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
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
#![no_std]

/// Represents the possible sizes in bytes of the discriminant of a variant type in the component model
#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
pub enum DiscriminantSize {
    /// 8-bit discriminant
    Size1,
    /// 16-bit discriminant
    Size2,
    /// 32-bit discriminant
    Size4,
}

impl DiscriminantSize {
    /// Calculate the size of discriminant needed to represent a variant with the specified number of cases.
    pub const fn from_count(count: usize) -> Option<Self> {
        if count <= 0xFF {
            Some(Self::Size1)
        } else if count <= 0xFFFF {
            Some(Self::Size2)
        } else if count <= 0xFFFF_FFFF {
            Some(Self::Size4)
        } else {
            None
        }
    }

    /// Returns the size, in bytes, of this discriminant
    pub const fn byte_size(&self) -> u32 {
        match self {
            DiscriminantSize::Size1 => 1,
            DiscriminantSize::Size2 => 2,
            DiscriminantSize::Size4 => 4,
        }
    }
}

impl From<DiscriminantSize> for u32 {
    /// Size of the discriminant as a `u32`
    fn from(size: DiscriminantSize) -> u32 {
        size.byte_size()
    }
}

impl From<DiscriminantSize> for usize {
    /// Size of the discriminant as a `usize`
    fn from(size: DiscriminantSize) -> usize {
        match size {
            DiscriminantSize::Size1 => 1,
            DiscriminantSize::Size2 => 2,
            DiscriminantSize::Size4 => 4,
        }
    }
}

/// Represents the number of bytes required to store a flags value in the component model
pub enum FlagsSize {
    /// There are no flags
    Size0,
    /// Flags can fit in a u8
    Size1,
    /// Flags can fit in a u16
    Size2,
    /// Flags can fit in a specified number of u32 fields
    Size4Plus(u8),
}

impl FlagsSize {
    /// Calculate the size needed to represent a value with the specified number of flags.
    pub const fn from_count(count: usize) -> FlagsSize {
        if count == 0 {
            FlagsSize::Size0
        } else if count <= 8 {
            FlagsSize::Size1
        } else if count <= 16 {
            FlagsSize::Size2
        } else {
            let amt = count.div_ceil(32);
            if amt > (u8::MAX as usize) {
                panic!("too many flags");
            }
            FlagsSize::Size4Plus(amt as u8)
        }
    }
}

/// A simple bump allocator which can be used with modules
pub const REALLOC_AND_FREE: &str = r#"
    (global $last (mut i32) (i32.const 8))
    (func $realloc (export "realloc")
        (param $old_ptr i32)
        (param $old_size i32)
        (param $align i32)
        (param $new_size i32)
        (result i32)

        (local $ret i32)

        ;; Test if the old pointer is non-null
        local.get $old_ptr
        if
            ;; If the old size is bigger than the new size then
            ;; this is a shrink and transparently allow it
            local.get $old_size
            local.get $new_size
            i32.gt_u
            if
                local.get $old_ptr
                return
            end

            ;; otherwise fall through to allocate a new chunk which will later
            ;; copy data over
        end

        ;; align up `$last`
        (global.set $last
            (i32.and
                (i32.add
                    (global.get $last)
                    (i32.add
                        (local.get $align)
                        (i32.const -1)))
                (i32.xor
                    (i32.add
                        (local.get $align)
                        (i32.const -1))
                    (i32.const -1))))

        ;; save the current value of `$last` as the return value
        global.get $last
        local.set $ret

        ;; bump our pointer
        (global.set $last
            (i32.add
                (global.get $last)
                (local.get $new_size)))

        ;; while `memory.size` is less than `$last`, grow memory
        ;; by one page
        (loop $loop
            (if
                (i32.lt_u
                    (i32.mul (memory.size) (i32.const 65536))
                    (global.get $last))
                (then
                    i32.const 1
                    memory.grow
                    ;; test to make sure growth succeeded
                    i32.const -1
                    i32.eq
                    if unreachable end

                    br $loop)))


        ;; ensure anything necessary is set to valid data by spraying a bit
        ;; pattern that is invalid
        local.get $ret
        i32.const 0xde
        local.get $new_size
        memory.fill

        ;; If the old pointer is present then that means this was a reallocation
        ;; of an existing chunk which means the existing data must be copied.
        local.get $old_ptr
        if
            local.get $ret          ;; destination
            local.get $old_ptr      ;; source
            local.get $old_size     ;; size
            memory.copy
        end

        local.get $ret
    )
"#;