zig/lib/compiler_rt/bswap.zig

const std = @import("std");
const builtin = @import("builtin");
const is_test = builtin.is_test;
const linkage: std.builtin.GlobalLinkage = if (builtin.is_test) .Internal else .Weak;
pub const panic = @import("common.zig").panic;

comptime {
    @export(__bswapsi2, .{ .name = "__bswapsi2", .linkage = linkage });
    @export(__bswapdi2, .{ .name = "__bswapdi2", .linkage = linkage });
    @export(__bswapti2, .{ .name = "__bswapti2", .linkage = linkage });
}

// bswap - byteswap
// - bswapXi2 for unoptimized big and little endian
// ie for u32
// DE AD BE EF   <- little|big endian
// FE BE AD DE   <- big|little endian
// ff 00 00 00 >> 3*8 (leftmost  byte)
// 00 ff 00 00 >> 1*8 (2nd left  byte)
// 00 00 ff 00 << 1*8 (2n right  byte)
// 00 00 00 ff << 3*8 (rightmost byte)

inline fn bswapXi2(comptime T: type, a: T) T {
    @setRuntimeSafety(builtin.is_test);
    switch (@bitSizeOf(T)) {
        32 => {
            // zig fmt: off
            return (((a & 0xff000000) >> 24)
                 |  ((a & 0x00ff0000) >> 8 )
                 |  ((a & 0x0000ff00) << 8 )
                 |  ((a & 0x000000ff) << 24));
            // zig fmt: on
        },
        64 => {
            // zig fmt: off
            return (((a & 0xff00000000000000) >> 56)
                 |  ((a & 0x00ff000000000000) >> 40 )
                 |  ((a & 0x0000ff0000000000) >> 24 )
                 |  ((a & 0x000000ff00000000) >> 8 )
                 |  ((a & 0x00000000ff000000) << 8 )
                 |  ((a & 0x0000000000ff0000) << 24 )
                 |  ((a & 0x000000000000ff00) << 40 )
                 |  ((a & 0x00000000000000ff) << 56));
            // zig fmt: on
        },
        128 => {
            // zig fmt: off
            return (((a & 0xff000000000000000000000000000000) >> 120)
                 |  ((a & 0x00ff0000000000000000000000000000) >> 104)
                 |  ((a & 0x0000ff00000000000000000000000000) >> 88 )
                 |  ((a & 0x000000ff000000000000000000000000) >> 72 )
                 |  ((a & 0x00000000ff0000000000000000000000) >> 56 )
                 |  ((a & 0x0000000000ff00000000000000000000) >> 40 )
                 |  ((a & 0x000000000000ff000000000000000000) >> 24 )
                 |  ((a & 0x00000000000000ff0000000000000000) >> 8  )
                 |  ((a & 0x0000000000000000ff00000000000000) << 8  )
                 |  ((a & 0x000000000000000000ff000000000000) << 24 )
                 |  ((a & 0x00000000000000000000ff0000000000) << 40 )
                 |  ((a & 0x0000000000000000000000ff00000000) << 56 )
                 |  ((a & 0x000000000000000000000000ff000000) << 72 )
                 |  ((a & 0x00000000000000000000000000ff0000) << 88 )
                 |  ((a & 0x0000000000000000000000000000ff00) << 104)
                 |  ((a & 0x000000000000000000000000000000ff) << 120));
            // zig fmt: on
        },
        else => unreachable,
    }
}

pub fn __bswapsi2(a: u32) callconv(.C) u32 {
    return bswapXi2(u32, a);
}

pub fn __bswapdi2(a: u64) callconv(.C) u64 {
    return bswapXi2(u64, a);
}

pub fn __bswapti2(a: u128) callconv(.C) u128 {
    return bswapXi2(u128, a);
}

test {
    _ = @import("bswapsi2_test.zig");
    _ = @import("bswapdi2_test.zig");
    _ = @import("bswapti2_test.zig");
}