cmov 0.4.3

Conditional move CPU intrinsics which are guaranteed on major platforms to execute in constant-time and not be rewritten as branches by the compiler. Provides wrappers for the CMOV family of instructions on x86/x86_64 and CSEL on AArch64, along with a portable "best-effort" fallback.
Documentation 
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161

use crate::{Cmov, CmovEq, Condition};
use core::arch::asm;

/// Conditional select
macro_rules! csel {
    ($cmp:expr, $csel:expr, $dst:expr, $src:expr, $condition:expr) => {
        unsafe {
            asm! {
                "cmp {0:w}, 0",
                $csel,
                in(reg) $condition,
                inlateout(reg) *$dst,
                in(reg) *$src,
                in(reg) *$dst,
                options(pure, nomem, nostack),
            };
        }
    };
}

/// Conditional select-based equality test
macro_rules! cseleq {
    ($eor:expr, $cmp:expr, $instruction:expr, $lhs:expr, $rhs:expr, $condition:expr, $dst:expr) => {
        let mut tmp = *$dst as u16;
        unsafe {
            asm! {
                $eor,
                $cmp,
                $instruction,
                out(reg) _,
                in(reg) *$lhs,
                in(reg) *$rhs,
                inlateout(reg) tmp,
                in(reg) $condition as u16,
                in(reg) tmp,
                options(pure, nomem, nostack),
            };
        };

        #[allow(clippy::cast_possible_truncation)]
        {
            *$dst = tmp as u8;
        }
    };
}

/// Conditional select using 32-bit `:w` registers
macro_rules! csel32 {
    ($csel:expr, $dst:expr, $src:expr, $condition:expr) => {
        csel!("cmp {0:w}, 0", $csel, $dst, $src, $condition)
    };
}

/// Conditional select using 64-bit `:x` registers
macro_rules! csel64 {
    ($csel:expr, $dst:expr, $src:expr, $condition:expr) => {
        csel!("cmp {0:x}, 0", $csel, $dst, $src, $condition)
    };
}

/// Conditional select equality test using 32-bit `:w` registers
macro_rules! cseleq32 {
    ($instruction:expr, $lhs:expr, $rhs:expr, $condition:expr, $dst:expr) => {
        cseleq!(
            "eor {0:w}, {1:w}, {2:w}",
            "cmp {0:w}, 0",
            $instruction,
            $lhs,
            $rhs,
            $condition,
            $dst
        )
    };
}

/// Conditional select equality test using 64-bit `:w` registers
macro_rules! cseleq64 {
    ($instruction:expr, $lhs:expr, $rhs:expr, $condition:expr, $dst:expr) => {
        cseleq!(
            "eor {0:x}, {1:x}, {2:x}",
            "cmp {0:x}, 0",
            $instruction,
            $lhs,
            $rhs,
            $condition,
            $dst
        )
    };
}

impl Cmov for u16 {
    #[inline]
    fn cmovnz(&mut self, value: &Self, condition: Condition) {
        csel32!("csel {1:w}, {2:w}, {3:w}, NE", self, value, condition);
    }

    #[inline]
    fn cmovz(&mut self, value: &Self, condition: Condition) {
        csel32!("csel {1:w}, {2:w}, {3:w}, EQ", self, value, condition);
    }
}

impl CmovEq for u16 {
    #[inline]
    fn cmovne(&self, rhs: &Self, input: Condition, output: &mut Condition) {
        cseleq32!("csel {3:w}, {4:w}, {5:w}, NE", self, rhs, input, output);
    }

    #[inline]
    fn cmoveq(&self, rhs: &Self, input: Condition, output: &mut Condition) {
        cseleq32!("csel {3:w}, {4:w}, {5:w}, EQ", self, rhs, input, output);
    }
}

impl Cmov for u32 {
    #[inline]
    fn cmovnz(&mut self, value: &Self, condition: Condition) {
        csel32!("csel {1:w}, {2:w}, {3:w}, NE", self, value, condition);
    }

    #[inline]
    fn cmovz(&mut self, value: &Self, condition: Condition) {
        csel32!("csel {1:w}, {2:w}, {3:w}, EQ", self, value, condition);
    }
}

impl CmovEq for u32 {
    #[inline]
    fn cmovne(&self, rhs: &Self, input: Condition, output: &mut Condition) {
        cseleq32!("csel {3:w}, {4:w}, {5:w}, NE", self, rhs, input, output);
    }

    #[inline]
    fn cmoveq(&self, rhs: &Self, input: Condition, output: &mut Condition) {
        cseleq32!("csel {3:w}, {4:w}, {5:w}, EQ", self, rhs, input, output);
    }
}

impl Cmov for u64 {
    #[inline]
    fn cmovnz(&mut self, value: &Self, condition: Condition) {
        csel64!("csel {1:x}, {2:x}, {3:x}, NE", self, value, condition);
    }

    #[inline]
    fn cmovz(&mut self, value: &Self, condition: Condition) {
        csel64!("csel {1:x}, {2:x}, {3:x}, EQ", self, value, condition);
    }
}

impl CmovEq for u64 {
    #[inline]
    fn cmovne(&self, rhs: &Self, input: Condition, output: &mut Condition) {
        cseleq64!("csel {3:x}, {4:x}, {5:x}, NE", self, rhs, input, output);
    }

    #[inline]
    fn cmoveq(&self, rhs: &Self, input: Condition, output: &mut Condition) {
        cseleq64!("csel {3:x}, {4:x}, {5:x}, EQ", self, rhs, input, output);
    }
}