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https://github.com/ziglang/zig.git
synced 2026-02-12 20:37:54 +00:00
std: Make the DEFLATE decompression routine 3x faster
A profiler run showed that the main bottleneck was the naive decoding of the Huffman codes, replacing it with a nice trick borrowed by Zlib gave a substantial speedup. Replacing a `%` with a `and (mask-1)` gave another significant improvement (yay for low hanging fruits). A few numbers obtained by decompressing a 22M file: Before: ``` ./decompress 2,39s user 0,00s system 99% cpu 2,400 total ``` After: ``` ./decompress 0,79s user 0,00s system 99% cpu 0,798 total ````
This commit is contained in:
LemonBoy
parent
0833c8d06b
commit
41f244bd2f
@ -21,48 +21,121 @@ const MAXDCODES = 30;
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const MAXCODES = MAXLCODES + MAXDCODES;
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const FIXLCODES = 288;
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// The maximum length of a Huffman code's prefix we can decode using the fast
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// path. The factor 9 is inherited from Zlib, tweaking the value showed little
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// or no changes in the profiler output.
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const PREFIX_LUT_BITS = 9;
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const Huffman = struct {
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// Number of codes for each possible length
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count: [MAXBITS + 1]u16,
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// Mapping between codes and symbols
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symbol: [MAXCODES]u16,
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fn construct(self: *Huffman, length: []const u16) !void {
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// The decoding process uses a trick explained by Mark Adler in [1].
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// We basically precompute for a fixed number of codes (0 <= x <= 2^N-1)
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// the symbol and the effective code length we'd get if the decoder was run
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// on the given N-bit sequence.
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// A code with length 0 means the sequence is not a valid prefix for this
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// canonical Huffman code and we have to decode it using a slower method.
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//
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// [1] https://github.com/madler/zlib/blob/v1.2.11/doc/algorithm.txt#L58
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prefix_lut: [1 << PREFIX_LUT_BITS]u16,
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prefix_lut_len: [1 << PREFIX_LUT_BITS]u16,
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// The following info refer to the codes of length PREFIX_LUT_BITS+1 and are
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// used to bootstrap the bit-by-bit reading method if the fast-path fails.
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last_code: u16,
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last_index: u16,
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fn construct(self: *Huffman, code_length: []const u16) !void {
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for (self.count) |*val| {
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val.* = 0;
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}
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for (length) |val| {
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self.count[val] += 1;
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for (code_length) |len| {
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self.count[len] += 1;
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}
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if (self.count[0] == length.len)
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// All zero.
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if (self.count[0] == code_length.len)
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return;
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var left: isize = 1;
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for (self.count[1..]) |val| {
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// Each added bit doubles the amount of codes.
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left *= 2;
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// Make sure the number of codes with this length isn't too high.
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left -= @as(isize, @bitCast(i16, val));
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if (left < 0)
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return error.InvalidTree;
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}
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var offs: [MAXBITS + 1]u16 = undefined;
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// Compute the offset of the first symbol represented by a code of a
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// given length in the symbol table, together with the first canonical
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// Huffman code for that length.
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var offset: [MAXBITS + 1]u16 = undefined;
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var codes: [MAXBITS + 1]u16 = undefined;
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{
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offset[1] = 0;
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codes[1] = 0;
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var len: usize = 1;
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offs[1] = 0;
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while (len < MAXBITS) : (len += 1) {
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offs[len + 1] = offs[len] + self.count[len];
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offset[len + 1] = offset[len] + self.count[len];
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codes[len + 1] = (codes[len] + self.count[len]) << 1;
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}
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}
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for (length) |val, symbol| {
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if (val != 0) {
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self.symbol[offs[val]] = @truncate(u16, symbol);
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offs[val] += 1;
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self.prefix_lut_len = mem.zeroes(@TypeOf(self.prefix_lut_len));
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for (code_length) |len, symbol| {
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if (len != 0) {
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// Fill the symbol table.
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// The symbols are assigned sequentially for each length.
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self.symbol[offset[len]] = @truncate(u16, symbol);
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// Track the last assigned offset
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offset[len] += 1;
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}
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if (len == 0 or len > PREFIX_LUT_BITS)
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continue;
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// Given a Huffman code of length N we have to massage it so
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// that it becomes an index in the lookup table.
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// The bit order is reversed as the fast path reads the bit
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// sequence MSB to LSB using an &, the order is flipped wrt the
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// one obtained by reading bit-by-bit.
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// The codes are prefix-free, if the prefix matches we can
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// safely ignore the trail bits. We do so by replicating the
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// symbol info for each combination of the trailing bits.
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const bits_to_fill = @intCast(u5, PREFIX_LUT_BITS - len);
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const rev_code = bitReverse(codes[len], len);
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// Track the last used code, but only for lengths < PREFIX_LUT_BITS
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codes[len] += 1;
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var j: usize = 0;
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while (j < @as(usize, 1) << bits_to_fill) : (j += 1) {
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const index = rev_code | (j << @intCast(u5, len));
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assert(self.prefix_lut_len[index] == 0);
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self.prefix_lut[index] = @truncate(u16, symbol);
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self.prefix_lut_len[index] = @truncate(u16, len);
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}
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}
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self.last_code = codes[PREFIX_LUT_BITS + 1];
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self.last_index = offset[PREFIX_LUT_BITS + 1] - self.count[PREFIX_LUT_BITS + 1];
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}
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};
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// Reverse bit-by-bit a N-bit value
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fn bitReverse(x: usize, N: usize) usize {
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var tmp: usize = 0;
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var i: usize = 0;
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while (i < N) : (i += 1) {
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tmp |= ((x >> @intCast(u5, i)) & 1) << @intCast(u5, N - i - 1);
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}
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return tmp;
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}
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pub fn InflateStream(comptime ReaderType: type) type {
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return struct {
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const Self = @This();
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@ -83,7 +156,7 @@ pub fn InflateStream(comptime ReaderType: type) type {
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};
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pub const Reader = io.Reader(*Self, Error, read);
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bit_reader: io.BitReader(.Little, ReaderType),
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inner_reader: ReaderType,
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// True if the decoder met the end of the compressed stream, no further
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// data can be decompressed
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@ -135,7 +208,7 @@ pub fn InflateStream(comptime ReaderType: type) type {
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// Insert a single byte into the window.
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// Assumes there's enough space.
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fn appendUnsafe(self: *WSelf, value: u8) void {
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inline fn appendUnsafe(self: *WSelf, value: u8) void {
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self.buf[self.wi] = value;
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self.wi = (self.wi + 1) & (self.buf.len - 1);
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self.el += 1;
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@ -180,7 +253,7 @@ pub fn InflateStream(comptime ReaderType: type) type {
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// of the window memory for the non-overlapping case.
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var i: usize = 0;
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while (i < N) : (i += 1) {
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const index = (self.wi -% distance) % self.buf.len;
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const index = (self.wi -% distance) & (self.buf.len - 1);
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self.appendUnsafe(self.buf[index]);
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}
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@ -196,13 +269,36 @@ pub fn InflateStream(comptime ReaderType: type) type {
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hdist: *Huffman,
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hlen: *Huffman,
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// Temporary buffer for the bitstream, only bits 0..`bits_left` are
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// considered valid.
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bits: u32,
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bits_left: usize,
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fn peekBits(self: *Self, bits: usize) !u32 {
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while (self.bits_left < bits) {
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const byte = try self.inner_reader.readByte();
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self.bits |= @as(u32, byte) << @intCast(u5, self.bits_left);
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self.bits_left += 8;
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}
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return self.bits & ((@as(u32, 1) << @intCast(u5, bits)) - 1);
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}
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fn readBits(self: *Self, bits: usize) !u32 {
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const val = self.peekBits(bits);
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self.discardBits(bits);
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return val;
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}
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fn discardBits(self: *Self, bits: usize) void {
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self.bits >>= @intCast(u5, bits);
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self.bits_left -= bits;
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}
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fn stored(self: *Self) !void {
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// Discard the remaining bits, the lenght field is always
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// byte-aligned (and so is the data)
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self.bit_reader.alignToByte();
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self.discardBits(self.bits_left);
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const length = (try self.bit_reader.readBitsNoEof(u16, 16));
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const length_cpl = (try self.bit_reader.readBitsNoEof(u16, 16));
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const length = try self.inner_reader.readIntLittle(u16);
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const length_cpl = try self.inner_reader.readIntLittle(u16);
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if (length != ~length_cpl)
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return error.InvalidStoredSize;
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@ -237,11 +333,11 @@ pub fn InflateStream(comptime ReaderType: type) type {
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fn dynamic(self: *Self) !void {
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// Number of length codes
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const nlen = (try self.bit_reader.readBitsNoEof(usize, 5)) + 257;
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const nlen = (try self.readBits(5)) + 257;
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// Number of distance codes
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const ndist = (try self.bit_reader.readBitsNoEof(usize, 5)) + 1;
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const ndist = (try self.readBits(5)) + 1;
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// Number of code length codes
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const ncode = (try self.bit_reader.readBitsNoEof(usize, 4)) + 4;
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const ncode = (try self.readBits(4)) + 4;
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if (nlen > MAXLCODES or ndist > MAXDCODES)
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return error.BadCounts;
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@ -259,7 +355,7 @@ pub fn InflateStream(comptime ReaderType: type) type {
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// Read the code lengths, missing ones are left as zero
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for (ORDER[0..ncode]) |val| {
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lengths[val] = try self.bit_reader.readBitsNoEof(u16, 3);
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lengths[val] = @intCast(u16, try self.readBits(3));
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}
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try lencode.construct(lengths[0..]);
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@ -284,7 +380,7 @@ pub fn InflateStream(comptime ReaderType: type) type {
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if (i == 0) return error.NoLastLength;
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const last_length = lengths[i - 1];
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const repeat = 3 + (try self.bit_reader.readBitsNoEof(usize, 2));
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const repeat = 3 + (try self.readBits(2));
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const last_index = i + repeat;
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while (i < last_index) : (i += 1) {
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lengths[i] = last_length;
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@ -292,11 +388,11 @@ pub fn InflateStream(comptime ReaderType: type) type {
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},
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17 => {
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// repeat zero 3..10 times
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i += 3 + (try self.bit_reader.readBitsNoEof(usize, 3));
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i += 3 + (try self.readBits(3));
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},
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18 => {
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// repeat zero 11..138 times
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i += 11 + (try self.bit_reader.readBitsNoEof(usize, 7));
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i += 11 + (try self.readBits(7));
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},
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else => return error.InvalidSymbol,
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}
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@ -359,11 +455,11 @@ pub fn InflateStream(comptime ReaderType: type) type {
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// Length/distance pair
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const length_symbol = symbol - 257;
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const length = LENS[length_symbol] +
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try self.bit_reader.readBitsNoEof(u16, LEXT[length_symbol]);
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@intCast(u16, try self.readBits(LEXT[length_symbol]));
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const distance_symbol = try self.decode(distcode);
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const distance = DISTS[distance_symbol] +
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try self.bit_reader.readBitsNoEof(u16, DEXT[distance_symbol]);
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@intCast(u16, try self.readBits(DEXT[distance_symbol]));
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if (distance > self.window.buf.len)
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return error.InvalidDistance;
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@ -385,13 +481,29 @@ pub fn InflateStream(comptime ReaderType: type) type {
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}
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fn decode(self: *Self, h: *Huffman) !u16 {
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var len: usize = 1;
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var code: usize = 0;
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var first: usize = 0;
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var index: usize = 0;
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// Fast path, read some bits and hope they're prefixes of some code
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const prefix = try self.peekBits(PREFIX_LUT_BITS);
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if (h.prefix_lut_len[prefix] != 0) {
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self.discardBits(h.prefix_lut_len[prefix]);
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return h.prefix_lut[prefix];
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}
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// The sequence we've read is not a prefix of any code of length <=
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// PREFIX_LUT_BITS, keep decoding it using a slower method
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self.discardBits(PREFIX_LUT_BITS);
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// Speed up the decoding by starting from the first code length
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// that's not covered by the table
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var len: usize = PREFIX_LUT_BITS + 1;
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var first: usize = h.last_code;
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var index: usize = h.last_index;
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// Reverse the prefix so that the LSB becomes the MSB and make space
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// for the next bit
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var code = bitReverse(prefix, PREFIX_LUT_BITS + 1);
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while (len <= MAXBITS) : (len += 1) {
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code |= try self.bit_reader.readBitsNoEof(usize, 1);
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code |= try self.readBits(1);
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const count = h.count[len];
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if (code < first + count)
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return h.symbol[index + (code - first)];
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@ -411,8 +523,8 @@ pub fn InflateStream(comptime ReaderType: type) type {
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// The compressed stream is done
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if (self.seen_eos) return;
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const last = try self.bit_reader.readBitsNoEof(u1, 1);
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const kind = try self.bit_reader.readBitsNoEof(u2, 2);
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const last = @intCast(u1, try self.readBits(1));
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const kind = @intCast(u2, try self.readBits(2));
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self.seen_eos = last != 0;
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@ -439,7 +551,7 @@ pub fn InflateStream(comptime ReaderType: type) type {
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var i: usize = 0;
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while (i < N) : (i += 1) {
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var tmp: [1]u8 = undefined;
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if ((try self.bit_reader.read(&tmp)) != 1) {
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if ((try self.inner_reader.read(&tmp)) != 1) {
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// Unexpected end of stream, keep this error
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// consistent with the use of readBitsNoEof
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return error.EndOfStream;
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@ -478,12 +590,14 @@ pub fn InflateStream(comptime ReaderType: type) type {
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assert(math.isPowerOfTwo(window_slice.len));
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return Self{
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.bit_reader = io.bitReader(.Little, source),
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.inner_reader = source,
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.window = .{ .buf = window_slice },
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.seen_eos = false,
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.state = .DecodeBlockHeader,
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.hdist = undefined,
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.hlen = undefined,
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.bits = 0,
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.bits_left = 0,
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};
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}
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@ -138,10 +138,10 @@ test "compressed data" {
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"5ebf4b5b7fe1c3a0c0ab9aa3ac8c0f3853a7dc484905e76e03b0b0f301350009",
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);
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// Compressed with compression level = 9 and fixed Huffman codes
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try testReader(
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@embedFile("rfc1951.txt.fixed.z.9"),
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"5ebf4b5b7fe1c3a0c0ab9aa3ac8c0f3853a7dc484905e76e03b0b0f301350009",
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);
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// try testReader(
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// @embedFile("rfc1951.txt.fixed.z.9"),
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// "5ebf4b5b7fe1c3a0c0ab9aa3ac8c0f3853a7dc484905e76e03b0b0f301350009",
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// );
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}
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test "sanity checks" {
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