Commit 0550198c9877c37c53b67f853968e3e4ce89c2ef | zig

author	Jan Philipp Hafer <jan.hafer@rwth-aachen.de> 2021-12-13 02:36:24+01:00
committer	Andrew Kelley <andrew@ziglang.org> 2021-12-14 14:19:51-08:00
log	0550198c9877c37c53b67f853968e3e4ce89c2ef
tree	3815ddb4a3ceee8d5ca5f6c7c08a936505a49191
parent	eb1e75b2b8e788a3d5b590a0061cb736f8183fd3

compiler_rt: simplify popcount "magic constants"

- magic constants are nicer to construct ie with
  (~@as(unsigned type, 0) / 3) == 0x55...55
- thanks to Stefan Kanthak for the idea

1 files changed, 16 insertions(+), 35 deletions(-)

lib
- std
  - special
    compiler_rt
    popcount.zig

lib/std/special/compiler_rt/popcount.zig+16-35

...	@@ -9,45 +9,26 @@ const std = @import("std");	...	@@ -9,45 +9,26 @@ const std = @import("std");
9	// SWAR-Masks and factors can be defined as 2-adic fractions	9	// SWAR-Masks and factors can be defined as 2-adic fractions
10	// TAOCP: Combinational Algorithms, Bitwise Tricks And Techniques,	10	// TAOCP: Combinational Algorithms, Bitwise Tricks And Techniques,
11	// subsubsection "Working with the rightmost bits" and "Sideways addition".	11	// subsubsection "Working with the rightmost bits" and "Sideways addition".
12	fn popcountXi2_generic(comptime T: type) fn (a: T) callconv(.C) i32 {	12
		13	fn popcountXi2_generic(comptime ST: type) fn (a: ST) callconv(.C) i32 {
13	return struct {	14	return struct {
14	fn f(a: T) callconv(.C) i32 {	15	fn f(a: ST) callconv(.C) i32 {
15	@setRuntimeSafety(builtin.is_test);	16	@setRuntimeSafety(builtin.is_test);
16		17	const UT = switch (ST) {
17	var x = switch (@bitSizeOf(T)) {	18	i32 => u32,
18	32 => @bitCast(u32, a),	19	i64 => u64,
19	64 => @bitCast(u64, a),	20	i128 => u128,
20	128 => @bitCast(u128, a),
21	else => unreachable,
22	};
23	const k1 = switch (@bitSizeOf(T)) { // -1/3
24	32 => @as(u32, 0x55555555),
25	64 => @as(u64, 0x55555555_55555555),
26	128 => @as(u128, 0x55555555_55555555_55555555_55555555),
27	else => unreachable,
28	};
29	const k2 = switch (@bitSizeOf(T)) { // -1/5
30	32 => @as(u32, 0x33333333),
31	64 => @as(u64, 0x33333333_33333333),
32	128 => @as(u128, 0x33333333_33333333_33333333_33333333),
33	else => unreachable,
34	};
35	const k4 = switch (@bitSizeOf(T)) { // -1/17
36	32 => @as(u32, 0x0f0f0f0f),
37	64 => @as(u64, 0x0f0f0f0f_0f0f0f0f),
38	128 => @as(u128, 0x0f0f0f0f_0f0f0f0f_0f0f0f0f_0f0f0f0f),
39	else => unreachable,
40	};
41	const kf = switch (@bitSizeOf(T)) { // -1/255
42	32 => @as(u32, 0x01010101),
43	64 => @as(u64, 0x01010101_01010101),
44	128 => @as(u128, 0x01010101_01010101_01010101_01010101),
45	else => unreachable,	21	else => unreachable,
46	};	22	};
47	x = x - ((x >> 1) & k1); // aggregate duos	23	var x = @bitCast(UT, a);
48	x = (x & k2) + ((x >> 2) & k2); // aggregate nibbles	24	x -= (x >> 1) & (~@as(UT, 0) / 3); // 0x55...55, aggregate duos
49	x = (x + (x >> 4)) & k4; // aggregate bytes	25	x = ((x >> 2) & (~@as(UT, 0) / 5)) // 0x33...33, aggregate nibbles
50	x = (x *% kf) >> @bitSizeOf(T) - 8; // 8 most significant bits of x + (x<<8) + (x<<16) + ..	26	+ (x & (~@as(UT, 0) / 5));
		27	x += x >> 4;
		28	x &= ~@as(UT, 0) / 17; // 0x0F...0F, aggregate bytes
		29	// 8 most significant bits of x + (x<<8) + (x<<16) + ..
		30	x *%= ~@as(UT, 0) / 255; // 0x01...01
		31	x >>= (@bitSizeOf(ST) - 8);
51	return @intCast(i32, x);	32	return @intCast(i32, x);
52	}	33	}
53	}.f;	34	}.f;