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|
//! CRC32C with hardware acceleration.
//!
//! The Zig standard library implementation processes one byte per table
//! lookup (as of Zig 0.16), which is more than an order of magnitude slower
//! than the dedicated CRC32C instructions available on aarch64 (CRC
//! extension) and x86_64 (SSE4.2). This module selects the best backend at
//! compile time and falls back to the standard library elsewhere, including
//! WebAssembly.
//!
//! The resulting value is identical across all backends: this is the
//! iSCSI CRC32C parameter set (reflected, initial and final XOR
//! `0xFFFFFFFF`), matching `std.hash.crc.Crc32Iscsi`.
const std = @import("std");
const builtin = @import("builtin");
/// The standard-library implementation of the same parameter set. This is
/// both the portable fallback and the reference the tests compare against.
const Software = std.hash.crc.Crc32Iscsi;
const Backend = enum {
aarch64_crc,
x86_64_sse42,
software,
};
const backend: Backend = backend: {
switch (builtin.cpu.arch) {
.aarch64,
.aarch64_be,
=> if (std.Target.aarch64.featureSetHas(
builtin.cpu.features,
.crc,
)) break :backend .aarch64_crc,
// The self-hosted x86_64 backend cannot encode the CRC32
// instruction forms used below, so that combination falls back to
// the portable implementation.
.x86_64 => if (builtin.zig_backend == .stage2_llvm and
std.Target.x86.featureSetHas(
builtin.cpu.features,
.sse4_2,
)) break :backend .x86_64_sse42,
else => {},
}
break :backend .software;
};
/// Streaming CRC32C with the same interface shape as `std.hash.crc` types.
pub const Crc32c = struct {
crc: u32,
pub fn init() Crc32c {
return .{ .crc = 0xFFFF_FFFF };
}
pub fn update(self: *Crc32c, bytes: []const u8) void {
self.crc = switch (comptime backend) {
.aarch64_crc, .x86_64_sse42 => updateHardware(self.crc, bytes),
.software => software: {
var crc: Software = .{ .crc = self.crc };
crc.update(bytes);
break :software crc.crc;
},
};
}
pub fn final(self: Crc32c) u32 {
return self.crc ^ 0xFFFF_FFFF;
}
pub fn hash(bytes: []const u8) u32 {
var c: Crc32c = .init();
c.update(bytes);
return c.final();
}
};
/// One update pass using the dedicated CRC32C instructions. Both supported
/// architectures handle unaligned loads efficiently, so the loop reads
/// little-endian words directly from the input.
fn updateHardware(initial: u32, bytes: []const u8) u32 {
var crc = initial;
var remaining = bytes;
while (remaining.len >= 8) : (remaining = remaining[8..]) {
crc = step(u64, crc, std.mem.readInt(
u64,
remaining[0..8],
.little,
));
}
if (remaining.len >= 4) {
crc = step(u32, crc, std.mem.readInt(
u32,
remaining[0..4],
.little,
));
remaining = remaining[4..];
}
for (remaining) |byte| crc = step(u8, crc, byte);
return crc;
}
/// One CRC32C instruction folding `value` into the running CRC.
inline fn step(comptime T: type, crc: u32, value: T) u32 {
return switch (comptime backend) {
.aarch64_crc => switch (T) {
u8 => asm ("crc32cb %[out:w], %[crc:w], %[value:w]"
: [out] "=r" (-> u32),
: [crc] "r" (crc),
[value] "r" (value),
),
u32 => asm ("crc32cw %[out:w], %[crc:w], %[value:w]"
: [out] "=r" (-> u32),
: [crc] "r" (crc),
[value] "r" (value),
),
u64 => asm ("crc32cx %[out:w], %[crc:w], %[value:x]"
: [out] "=r" (-> u32),
: [crc] "r" (crc),
[value] "r" (value),
),
else => comptime unreachable,
},
.x86_64_sse42 => switch (T) {
u8 => asm ("crc32b %[value], %[out]"
: [out] "=r" (-> u32),
: [value] "r" (value),
[crc_in] "0" (crc),
),
u32 => asm ("crc32l %[value], %[out]"
: [out] "=r" (-> u32),
: [value] "r" (value),
[crc_in] "0" (crc),
),
u64 => @truncate(asm ("crc32q %[value], %[out]"
: [out] "=r" (-> u64),
: [value] "r" (value),
[crc_in] "0" (@as(u64, crc)),
)),
else => comptime unreachable,
},
.software => comptime unreachable,
};
}
test "matches the check value" {
// The catalog check value for CRC-32/ISCSI.
try std.testing.expectEqual(
@as(u32, 0xE3069283),
Crc32c.hash("123456789"),
);
}
test "matches the standard library at every length and split" {
var bytes: [259]u8 = undefined;
var prng = std.Random.DefaultPrng.init(0xC5C32C);
prng.random().bytes(&bytes);
for (0..bytes.len + 1) |len| {
const input = bytes[0..len];
try std.testing.expectEqual(
Software.hash(input),
Crc32c.hash(input),
);
// Streaming across arbitrary split points must not change the
// result: word batching may not leak state between updates.
var split: Crc32c = .init();
split.update(input[0 .. len / 3]);
split.update(input[len / 3 .. len - len / 3]);
split.update(input[len - len / 3 ..]);
try std.testing.expectEqual(Software.hash(input), split.final());
}
}
test "matches the standard library at every alignment" {
var bytes: [64 + 16]u8 = undefined;
var prng = std.Random.DefaultPrng.init(0xA11C);
prng.random().bytes(&bytes);
for (0..16) |offset| {
const input = bytes[offset..][0..64];
try std.testing.expectEqual(
Software.hash(input),
Crc32c.hash(input),
);
}
}
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