c7804277bf
when the integer part does not fit in the destination integer type * Also fix incorrect safety triggered for integer casting an `i32` to a `u7`. closes #1138 * adds compiler-rt function: `__floatuntidf`
61 lines
2.1 KiB
Zig
61 lines
2.1 KiB
Zig
const builtin = @import("builtin");
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const is_test = builtin.is_test;
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const DBL_MANT_DIG = 53;
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pub extern fn __floatuntidf(arg: u128) 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 a = arg;
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const N: u32 = @sizeOf(u128) * 8;
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const sd = @bitCast(i32, N -% @clz(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 shift_amt = @bitCast(i32, N +% (DBL_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 -% (DBL_MANT_DIG + 2))) |
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@boolToInt((a & (u128(@maxValue(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 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 high: u64 = @bitCast(u32, (e +% 1023) << 20) | // exponent
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(@truncate(u32, a >> 32) & 0x000FFFFF); // mantissa-high
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const low = @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 floatuntidf" {
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_ = @import("floatuntidf_test.zig");
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}
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