implement exp

library/compiler-builtins/libm/src/lib.rs

@@ -378,7 +378,6 @@ pub trait F64Ext: private::Sealed {
 
     fn sqrt(self) -> Self;
 
-    #[cfg(todo)]
     fn exp(self) -> Self;
 
     #[cfg(todo)]
@@ -518,7 +517,6 @@ impl F64Ext for f64 {
         sqrt(self)
     }
 
-    #[cfg(todo)]
     #[inline]
     fn exp(self) -> Self {
         exp(self)

library/compiler-builtins/libm/src/math/exp.rs

@@ -0,0 +1,150 @@
+/* origin: FreeBSD /usr/src/lib/msun/src/e_exp.c */
+/*
+ * ====================================================
+ * Copyright (C) 2004 by Sun Microsystems, Inc. All rights reserved.
+ *
+ * Permission to use, copy, modify, and distribute this
+ * software is freely granted, provided that this notice
+ * is preserved.
+ * ====================================================
+ */
+/* exp(x)
+ * Returns the exponential of x.
+ *
+ * Method
+ *   1. Argument reduction:
+ *      Reduce x to an r so that |r| <= 0.5*ln2 ~ 0.34658.
+ *      Given x, find r and integer k such that
+ *
+ *               x = k*ln2 + r,  |r| <= 0.5*ln2.
+ *
+ *      Here r will be represented as r = hi-lo for better
+ *      accuracy.
+ *
+ *   2. Approximation of exp(r) by a special rational function on
+ *      the interval [0,0.34658]:
+ *      Write
+ *          R(r**2) = r*(exp(r)+1)/(exp(r)-1) = 2 + r*r/6 - r**4/360 + ...
+ *      We use a special Remez algorithm on [0,0.34658] to generate
+ *      a polynomial of degree 5 to approximate R. The maximum error
+ *      of this polynomial approximation is bounded by 2**-59. In
+ *      other words,
+ *          R(z) ~ 2.0 + P1*z + P2*z**2 + P3*z**3 + P4*z**4 + P5*z**5
+ *      (where z=r*r, and the values of P1 to P5 are listed below)
+ *      and
+ *          |                  5          |     -59
+ *          | 2.0+P1*z+...+P5*z   -  R(z) | <= 2
+ *          |                             |
+ *      The computation of exp(r) thus becomes
+ *                              2*r
+ *              exp(r) = 1 + ----------
+ *                            R(r) - r
+ *                                 r*c(r)
+ *                     = 1 + r + ----------- (for better accuracy)
+ *                                2 - c(r)
+ *      where
+ *                              2       4             10
+ *              c(r) = r - (P1*r  + P2*r  + ... + P5*r   ).
+ *
+ *   3. Scale back to obtain exp(x):
+ *      From step 1, we have
+ *         exp(x) = 2^k * exp(r)
+ *
+ * Special cases:
+ *      exp(INF) is INF, exp(NaN) is NaN;
+ *      exp(-INF) is 0, and
+ *      for finite argument, only exp(0)=1 is exact.
+ *
+ * Accuracy:
+ *      according to an error analysis, the error is always less than
+ *      1 ulp (unit in the last place).
+ *
+ * Misc. info.
+ *      For IEEE double
+ *          if x >  709.782712893383973096 then exp(x) overflows
+ *          if x < -745.133219101941108420 then exp(x) underflows
+ */
+
+use super::scalbn;
+
+const HALF: [f64; 2] = [0.5, -0.5];
+const LN2HI: f64 = 6.93147180369123816490e-01; /* 0x3fe62e42, 0xfee00000 */
+const LN2LO: f64 = 1.90821492927058770002e-10; /* 0x3dea39ef, 0x35793c76 */
+const INVLN2: f64 = 1.44269504088896338700e+00; /* 0x3ff71547, 0x652b82fe */
+const P1: f64 = 1.66666666666666019037e-01; /* 0x3FC55555, 0x5555553E */
+const P2: f64 = -2.77777777770155933842e-03; /* 0xBF66C16C, 0x16BEBD93 */
+const P3: f64 = 6.61375632143793436117e-05; /* 0x3F11566A, 0xAF25DE2C */
+const P4: f64 = -1.65339022054652515390e-06; /* 0xBEBBBD41, 0xC5D26BF1 */
+const P5: f64 = 4.13813679705723846039e-08; /* 0x3E663769, 0x72BEA4D0 */
+
+#[inline]
+pub fn exp(mut x: f64) -> f64 {
+    let x1p1023 = f64::from_bits(0x7fe0000000000000); // 0x1p1023 === 2 ^ 1023
+    let x1p_149 = f64::from_bits(0x36a0000000000000); // 0x1p-149 === 2 ^ -149
+
+    let hi: f64;
+    let lo: f64;
+    let c: f64;
+    let xx: f64;
+    let y: f64;
+    let k: i32;
+    let sign: i32;
+    let mut hx: u32;
+
+    hx = (x.to_bits() >> 32) as u32;
+    sign = (hx >> 31) as i32;
+    hx &= 0x7fffffff; /* high word of |x| */
+
+    /* special cases */
+    if hx >= 0x4086232b {
+        /* if |x| >= 708.39... */
+        if x.is_nan() {
+            return x;
+        }
+        if x > 709.782712893383973096 {
+            /* overflow if x!=inf */
+            x *= x1p1023;
+            return x;
+        }
+        if x < -708.39641853226410622 {
+            /* underflow if x!=-inf */
+            force_eval!((-x1p_149 / x) as f32);
+            if x < -745.13321910194110842 {
+                return 0.;
+            }
+        }
+    }
+
+    /* argument reduction */
+    if hx > 0x3fd62e42 {
+        /* if |x| > 0.5 ln2 */
+        if hx >= 0x3ff0a2b2 {
+            /* if |x| >= 1.5 ln2 */
+            k = (INVLN2 * x + HALF[sign as usize]) as i32;
+        } else {
+            k = 1 - sign - sign;
+        }
+        hi = x - k as f64 * LN2HI; /* k*ln2hi is exact here */
+        lo = k as f64 * LN2LO;
+        x = hi - lo;
+    } else if hx > 0x3e300000 {
+        /* if |x| > 2**-28 */
+        k = 0;
+        hi = x;
+        lo = 0.;
+    } else {
+        /* inexact if x!=0 */
+        force_eval!(x1p1023 + x);
+        return 1. + x;
+    }
+
+    /* x is now in primary range */
+    xx = x * x;
+    c = x - xx * (P1 + xx * (P2 + xx * (P3 + xx * (P4 + xx * P5))));
+    y = 1. + (x * c / (2. - c) - lo + hi);
+    if k == 0 {
+        y
+    } else {
+        scalbn(y, k)
+    }
+}

library/compiler-builtins/libm/src/math/mod.rs

@@ -9,6 +9,7 @@ macro_rules! force_eval {
 mod ceil;
 mod ceilf;
 mod cosf;
+mod exp;
 mod expf;
 mod fabs;
 mod fabsf;
@@ -39,6 +40,7 @@ mod truncf;
 pub use self::ceil::ceil;
 pub use self::ceilf::ceilf;
 pub use self::cosf::cosf;
+pub use self::exp::exp;
 pub use self::expf::expf;
 pub use self::fabs::fabs;
 pub use self::fabsf::fabsf;

library/compiler-builtins/libm/test-generator/src/main.rs

@@ -703,7 +703,7 @@ f64_f64! {
     ceil,
     // cos,
     // cosh,
-    // exp,
+    exp,
     // exp2,
     // expm1,
     floor,