72 lines
2.4 KiB
Zig
72 lines
2.4 KiB
Zig
const builtin = @import("builtin");
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const is_test = builtin.is_test;
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const std = @import("std");
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const maxInt = std.math.maxInt;
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const DBL_MANT_DIG = 53;
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pub extern fn __floattidf(arg: i128) f64 {
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@setRuntimeSafety(is_test);
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if (arg == 0)
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return 0.0;
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var ai = arg;
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const N: u32 = 128;
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const si = ai >> @intCast(u7, (N - 1));
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ai = ((ai ^ si) -% si);
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var a = @bitCast(u128, ai);
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const sd = @bitCast(i32, N - @clz(u128, a)); // number of significant digits
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var e: i32 = sd - 1; // exponent
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if (sd > DBL_MANT_DIG) {
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// start: 0000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQxxxxxxxxxxxxxxxxxx
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// finish: 000000000000000000000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQR
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// 12345678901234567890123456
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// 1 = msb 1 bit
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// P = bit DBL_MANT_DIG-1 bits to the right of 1
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// Q = bit DBL_MANT_DIG bits to the right of 1
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// R = "or" of all bits to the right of Q
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switch (sd) {
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DBL_MANT_DIG + 1 => {
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a <<= 1;
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},
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DBL_MANT_DIG + 2 => {},
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else => {
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const shift1_amt = @intCast(i32, sd - (DBL_MANT_DIG + 2));
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const shift1_amt_u7 = @intCast(u7, shift1_amt);
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const shift2_amt = @intCast(i32, N + (DBL_MANT_DIG + 2)) - sd;
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const shift2_amt_u7 = @intCast(u7, shift2_amt);
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a = (a >> shift1_amt_u7) | @boolToInt((a & (@intCast(u128, maxInt(u128)) >> shift2_amt_u7)) != 0);
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},
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}
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// finish
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a |= @boolToInt((a & 4) != 0); // Or P into R
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a += 1; // round - this step may add a significant bit
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a >>= 2; // dump Q and R
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// a is now rounded to DBL_MANT_DIG or DBL_MANT_DIG+1 bits
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if ((a & (u128(1) << DBL_MANT_DIG)) != 0) {
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a >>= 1;
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e += 1;
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}
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// a is now rounded to DBL_MANT_DIG bits
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} else {
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a <<= @intCast(u7, DBL_MANT_DIG - sd);
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// a is now rounded to DBL_MANT_DIG bits
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}
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const s = @bitCast(u128, arg) >> (128 - 32);
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const high: u64 = (@intCast(u64, s) & 0x80000000) | // sign
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(@intCast(u32, (e + 1023)) << 20) | // exponent
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(@truncate(u32, a >> 32) & 0x000fffff); // mantissa-high
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const low: u64 = @truncate(u32, a); // mantissa-low
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return @bitCast(f64, low | (high << 32));
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}
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test "import floattidf" {
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_ = @import("floattidf_test.zig");
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}
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