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feat: branchless sw pext for 64-bit integers

This commit is contained in:
Rekai Nyangadzayi Musuka 2023-12-02 16:26:25 -06:00
parent fbbd25a4c8
commit 611549c8e9
1 changed files with 63 additions and 23 deletions
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86
src/lib.zig
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@ -58,7 +58,7 @@ pub fn match(comptime bit_string: []const u8, value: anytype) bool {
var clr: ValT = 0;
var offset = 0;
// FIXME: I linear search like this 5 times across the entie lib. Consider structuring this like a regex lib (compiling a match)
// FIXME: I linear search like this 5 times across the entire lib. Consider structuring this like a regex lib (compiling a match)
for (bit_string, 0..) |char, i| {
switch (char) {
'0' => clr |= @as(ValT, 1) << @intCast((bit_count - 1 - (i - offset))),
@ -99,6 +99,11 @@ test match {
try std.testing.expectEqual(true, match("1111_1111", @as(u8, 0b11111111)));
try std.testing.expectEqual(true, match("________11111111", @as(u8, 0b11111111)));
try std.testing.expectEqual(true, match("11111111________", @as(u8, 0b11111111)));
try std.testing.expectEqual(true, match(
"11111111_11111111_11111111_11111111_11111111_11111111_11111111_11111111",
@as(u64, 0xFFFF_FFFF_FFFF_FFFF),
));
}
/// Extracts the variables (defined in the bit string) from a value.
@ -134,15 +139,10 @@ pub fn extract(comptime bit_string: []const u8, value: anytype) Bitfield(bit_str
}
}
// TODO: decide at compile time if we're calling the 32-bit or 64-bit version of `PEXT`
// TODO: rewrite this invariant thing to account for underscores
const PextT = if (@typeInfo(ValT).Int.bits > 32) u64 else u32;
const use_hw = bmi2 and !@inComptime();
// invariant: the bit count in the field we're writing to and the
// # of bits we happened to find in this linear search are identical
//
// we're confident in this because it's guaranteed to be the same bit_string,
// and it's the same linear search. If you're reading this double check that this is still the case lol
break :blk @truncate(if (bmi2 and !@inComptime()) pext.hw(u32, value, masked_val) else pext.sw(u32, value, masked_val));
break :blk @truncate(if (use_hw) pext.hw(PextT, value, masked_val) else pext.sw(PextT, value, masked_val));
};
}
@ -211,6 +211,18 @@ test extract {
try std.testing.expectEqual(@as(usize, 0), @typeInfo(T).Struct.fields.len);
}
{
const ret = extract(
"11111111_ssssssss_11111111_dddddddd_11111111_vvvvvvvv_11111111_xxxxxxxx",
@as(u64, 0xFF55_FF77_FF33_FF00),
);
try std.testing.expectEqual(@as(u8, 0x55), ret.s);
try std.testing.expectEqual(@as(u8, 0x77), ret.d);
try std.testing.expectEqual(@as(u8, 0x33), ret.v);
try std.testing.expectEqual(@as(u8, 0x00), ret.x);
}
}
pub fn matchExtract(comptime bit_string: []const u8, value: anytype) ?Bitfield(bit_string) {
@ -285,9 +297,6 @@ pub fn Bitfield(comptime bit_string: []const u8) type {
fn verify(comptime T: type, comptime bit_string: []const u8) void {
const info = @typeInfo(T);
// FIXME: remove the need for this
if (info.Int.bits > 32) @compileError("TODO: 64-bit `PEXT` software implementation");
std.debug.assert(info != .ComptimeInt);
std.debug.assert(info.Int.signedness == .unsigned);
std.debug.assert(info.Int.bits <= 64); // x86 PEXT u32 and u64 operands only
@ -317,13 +326,13 @@ const pext = struct {
};
}
// why we need this: https://github.com/ziglang/zig/issues/14995 (ideally compiler-rt implements this for us)
fn sw(comptime T: type, value: T, mask: T) T {
// TODO: rewrite in idiomatic zig / understand the algorithm and write an original implementation
// code source: https://stackoverflow.com/questions/41720249/detecting-matching-bits-in-c
// note: will be replaced in the future by https://github.com/ziglang/zig/issues/14995 (hopefully?)
return switch (T) {
u32 => {
// TODO: Looks (and is) like C code :pensive:
// code source: https://stackoverflow.com/questions/41720249/detecting-matching-bits-in-c
var _value: T = value;
var _mask: T = mask;
@ -333,7 +342,7 @@ const pext = struct {
var mv: T = undefined;
var t: T = undefined;
inline for (0..@typeInfo(u5).Int.bits) |i| {
inline for (0..@typeInfo(Log2Int(T)).Int.bits) |i| {
mp = mk ^ (mk << 1); // parallel suffix
mp = mp ^ (mp << 2);
mp = mp ^ (mp << 4);
@ -348,7 +357,32 @@ const pext = struct {
return _value;
},
u64 => @compileError("TODO: find/write branchless software impl of `PEXT` for 64-bit values"),
u64 => {
var _value: T = value;
var _mask: T = mask;
_value &= _mask;
var mk: T = ~_mask << 1;
var mp: T = undefined;
var mv: T = undefined;
var t: T = undefined;
inline for (0..@typeInfo(Log2Int(T)).Int.bits) |i| {
mp = mk ^ (mk << 1); // parallel suffix
mp = mp ^ (mp << 2);
mp = mp ^ (mp << 4);
mp = mp ^ (mp << 8);
mp = mp ^ (mp << 16);
mp = mp ^ (mp << 32);
mv = (mp & _mask); // bits to move
_mask = ((_mask ^ mv) | (mv >> (1 << i))); // compress _mask
t = (_value & mv);
_value = ((_value ^ t) | (t >> (1 << i))); // compress _value
mk &= ~mp;
}
return _value;
},
else => @compileError("pext is sunsupported for " ++ @typeName(T) ++ "."),
};
}
@ -356,20 +390,26 @@ const pext = struct {
test pext {
const builtin = @import("builtin");
try std.testing.expectEqual(@as(u32, 0x0001_2567), pext.hw(u32, 0x12345678, 0xFF00FFF0));
try std.testing.expectEqual(@as(u64, 0x0001_2567), pext.hw(u64, 0x12345678, 0xFF00FFF0));
switch (builtin.cpu.arch) {
.x86_64 => if (std.Target.x86.featureSetHas(builtin.cpu.features, .bmi2)) {
try std.testing.expectEqual(@as(u32, 0x0001_2567), pext.hw(u32, 0x12345678, 0xFF00FFF0));
try std.testing.expectEqual(@as(u64, 0x0001_2567), pext.hw(u64, 0x12345678, 0xFF00FFF0));
// random tests
// TODO: when implemented, test 64-bit fallback `PEXT` as well
var rand_impl = std.rand.DefaultPrng.init(0xBAADF00D_DEADCAFE);
for (0..100) |_| {
const value = rand_impl.random().int(u32);
const mask = rand_impl.random().int(u32);
try std.testing.expectEqual(pext.hw(u32, value, mask), pext.sw(u32, value, mask));
}
for (0..100) |_| {
const value = rand_impl.random().int(u64);
const mask = rand_impl.random().int(u64);
try std.testing.expectEqual(pext.hw(u64, value, mask), pext.sw(u64, value, mask));
}
},
else => return error.SkipZigTest,
}