mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
synced 2024-10-29 23:53:32 +00:00
2122e2a4ef
Currently the clamp algorithm does:
if (val > hi)
val = hi;
if (val < lo)
val = lo;
But since hi > lo by definition, this can be made more efficient with:
if (val > hi)
val = hi;
else if (val < lo)
val = lo;
So fix up the clamp and clamp_t functions to do this, adding the same
argument checking as for min and min_t.
For simple cases, code generation on x86_64 and aarch64 stay about the
same:
before:
cmp edi, edx
mov eax, esi
cmova edi, edx
cmp edi, esi
cmovnb eax, edi
ret
after:
cmp edi, esi
mov eax, edx
cmovnb esi, edi
cmp edi, edx
cmovb eax, esi
ret
before:
cmp w0, w2
csel w8, w0, w2, lo
cmp w8, w1
csel w0, w8, w1, hi
ret
after:
cmp w0, w1
csel w8, w0, w1, hi
cmp w0, w2
csel w0, w8, w2, lo
ret
On MIPS64, however, code generation improves, by removing arithmetic in
the second branch:
before:
sltu $3,$6,$4
bne $3,$0,.L2
move $2,$6
move $2,$4
.L2:
sltu $3,$2,$5
bnel $3,$0,.L7
move $2,$5
.L7:
jr $31
nop
after:
sltu $3,$4,$6
beq $3,$0,.L13
move $2,$6
sltu $3,$4,$5
bne $3,$0,.L12
move $2,$4
.L13:
jr $31
nop
.L12:
jr $31
move $2,$5
For more complex cases with surrounding code, the effects are a bit
more complicated. For example, consider this simplified version of
timestamp_truncate() from fs/inode.c on x86_64:
struct timespec64 timestamp_truncate(struct timespec64 t, struct inode *inode)
{
struct super_block *sb = inode->i_sb;
unsigned int gran = sb->s_time_gran;
t.tv_sec = clamp(t.tv_sec, sb->s_time_min, sb->s_time_max);
if (t.tv_sec == sb->s_time_max || t.tv_sec == sb->s_time_min)
t.tv_nsec = 0;
return t;
}
before:
mov r8, rdx
mov rdx, rsi
mov rcx, QWORD PTR [r8]
mov rax, QWORD PTR [rcx+8]
mov rcx, QWORD PTR [rcx+16]
cmp rax, rdi
mov r8, rcx
cmovge rdi, rax
cmp rdi, rcx
cmovle r8, rdi
cmp rax, r8
je .L4
cmp rdi, rcx
jge .L4
mov rax, r8
ret
.L4:
xor edx, edx
mov rax, r8
ret
after:
mov rax, QWORD PTR [rdx]
mov rdx, QWORD PTR [rax+8]
mov rax, QWORD PTR [rax+16]
cmp rax, rdi
jg .L6
mov r8, rax
xor edx, edx
.L2:
mov rax, r8
ret
.L6:
cmp rdx, rdi
mov r8, rdi
cmovge r8, rdx
cmp rax, r8
je .L4
xor eax, eax
cmp rdx, rdi
cmovl rax, rsi
mov rdx, rax
mov rax, r8
ret
.L4:
xor edx, edx
jmp .L2
In this case, we actually gain a branch, unfortunately, because the
compiler's replacement axioms no longer as cleanly apply.
So all and all, this change is a bit of a mixed bag.
Link: https://lkml.kernel.org/r/20220926133435.1333846-2-Jason@zx2c4.com
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
169 lines
4.9 KiB
C
169 lines
4.9 KiB
C
/* SPDX-License-Identifier: GPL-2.0 */
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#ifndef _LINUX_MINMAX_H
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#define _LINUX_MINMAX_H
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#include <linux/const.h>
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/*
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* min()/max()/clamp() macros must accomplish three things:
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*
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* - avoid multiple evaluations of the arguments (so side-effects like
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* "x++" happen only once) when non-constant.
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* - perform strict type-checking (to generate warnings instead of
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* nasty runtime surprises). See the "unnecessary" pointer comparison
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* in __typecheck().
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* - retain result as a constant expressions when called with only
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* constant expressions (to avoid tripping VLA warnings in stack
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* allocation usage).
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*/
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#define __typecheck(x, y) \
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(!!(sizeof((typeof(x) *)1 == (typeof(y) *)1)))
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#define __no_side_effects(x, y) \
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(__is_constexpr(x) && __is_constexpr(y))
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#define __safe_cmp(x, y) \
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(__typecheck(x, y) && __no_side_effects(x, y))
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#define __cmp(x, y, op) ((x) op (y) ? (x) : (y))
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#define __cmp_once(x, y, unique_x, unique_y, op) ({ \
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typeof(x) unique_x = (x); \
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typeof(y) unique_y = (y); \
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__cmp(unique_x, unique_y, op); })
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#define __careful_cmp(x, y, op) \
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__builtin_choose_expr(__safe_cmp(x, y), \
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__cmp(x, y, op), \
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__cmp_once(x, y, __UNIQUE_ID(__x), __UNIQUE_ID(__y), op))
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#define __clamp(val, lo, hi) \
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((val) >= (hi) ? (hi) : ((val) <= (lo) ? (lo) : (val)))
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#define __clamp_once(val, lo, hi, unique_val, unique_lo, unique_hi) ({ \
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typeof(val) unique_val = (val); \
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typeof(lo) unique_lo = (lo); \
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typeof(hi) unique_hi = (hi); \
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__clamp(unique_val, unique_lo, unique_hi); })
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#define __clamp_input_check(lo, hi) \
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(BUILD_BUG_ON_ZERO(__builtin_choose_expr( \
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__is_constexpr((lo) > (hi)), (lo) > (hi), false)))
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#define __careful_clamp(val, lo, hi) ({ \
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__clamp_input_check(lo, hi) + \
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__builtin_choose_expr(__typecheck(val, lo) && __typecheck(val, hi) && \
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__typecheck(hi, lo) && __is_constexpr(val) && \
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__is_constexpr(lo) && __is_constexpr(hi), \
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__clamp(val, lo, hi), \
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__clamp_once(val, lo, hi, __UNIQUE_ID(__val), \
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__UNIQUE_ID(__lo), __UNIQUE_ID(__hi))); })
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/**
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* min - return minimum of two values of the same or compatible types
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* @x: first value
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* @y: second value
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*/
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#define min(x, y) __careful_cmp(x, y, <)
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/**
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* max - return maximum of two values of the same or compatible types
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* @x: first value
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* @y: second value
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*/
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#define max(x, y) __careful_cmp(x, y, >)
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/**
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* min3 - return minimum of three values
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* @x: first value
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* @y: second value
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* @z: third value
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*/
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#define min3(x, y, z) min((typeof(x))min(x, y), z)
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/**
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* max3 - return maximum of three values
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* @x: first value
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* @y: second value
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* @z: third value
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*/
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#define max3(x, y, z) max((typeof(x))max(x, y), z)
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/**
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* min_not_zero - return the minimum that is _not_ zero, unless both are zero
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* @x: value1
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* @y: value2
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*/
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#define min_not_zero(x, y) ({ \
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typeof(x) __x = (x); \
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typeof(y) __y = (y); \
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__x == 0 ? __y : ((__y == 0) ? __x : min(__x, __y)); })
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/**
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* clamp - return a value clamped to a given range with strict typechecking
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* @val: current value
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* @lo: lowest allowable value
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* @hi: highest allowable value
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*
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* This macro does strict typechecking of @lo/@hi to make sure they are of the
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* same type as @val. See the unnecessary pointer comparisons.
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*/
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#define clamp(val, lo, hi) __careful_clamp(val, lo, hi)
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/*
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* ..and if you can't take the strict
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* types, you can specify one yourself.
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*
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* Or not use min/max/clamp at all, of course.
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*/
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/**
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* min_t - return minimum of two values, using the specified type
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* @type: data type to use
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* @x: first value
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* @y: second value
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*/
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#define min_t(type, x, y) __careful_cmp((type)(x), (type)(y), <)
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/**
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* max_t - return maximum of two values, using the specified type
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* @type: data type to use
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* @x: first value
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* @y: second value
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*/
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#define max_t(type, x, y) __careful_cmp((type)(x), (type)(y), >)
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/**
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* clamp_t - return a value clamped to a given range using a given type
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* @type: the type of variable to use
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* @val: current value
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* @lo: minimum allowable value
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* @hi: maximum allowable value
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*
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* This macro does no typechecking and uses temporary variables of type
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* @type to make all the comparisons.
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*/
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#define clamp_t(type, val, lo, hi) __careful_clamp((type)(val), (type)(lo), (type)(hi))
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/**
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* clamp_val - return a value clamped to a given range using val's type
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* @val: current value
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* @lo: minimum allowable value
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* @hi: maximum allowable value
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*
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* This macro does no typechecking and uses temporary variables of whatever
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* type the input argument @val is. This is useful when @val is an unsigned
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* type and @lo and @hi are literals that will otherwise be assigned a signed
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* integer type.
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*/
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#define clamp_val(val, lo, hi) clamp_t(typeof(val), val, lo, hi)
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/**
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* swap - swap values of @a and @b
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* @a: first value
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* @b: second value
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*/
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#define swap(a, b) \
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do { typeof(a) __tmp = (a); (a) = (b); (b) = __tmp; } while (0)
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#endif /* _LINUX_MINMAX_H */
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