61 lines
2.1 KiB
Zig
61 lines
2.1 KiB
Zig
const builtin = @import("builtin");
|
|
const is_test = builtin.is_test;
|
|
|
|
const DBL_MANT_DIG = 53;
|
|
|
|
pub extern fn __floatuntidf(arg: u128) f64 {
|
|
@setRuntimeSafety(is_test);
|
|
|
|
if (arg == 0)
|
|
return 0.0;
|
|
|
|
var a = arg;
|
|
const N: u32 = @sizeOf(u128) * 8;
|
|
const sd = @bitCast(i32, N - @clz(a)); // number of significant digits
|
|
var e: i32 = sd - 1; // exponent
|
|
if (sd > DBL_MANT_DIG) {
|
|
// start: 0000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQxxxxxxxxxxxxxxxxxx
|
|
// finish: 000000000000000000000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQR
|
|
// 12345678901234567890123456
|
|
// 1 = msb 1 bit
|
|
// P = bit DBL_MANT_DIG-1 bits to the right of 1
|
|
// Q = bit DBL_MANT_DIG bits to the right of 1
|
|
// R = "or" of all bits to the right of Q
|
|
switch (sd) {
|
|
DBL_MANT_DIG + 1 => {
|
|
a <<= 1;
|
|
},
|
|
DBL_MANT_DIG + 2 => {},
|
|
else => {
|
|
const shift_amt = @bitCast(i32, N + (DBL_MANT_DIG + 2)) - sd;
|
|
const shift_amt_u7 = @intCast(u7, shift_amt);
|
|
a = (a >> @intCast(u7, sd - (DBL_MANT_DIG + 2))) |
|
|
@boolToInt((a & (u128(@maxValue(u128)) >> shift_amt_u7)) != 0);
|
|
},
|
|
}
|
|
// finish
|
|
a |= @boolToInt((a & 4) != 0); // Or P into R
|
|
a += 1; // round - this step may add a significant bit
|
|
a >>= 2; // dump Q and R
|
|
// a is now rounded to DBL_MANT_DIG or DBL_MANT_DIG+1 bits
|
|
if ((a & (u128(1) << DBL_MANT_DIG)) != 0) {
|
|
a >>= 1;
|
|
e += 1;
|
|
}
|
|
// a is now rounded to DBL_MANT_DIG bits
|
|
} else {
|
|
a <<= @intCast(u7, DBL_MANT_DIG - sd);
|
|
// a is now rounded to DBL_MANT_DIG bits
|
|
}
|
|
|
|
const high: u64 = @bitCast(u32, (e + 1023) << 20) | // exponent
|
|
(@truncate(u32, a >> 32) & 0x000FFFFF); // mantissa-high
|
|
const low = @truncate(u32, a); // mantissa-low
|
|
|
|
return @bitCast(f64, low | (high << 32));
|
|
}
|
|
|
|
test "import floatuntidf" {
|
|
_ = @import("floatuntidf_test.zig");
|
|
}
|