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Message-ID: <6228a4ff9c074075bf07cca0e9d3eab2@AcuMS.aculab.com>
Date: Fri, 4 Aug 2023 10:54:23 +0000
From: David Laight <David.Laight@...LAB.COM>
To: "'linux-kernel@...r.kernel.org'" <linux-kernel@...r.kernel.org>,
"'Andy Shevchenko'" <andriy.shevchenko@...ux.intel.com>,
'Andrew Morton' <akpm@...ux-foundation.org>,
"'Matthew Wilcox (Oracle)'" <willy@...radead.org>,
'Christoph Hellwig' <hch@...radead.org>,
"'Jason A. Donenfeld'" <Jason@...c4.com>,
'Linus Torvalds' <torvalds@...ux-foundation.org>
Subject: [PATCH v3 2/5] minmax: Allow min()/max()/clamp() if the arguments
have the same signedness.
The type-check in min()/max() is there to stop unexpected results if a
negative value gets converted to a large unsigned value.
However it also rejects 'unsigned int' v 'unsigned long' compares
which are common and never problematc.
Replace the 'same type' check with a 'same signedness' check.
The new test isn't itself a compile time error, so use static_assert()
to report the error and give a meaningful error message.
Due to the way builtin_choose_expr() works detecting the error in the
'non-constant' side (where static_assert() can be used) also detects
errors when the arguments are constant.
Signed-off-by: David Laight <david.laight@...lab.com>
---
Changes for v3:
- Wrap is_signed_type() in __builtin_choose_expr() and __is_constexpr().
Generates a compile-time 0 for pointer types avoiding a bug in clang < 16.
Changes for v2:
- #include <linux/build_bug.h> to fix 'um' build.
This matches a separate commit from Andy.
- Do not delete __typecheck() - used outside this file.
- Use __builtin_choose_expr() in __clamp_once() to fix clang builds.
include/linux/minmax.h | 65 +++++++++++++++++++++++-------------------
1 file changed, 35 insertions(+), 30 deletions(-)
diff --git a/include/linux/minmax.h b/include/linux/minmax.h
index 531860e9cc55..6f79e44aad86 100644
--- a/include/linux/minmax.h
+++ b/include/linux/minmax.h
@@ -2,6 +2,7 @@
#ifndef _LINUX_MINMAX_H
#define _LINUX_MINMAX_H
+#include <linux/build_bug.h>
#include <linux/const.h>
/*
@@ -9,9 +10,8 @@
*
* - avoid multiple evaluations of the arguments (so side-effects like
* "x++" happen only once) when non-constant.
- * - perform strict type-checking (to generate warnings instead of
- * nasty runtime surprises). See the "unnecessary" pointer comparison
- * in __typecheck().
+ * - perform signed v unsigned type-checking (to generate compile
+ * errors instead of nasty runtime surprises).
* - retain result as a constant expressions when called with only
* constant expressions (to avoid tripping VLA warnings in stack
* allocation usage).
@@ -19,23 +19,30 @@
#define __typecheck(x, y) \
(!!(sizeof((typeof(x) *)1 == (typeof(y) *)1)))
-#define __no_side_effects(x, y) \
- (__is_constexpr(x) && __is_constexpr(y))
+/* is_signed_type() isn't a constexpr for pointer types */
+#define __is_signed(x) \
+ __builtin_choose_expr(__is_constexpr(is_signed_type(typeof(x))), \
+ is_signed_type(typeof(x)), 0)
-#define __safe_cmp(x, y) \
- (__typecheck(x, y) && __no_side_effects(x, y))
+#define __types_ok(x, y) \
+ (__is_signed(x) == __is_signed(y))
-#define __cmp(x, y, op) ((x) op (y) ? (x) : (y))
+#define __cmp_op_min <
+#define __cmp_op_max >
-#define __cmp_once(x, y, unique_x, unique_y, op) ({ \
+#define __cmp(op, x, y) ((x) __cmp_op_##op (y) ? (x) : (y))
+
+#define __cmp_once(op, x, y, unique_x, unique_y) ({ \
typeof(x) unique_x = (x); \
typeof(y) unique_y = (y); \
- __cmp(unique_x, unique_y, op); })
+ static_assert(__types_ok(x, y), \
+ #op "(" #x ", " #y ") signedness error, fix types or consider " #op "_unsigned() before " #op "_t()"); \
+ __cmp(op, unique_x, unique_y); })
-#define __careful_cmp(x, y, op) \
- __builtin_choose_expr(__safe_cmp(x, y), \
- __cmp(x, y, op), \
- __cmp_once(x, y, __UNIQUE_ID(__x), __UNIQUE_ID(__y), op))
+#define __careful_cmp(op, x, y) \
+ __builtin_choose_expr(__is_constexpr((x) - (y)), \
+ __cmp(op, x, y), \
+ __cmp_once(op, x, y, __UNIQUE_ID(__x), __UNIQUE_ID(__y)))
#define __clamp(val, lo, hi) \
((val) >= (hi) ? (hi) : ((val) <= (lo) ? (lo) : (val)))
@@ -44,17 +51,15 @@
typeof(val) unique_val = (val); \
typeof(lo) unique_lo = (lo); \
typeof(hi) unique_hi = (hi); \
+ static_assert(__builtin_choose_expr(__is_constexpr((lo) > (hi)), \
+ (lo) <= (hi), true), \
+ "clamp() low limit " #lo " greater than high limit " #hi); \
+ static_assert(__types_ok(val, lo), "clamp() 'lo' signedness error"); \
+ static_assert(__types_ok(val, hi), "clamp() 'hi' signedness error"); \
__clamp(unique_val, unique_lo, unique_hi); })
-#define __clamp_input_check(lo, hi) \
- (BUILD_BUG_ON_ZERO(__builtin_choose_expr( \
- __is_constexpr((lo) > (hi)), (lo) > (hi), false)))
-
#define __careful_clamp(val, lo, hi) ({ \
- __clamp_input_check(lo, hi) + \
- __builtin_choose_expr(__typecheck(val, lo) && __typecheck(val, hi) && \
- __typecheck(hi, lo) && __is_constexpr(val) && \
- __is_constexpr(lo) && __is_constexpr(hi), \
+ __builtin_choose_expr(__is_constexpr((val) - (lo) + (hi)), \
__clamp(val, lo, hi), \
__clamp_once(val, lo, hi, __UNIQUE_ID(__val), \
__UNIQUE_ID(__lo), __UNIQUE_ID(__hi))); })
@@ -64,14 +69,14 @@
* @x: first value
* @y: second value
*/
-#define min(x, y) __careful_cmp(x, y, <)
+#define min(x, y) __careful_cmp(min, x, y)
/**
* max - return maximum of two values of the same or compatible types
* @x: first value
* @y: second value
*/
-#define max(x, y) __careful_cmp(x, y, >)
+#define max(x, y) __careful_cmp(max, x, y)
/**
* min_unsigned - return minimum of two non-negative values
@@ -79,16 +84,16 @@
* @x: first value
* @y: second value
*/
-#define min_unsigned(x, y) \
- __careful_cmp((x) + 0u + 0ul + 0ull, (y) + 0u + 0ul + 0ull, <)
+#define min_unsigned(x, y) \
+ __careful_cmp(min, (x) + 0u + 0ul + 0ull, (y) + 0u + 0ul + 0ull)
/**
* max_unsigned - return maximum of two non-negative values
* @x: first value
* @y: second value
*/
-#define max_unsigned(x, y) \
- __careful_cmp((x) + 0u + 0ul + 0ull, (y) + 0u + 0ul + 0ull, >)
+#define max_unsigned(x, y) \
+ __careful_cmp(max, (x) + 0u + 0ul + 0ull, (y) + 0u + 0ul + 0ull)
/**
* min3 - return minimum of three values
@@ -140,7 +145,7 @@
* @x: first value
* @y: second value
*/
-#define min_t(type, x, y) __careful_cmp((type)(x), (type)(y), <)
+#define min_t(type, x, y) __careful_cmp(min, (type)(x), (type)(y))
/**
* max_t - return maximum of two values, using the specified type
@@ -148,7 +153,7 @@
* @x: first value
* @y: second value
*/
-#define max_t(type, x, y) __careful_cmp((type)(x), (type)(y), >)
+#define max_t(type, x, y) __careful_cmp(max, (type)(x), (type)(y))
/**
* clamp_t - return a value clamped to a given range using a given type
--
2.17.1
-
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