pointers to integers
* of different sizes without truncating 64-bit values so 'long' or 'long long'
* must be used depending on the size of the value.
*
* NOTE! The cast is there only to avoid any warnings
* from when values that aren't signed integer types.
* This does not work for 128-bit signed integers since the cast would truncate
* them, but we do not use s128 types in the kernel (we do use 'u128',
* but they are handled by the !is_signed_type() case).
*/
#ifdef CONFIG_64BIT
#define __signed_type(ux) long