67 lines
2.7 KiB
Zig
67 lines
2.7 KiB
Zig
const math = @import("std").math;
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const common = @import("./common.zig");
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const trunc_f80 = @import("./truncf.zig").trunc_f80;
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pub const panic = common.panic;
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comptime {
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@export(__trunctfxf2, .{ .name = "__trunctfxf2", .linkage = common.linkage });
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}
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pub fn __trunctfxf2(a: f128) callconv(.C) f80 {
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const src_sig_bits = math.floatMantissaBits(f128);
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const dst_sig_bits = math.floatMantissaBits(f80) - 1; // -1 for the integer bit
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// Various constants whose values follow from the type parameters.
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// Any reasonable optimizer will fold and propagate all of these.
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const src_bits = @typeInfo(f128).Float.bits;
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const src_exp_bits = src_bits - src_sig_bits - 1;
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const src_inf_exp = 0x7FFF;
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const src_inf = src_inf_exp << src_sig_bits;
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const src_sign_mask = 1 << (src_sig_bits + src_exp_bits);
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const src_abs_mask = src_sign_mask - 1;
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const round_mask = (1 << (src_sig_bits - dst_sig_bits)) - 1;
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const halfway = 1 << (src_sig_bits - dst_sig_bits - 1);
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// Break a into a sign and representation of the absolute value
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const a_rep = @bitCast(u128, a);
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const a_abs = a_rep & src_abs_mask;
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const sign: u16 = if (a_rep & src_sign_mask != 0) 0x8000 else 0;
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const integer_bit = 1 << 63;
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var res: math.F80 = undefined;
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if (a_abs > src_inf) {
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// a is NaN.
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// Conjure the result by beginning with infinity, setting the qNaN
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// bit and inserting the (truncated) trailing NaN field.
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res.exp = 0x7fff;
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res.fraction = 0x8000000000000000;
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res.fraction |= @truncate(u64, a_abs >> (src_sig_bits - dst_sig_bits));
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} else {
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// The exponent of a is within the range of normal numbers in the
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// destination format. We can convert by simply right-shifting with
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// rounding, adding the explicit integer bit, and adjusting the exponent
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res.fraction = @truncate(u64, a_abs >> (src_sig_bits - dst_sig_bits)) | integer_bit;
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res.exp = @truncate(u16, a_abs >> src_sig_bits);
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const round_bits = a_abs & round_mask;
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if (round_bits > halfway) {
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// Round to nearest
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const carry = @boolToInt(@addWithOverflow(u64, res.fraction, 1, &res.fraction));
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res.exp += carry;
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res.fraction |= @as(u64, carry) << 63; // Restore integer bit after carry
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} else if (round_bits == halfway) {
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// Ties to even
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const carry = @boolToInt(@addWithOverflow(u64, res.fraction, res.fraction & 1, &res.fraction));
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res.exp += carry;
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res.fraction |= @as(u64, carry) << 63; // Restore integer bit after carry
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}
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if (res.exp == 0) res.fraction &= ~@as(u64, integer_bit); // Remove integer bit for de-normals
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}
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res.exp |= sign;
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return math.make_f80(res);
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}
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