368 lines
14 KiB
Zig
368 lines
14 KiB
Zig
//! A library to check and extract values from integers based on a "bit string". Primarily intended for (my) emulator instruction decoding, but maybe someone else can find a use for it?
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//!
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//! ## Example
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//! ```zig
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//! const std = @import("std");
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//! test "doc test" {
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//! const value: u8 = 0b10001011;
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//!
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//! try std.testing.expectEqual(true, match("10001011", value));
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//! try std.testing.expectEqual(false, match("11111011", value));
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//! try std.testing.expectEqual(true, match("1---1011", value));
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//!
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//! {
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//! const ret = extract("1000aaaa", value);
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//! try std.testing.expectEqual(@as(u4, 0b1011), ret.a);
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//! }
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//! {
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//! const ret = extract("1aaa1aaa", value);
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//! try std.testing.expectEqual(@as(u6, 0b000011), ret.a);
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//! }
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//! {
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//! const ret = extract("1---abcd", value);
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//! try std.testing.expectEqual(@as(u3, 0b1), ret.a);
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//! try std.testing.expectEqual(@as(u3, 0b0), ret.b);
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//! try std.testing.expectEqual(@as(u3, 0b1), ret.c);
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//! try std.testing.expectEqual(@as(u3, 0b1), ret.d);
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//! }
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//! }
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//! ```
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//! ## Syntax
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//! | Token | Meaning | Description
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//! | ------- | --------- | -----------
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//! | `0` | Unset bit | In the equivalent position, the value's bit must be set.
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//! | `1` | Set bit | In the equivalent position, the value's bit must be set.
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//! | `a..=z` | Variable | Given the 4-bit bit string, `"1aa0"`, the value `0b1010` would produce the variable `a` with the value `0b01`
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//! | `-` | Ignored | In the equivalent position, the value's bit does not matter.
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//!
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//! ## Notes
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//! - This library does the majority of it's work at `comptime`. Due to this, you cannot create strings to match against at runtime.
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//! - Variables do not have to be "sequential". This means the 5-bit bit string `"1aa0a"` with the value `0b10101` will produce the variable `a` with the value `0b011`.
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const std = @import("std");
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const Log2Int = std.math.Log2Int;
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/// Test to see if a value matches the provided bit-string
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///
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/// ### Example
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/// ```zig
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/// match("1100", @as(u4, 0b1100)) // true
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/// match("1100", @as(u4, 0b1110)) // false
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/// match("1--0", @as(u4, 0b1010)) // true
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/// match("1ab0", @as(u4, 0b1010)) // true
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/// ```
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pub fn match(comptime bit_string: []const u8, value: anytype) bool {
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const ValT = @TypeOf(value);
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comptime verify(ValT, bit_string);
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const masks: struct { ValT, ValT } = comptime blk: {
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const bit_count = @typeInfo(ValT).Int.bits;
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var set: ValT = 0;
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var clr: ValT = 0;
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// FIXME: I linear search bit_string 4 separate times. Consider doing a single search and compromizing on memory + stateless API? (imagine a "regex compile"-like API)
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for (bit_string, 0..) |char, i| {
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switch (char) {
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'0' => clr |= @as(ValT, 1) << @intCast(bit_count - 1 - i),
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'1' => set |= @as(ValT, 1) << @intCast(bit_count - 1 - i),
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'a'...'z', '-' => continue,
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else => @compileError("'" ++ [_]u8{char} ++ "' was unexpected when parsing bitstring"),
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}
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}
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break :blk .{ set, clr };
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};
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const set_mask = masks[0];
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const clr_mask = masks[1];
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return (value & set_mask) == set_mask and (~value & clr_mask) == clr_mask;
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}
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test "match" {
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// doc tests
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try std.testing.expectEqual(true, match("1100", @as(u4, 0b1100))); // true
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try std.testing.expectEqual(false, match("1100", @as(u4, 0b1110))); // false
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try std.testing.expectEqual(true, match("1--0", @as(u4, 0b1010))); // true
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try std.testing.expectEqual(true, match("1ab0", @as(u4, 0b1010))); // true
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// other tests
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try std.testing.expectEqual(true, match("11111111", @as(u8, 0b11111111)));
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try std.testing.expectEqual(true, match("10110011", @as(u8, 0b10110011)));
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try std.testing.expectEqual(true, match("101aaabb", @as(u8, 0b10110001)));
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try std.testing.expectEqual(true, match("abcdefgh", @as(u8, 0b10110101)));
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try std.testing.expectEqual(true, match("aaa---11", @as(u8, 0b01011111)));
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try std.testing.expectEqual(true, match("1a0b1c0d", @as(u8, 0b10011101)));
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try std.testing.expectEqual(false, match("aaa---11", @as(u8, 0b01011110)));
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}
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/// Extracts the variables (defined in the bit string) from a value.
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///
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/// ### Examples
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/// ```
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/// const ret = extract("aaaa", @as(u4, 0b1001)); // ret.a == 0b1001
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/// const ret = extract("abcd", @as(u4, 0b1001)); // ret.a == 0b1, ret.b == 0b0, ret.c == 0b0, ret.d == 0b1
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/// const ret = extract("a0ab", @as(u4, 0b1001)); // ret.a == 0b10, ret.b == 0b1
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/// const ret = extract("-a-a", @as(u4, 0b1001)); // ret.a == 0b01
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/// ```
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///
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/// Note: In Debug and ReleaseSafe builds, there's a runtime assert that
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/// ensures that the value matches against the bit string.
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pub fn extract(comptime bit_string: []const u8, value: anytype) Bitfield(bit_string) {
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const builtin = @import("builtin");
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const ValT = @TypeOf(value);
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const ReturnT = Bitfield(bit_string);
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const bmi2 = switch (builtin.target.cpu.arch) {
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.x86_64 => std.Target.x86.featureSetHas(builtin.cpu.features, .bmi2),
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else => false,
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};
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comptime verify(ValT, bit_string);
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std.debug.assert(match(bit_string, value)); // prevents branchless impl in ReleaseSafe
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var ret: ReturnT = undefined;
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inline for (@typeInfo(ReturnT).Struct.fields) |field| {
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@field(ret, field.name) = blk: {
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var masked_val: ValT = 0;
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for (bit_string, 0..) |char, i| {
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const rev = @typeInfo(ValT).Int.bits - 1 - i;
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if (char == field.name[0]) masked_val |= @as(ValT, 1) << @intCast(rev); // no penalty
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}
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// TODO: decide at compile time if we're calling the 32-bit or 64-bit version of `PEXT`
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// invariant: the bit count in the field we're writing to and the
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// # of bits we happened to find in this linear search are identical
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//
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// we're confident in this because it's guaranteed to be the same bit_string,
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// and it's the same linear search. If you're reading this double check that this is still the case lol
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break :blk @truncate(if (bmi2) pext.hardware(u32, value, masked_val) else pext.software(u32, value, masked_val));
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};
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}
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return ret;
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}
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test "extract" {
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// doc tests
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{
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const ret = extract("aaaa", @as(u4, 0b1001));
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try std.testing.expectEqual(@as(u4, 0b1001), ret.a);
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}
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{
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const ret = extract("abcd", @as(u4, 0b1001));
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try std.testing.expectEqual(@as(u1, 0b1), ret.a);
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try std.testing.expectEqual(@as(u1, 0b0), ret.b);
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try std.testing.expectEqual(@as(u1, 0b0), ret.c);
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try std.testing.expectEqual(@as(u1, 0b1), ret.d);
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}
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{
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const ret = extract("a0ab", @as(u4, 0b1001));
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try std.testing.expectEqual(@as(u2, 0b10), ret.a);
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try std.testing.expectEqual(@as(u1, 0b01), ret.b);
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}
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{
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const ret = extract("-a-a", @as(u4, 0b1001));
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try std.testing.expectEqual(@as(u2, 0b01), ret.a);
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}
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// other tests
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{
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const ret = extract("10aaabbc", @as(u8, 0b10110011));
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try std.testing.expectEqual(@as(u3, 0b110), ret.a);
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try std.testing.expectEqual(@as(u2, 0b01), ret.b);
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try std.testing.expectEqual(@as(u1, 0b1), ret.c);
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}
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{
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const ret = extract("1111abababab1010", @as(u16, 0b1111_1110_1101_1010));
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try std.testing.expectEqual(@as(u4, 0b1110), ret.a);
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try std.testing.expectEqual(@as(u4, 0b1011), ret.b);
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}
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}
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/// Parses a bit string and reifies a struct that will contain fields that correspond to the variables present in the bit string.
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///
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///
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/// Note: If it weren't for the return type of `extract()`, this type would be a private implementation detail
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///
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/// TODO: I will probably rename this type
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pub fn Bitfield(comptime bit_string: []const u8) type {
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const StructField = std.builtin.Type.StructField;
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const alphabet_set: u26 = tmp: {
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var bit_set: u26 = 0;
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for (bit_string) |char| {
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switch (char) {
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'a'...'z' => |c| bit_set |= @as(u26, 1) << @intCast(c - 'a'),
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else => continue,
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}
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}
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break :tmp bit_set;
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};
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const field_len = @popCount(alphabet_set);
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const fields = blk: {
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var tmp: [field_len]StructField = undefined;
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const Tmp = struct { bits: u8 = 0, char: ?u8 = null };
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var things: [field_len]Tmp = [_]Tmp{.{}} ** field_len;
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for (bit_string) |char| {
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switch (char) {
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'a'...'z' => |c| {
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const bit_in_set = @as(u26, 1) << @intCast(c - 'a');
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const pos = @ctz(alphabet_set & ~(bit_in_set - 1));
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things[pos].bits += 1;
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things[pos].char = c;
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},
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'1', '0', '-' => continue,
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else => @compileError("error when parsing bitset string"),
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}
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}
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for (things, &tmp) |th, *field| {
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const FieldInt = @Type(.{ .Int = .{ .signedness = .unsigned, .bits = th.bits } });
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field.* = .{
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.name = &.{th.char.?},
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.type = FieldInt,
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.default_value = null,
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.is_comptime = false,
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.alignment = @alignOf(FieldInt),
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};
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}
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break :blk tmp;
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};
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return @Type(.{ .Struct = .{
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.layout = .Auto,
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.fields = &fields,
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.decls = &.{},
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.is_tuple = false,
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} });
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}
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fn verify(comptime T: type, comptime bit_string: []const u8) void {
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const info = @typeInfo(T);
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// FIXME: remove the need for this
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if (info.Int.bits > 32) @compileError("TODO: 64-bit `PEXT` software implementation");
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std.debug.assert(info != .ComptimeInt);
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std.debug.assert(info.Int.signedness == .unsigned);
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std.debug.assert(info.Int.bits <= 64); // x86 PEXT u32 and u64 operands only
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std.debug.assert(bit_string.len == info.Int.bits); // TODO: Support Underscores?
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}
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const pext = struct {
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fn hardware(comptime T: type, value: T, mask: T) T {
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return switch (T) {
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u32 => asm ("pextl %[mask], %[value], %[ret]"
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: [ret] "=r" (-> T),
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: [value] "r" (value),
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[mask] "r" (mask),
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),
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u64 => asm ("pextq %[mask], %[value], %[ret]"
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: [ret] "=r" (-> T),
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: [value] "r" (value),
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[mask] "r" (mask),
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),
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else => @compileError("pext is sunsupported for " ++ @typeName(T) ++ "."),
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};
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}
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// why we need this: https://github.com/ziglang/zig/issues/14995 (ideally compiler-rt implements this for us)
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fn software(comptime T: type, value: T, mask: T) T {
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return switch (T) {
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u32 => {
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// TODO: Looks (and is) like C code :pensive:
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// code source: https://stackoverflow.com/questions/41720249/detecting-matching-bits-in-c
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var _value: T = value;
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var _mask: T = mask;
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_value &= _mask;
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var mk: T = ~_mask << 1;
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var mp: T = undefined;
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var mv: T = undefined;
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var t: T = undefined;
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inline for (0..@typeInfo(u5).Int.bits) |i| {
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mp = mk ^ (mk << 1); // parallel suffix
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mp = mp ^ (mp << 2);
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mp = mp ^ (mp << 4);
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mp = mp ^ (mp << 8);
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mp = mp ^ (mp << 16);
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mv = (mp & _mask); // bits to move
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_mask = ((_mask ^ mv) | (mv >> (1 << i))); // compress _mask
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t = (_value & mv);
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_value = ((_value ^ t) | (t >> (1 << i))); // compress _value
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mk &= ~mp;
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}
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return _value;
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},
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u64 => @compileError("TODO: find/write branchless software impl of `PEXT` for 64-bit values"),
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else => @compileError("pext is sunsupported for " ++ @typeName(T) ++ "."),
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};
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}
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test "pext" {
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const builtin = @import("builtin");
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switch (builtin.cpu.arch) {
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.x86_64 => if (std.Target.x86.featureSetHas(builtin.cpu.features, .bmi2)) {
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try std.testing.expectEqual(@as(u32, 0x0001_2567), pext.hardware(u32, 0x12345678, 0xFF00FFF0));
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try std.testing.expectEqual(@as(u64, 0x0001_2567), pext.hardware(u64, 0x12345678, 0xFF00FFF0));
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// random tests
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// TODO: when implemented, test 64-bit fallback `PEXT` as well
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var rand_impl = std.rand.DefaultPrng.init(0xBAADF00D_DEADCAFE);
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for (0..100) |_| {
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const value = rand_impl.random().int(u32);
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const mask = rand_impl.random().int(u32);
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try std.testing.expectEqual(pext.hardware(u32, value, mask), pext.software(u32, value, mask));
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}
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},
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else => {},
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}
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// example values from: https://en.wikipedia.org/w/index.php?title=X86_Bit_manipulation_instruction_set&oldid=1170426748
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try std.testing.expectEqual(@as(u32, 0x0001_2567), pext.software(u32, 0x12345678, 0xFF00FFF0));
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}
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};
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test "doc test" {
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const value: u8 = 0b10001011;
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try std.testing.expectEqual(true, match("10001011", value));
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try std.testing.expectEqual(false, match("11111011", value));
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try std.testing.expectEqual(true, match("1---1011", value));
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{
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const ret = extract("1000aaaa", value);
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try std.testing.expectEqual(@as(u4, 0b1011), ret.a);
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}
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{
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const ret = extract("1aaa1aaa", value);
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try std.testing.expectEqual(@as(u6, 0b000011), ret.a);
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}
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{
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const ret = extract("1---abcd", value);
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try std.testing.expectEqual(@as(u3, 0b1), ret.a);
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try std.testing.expectEqual(@as(u3, 0b0), ret.b);
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try std.testing.expectEqual(@as(u3, 0b1), ret.c);
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try std.testing.expectEqual(@as(u3, 0b1), ret.d);
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}
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}
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