62 lines
2.1 KiB
Zig
62 lines
2.1 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 FLT_MANT_DIG = 24;
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pub extern fn __floatuntisf(arg: u128) f32 {
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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 a = arg;
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const N: u32 = @sizeOf(u128) * 8;
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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 > FLT_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 FLT_MANT_DIG-1 bits to the right of 1
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// Q = bit FLT_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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FLT_MANT_DIG + 1 => {
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a <<= 1;
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},
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FLT_MANT_DIG + 2 => {},
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else => {
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const shift_amt = @bitCast(i32, N + (FLT_MANT_DIG + 2)) - sd;
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const shift_amt_u7 = @intCast(u7, shift_amt);
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a = (a >> @intCast(u7, sd - (FLT_MANT_DIG + 2))) |
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@boolToInt((a & (u128(maxInt(u128)) >> shift_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 FLT_MANT_DIG or FLT_MANT_DIG+1 bits
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if ((a & (u128(1) << FLT_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 FLT_MANT_DIG bits
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} else {
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a <<= @intCast(u7, FLT_MANT_DIG - sd);
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// a is now rounded to FLT_MANT_DIG bits
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}
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const high = @bitCast(u32, (e + 127) << 23); // exponent
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const low = @truncate(u32, a) & 0x007fffff; // mantissa
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return @bitCast(f32, high | low);
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}
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test "import floatuntisf" {
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_ = @import("floatuntisf_test.zig");
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}
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