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compiler_rt: simplify popcount "magic constants"

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- magic constants are nicer to construct ie with
  (~@as(unsigned type, 0) / 3) == 0x55...55
- thanks to Stefan Kanthak for the idea
This commit is contained in:
Jan Philipp Hafer 2021-12-13 02:36:24 +01:00 committed by Andrew Kelley
parent eb1e75b2b8
commit 0550198c98
1 changed files with 17 additions and 36 deletions
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53
lib/std/special/compiler_rt/popcount.zig
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@ -9,45 +9,26 @@ const std = @import("std");
// SWAR-Masks and factors can be defined as 2-adic fractions
// TAOCP: Combinational Algorithms, Bitwise Tricks And Techniques,
// subsubsection "Working with the rightmost bits" and "Sideways addition".
fn popcountXi2_generic(comptime T: type) fn (a: T) callconv(.C) i32 {
return struct {
fn f(a: T) callconv(.C) i32 {
@setRuntimeSafety(builtin.is_test);
var x = switch (@bitSizeOf(T)) {
32 => @bitCast(u32, a),
64 => @bitCast(u64, a),
128 => @bitCast(u128, a),
fn popcountXi2_generic(comptime ST: type) fn (a: ST) callconv(.C) i32 {
return struct {
fn f(a: ST) callconv(.C) i32 {
@setRuntimeSafety(builtin.is_test);
const UT = switch (ST) {
i32 => u32,
i64 => u64,
i128 => u128,
else => unreachable,
};
const k1 = switch (@bitSizeOf(T)) { // -1/3
32 => @as(u32, 0x55555555),
64 => @as(u64, 0x55555555_55555555),
128 => @as(u128, 0x55555555_55555555_55555555_55555555),
else => unreachable,
};
const k2 = switch (@bitSizeOf(T)) { // -1/5
32 => @as(u32, 0x33333333),
64 => @as(u64, 0x33333333_33333333),
128 => @as(u128, 0x33333333_33333333_33333333_33333333),
else => unreachable,
};
const k4 = switch (@bitSizeOf(T)) { // -1/17
32 => @as(u32, 0x0f0f0f0f),
64 => @as(u64, 0x0f0f0f0f_0f0f0f0f),
128 => @as(u128, 0x0f0f0f0f_0f0f0f0f_0f0f0f0f_0f0f0f0f),
else => unreachable,
};
const kf = switch (@bitSizeOf(T)) { // -1/255
32 => @as(u32, 0x01010101),
64 => @as(u64, 0x01010101_01010101),
128 => @as(u128, 0x01010101_01010101_01010101_01010101),
else => unreachable,
};
x = x - ((x >> 1) & k1); // aggregate duos
x = (x & k2) + ((x >> 2) & k2); // aggregate nibbles
x = (x + (x >> 4)) & k4; // aggregate bytes
x = (x *% kf) >> @bitSizeOf(T) - 8; // 8 most significant bits of x + (x<<8) + (x<<16) + ..
var x = @bitCast(UT, a);
x -= (x >> 1) & (~@as(UT, 0) / 3); // 0x55...55, aggregate duos
x = ((x >> 2) & (~@as(UT, 0) / 5)) // 0x33...33, aggregate nibbles
+ (x & (~@as(UT, 0) / 5));
x += x >> 4;
x &= ~@as(UT, 0) / 17; // 0x0F...0F, aggregate bytes
// 8 most significant bits of x + (x<<8) + (x<<16) + ..
x *%= ~@as(UT, 0) / 255; // 0x01...01
x >>= (@bitSizeOf(ST) - 8);
return @intCast(i32, x);
}
}.f;