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eqlBytes

std.mem.eql heavily optimized for slices of bytes.

mem.eqlBytes

fn eqlBytes(a: []const u8, b: []const u8) bool

File

lib/std/mem.zig:787

Code

fn eqlBytes(a: []const u8, b: []const u8) bool {
    comptime assert(use_vectors_for_comparison);

    if (a.len != b.len) return false;
    if (a.len == 0 or a.ptr == b.ptr) return true;

    if (a.len <= 16) {
        if (a.len < 4) {
            const x = (a[0] ^ b[0]) | (a[a.len - 1] ^ b[a.len - 1]) | (a[a.len / 2] ^ b[a.len / 2]);
            return x == 0;
        }
        var x: u32 = 0;
        for ([_]usize{ 0, a.len - 4, (a.len / 8) * 4, a.len - 4 - ((a.len / 8) * 4) }) |n| {
            x |= @as(u32, @bitCast(a[n..][0..4].*)) ^ @as(u32, @bitCast(b[n..][0..4].*));
        }
        return x == 0;
    }

    // Figure out the fastest way to scan through the input in chunks.
    // Uses vectors when supported and falls back to usize/words when not.
    const Scan = if (std.simd.suggestVectorLength(u8)) |vec_size|
        struct {
            pub const size = vec_size;
            pub const Chunk = @Vector(size, u8);
            pub inline fn isNotEqual(chunk_a: Chunk, chunk_b: Chunk) bool {
                return @reduce(.Or, chunk_a != chunk_b);
            }
        }
    else
        struct {
            pub const size = @sizeOf(usize);
            pub const Chunk = usize;
            pub inline fn isNotEqual(chunk_a: Chunk, chunk_b: Chunk) bool {
                return chunk_a != chunk_b;
            }
        };

    inline for (1..6) |s| {
        const n = 16 << s;
        if (n <= Scan.size and a.len <= n) {
            const V = @Vector(n / 2, u8);
            var x = @as(V, a[0 .. n / 2].*) ^ @as(V, b[0 .. n / 2].*);
            x |= @as(V, a[a.len - n / 2 ..][0 .. n / 2].*) ^ @as(V, b[a.len - n / 2 ..][0 .. n / 2].*);
            const zero: V = @splat(0);
            return !@reduce(.Or, x != zero);
        }
    }
    // Compare inputs in chunks at a time (excluding the last chunk).
    for (0..(a.len - 1) / Scan.size) |i| {
        const a_chunk: Scan.Chunk = @bitCast(a[i * Scan.size ..][0..Scan.size].*);
        const b_chunk: Scan.Chunk = @bitCast(b[i * Scan.size ..][0..Scan.size].*);
        if (Scan.isNotEqual(a_chunk, b_chunk)) return false;
    }

    // Compare the last chunk using an overlapping read (similar to the previous size strategies).
    const last_a_chunk: Scan.Chunk = @bitCast(a[a.len - Scan.size ..][0..Scan.size].*);
    const last_b_chunk: Scan.Chunk = @bitCast(b[a.len - Scan.size ..][0..Scan.size].*);
    return !Scan.isNotEqual(last_a_chunk, last_b_chunk);
}