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expf

exp.expf
pub fn expf(x_: f32) callconv(.c) f32

File

lib/compiler_rt/exp.zig:36

Code

pub fn expf(x_: f32) callconv(.c) f32 {
    const half = [_]f32{ 0.5, -0.5 };
    const ln2hi = 6.9314575195e-1;
    const ln2lo = 1.4286067653e-6;
    const invln2 = 1.4426950216e+0;
    const P1 = 1.6666625440e-1;
    const P2 = -2.7667332906e-3;

    var x = x_;
    var hx: u32 = @bitCast(x);
    const sign: i32 = @intCast(hx >> 31);
    hx &= 0x7FFFFFFF;

    if (math.isNan(x)) {
        return x;
    }

    // |x| >= -87.33655 or nan
    if (hx >= 0x42AEAC50) {
        // nan
        if (hx > 0x7F800000) {
            return x;
        }
        // x >= 88.722839
        if (hx >= 0x42b17218 and sign == 0) {
            return x * 0x1.0p127;
        }
        if (sign != 0) {
            if (compiler_rt.want_float_exceptions) mem.doNotOptimizeAway(-0x1.0p-149 / x); // overflow
            // x <= -103.972084
            if (hx >= 0x42CFF1B5) {
                return 0;
            }
        }
    }

    var k: i32 = undefined;
    var hi: f32 = undefined;
    var lo: f32 = undefined;

    // |x| > 0.5 * ln2
    if (hx > 0x3EB17218) {
        // |x| > 1.5 * ln2
        if (hx > 0x3F851592) {
            k = @intFromFloat(invln2 * x + half[@intCast(sign)]);
        } else {
            k = 1 - sign - sign;
        }

        const fk: f32 = @floatFromInt(k);
        hi = x - fk * ln2hi;
        lo = fk * ln2lo;
        x = hi - lo;
    }
    // |x| > 2^(-14)
    else if (hx > 0x39000000) {
        k = 0;
        hi = x;
        lo = 0;
    } else {
        if (compiler_rt.want_float_exceptions) mem.doNotOptimizeAway(0x1.0p127 + x); // inexact
        return 1 + x;
    }

    const xx = x * x;
    const c = x - xx * (P1 + xx * P2);
    const y = 1 + (x * c / (2 - c) - lo + hi);

    if (k == 0) {
        return y;
    } else {
        return math.scalbn(y, k);
    }
}