implement log10 and log10f

library/compiler-builtins/libm/src/math/log10f.rs

@@ -0,0 +1,76 @@
+use core::f32;
+
+const IVLN10HI: f32 = 4.3432617188e-01; /* 0x3ede6000 */
+const IVLN10LO: f32 = -3.1689971365e-05; /* 0xb804ead9 */
+const LOG10_2HI: f32 = 3.0102920532e-01; /* 0x3e9a2080 */
+const LOG10_2LO: f32 = 7.9034151668e-07; /* 0x355427db */
+/* |(log(1+s)-log(1-s))/s - Lg(s)| < 2**-34.24 (~[-4.95e-11, 4.97e-11]). */
+const LG1: f32 = 0.66666662693; /* 0xaaaaaa.0p-24 */
+const LG2: f32 = 0.40000972152; /* 0xccce13.0p-25 */
+const LG3: f32 = 0.28498786688; /* 0x91e9ee.0p-25 */
+const LG4: f32 = 0.24279078841; /* 0xf89e26.0p-26 */
+
+#[inline]
+pub fn log10f(mut x: f32) -> f32 {
+    let x1p25f = f32::from_bits(0x4c000000); // 0x1p25f === 2 ^ 25
+
+    let mut ui: u32 = x.to_bits();
+    let hfsq: f32;
+    let f: f32;
+    let s: f32;
+    let z: f32;
+    let r: f32;
+    let w: f32;
+    let t1: f32;
+    let t2: f32;
+    let dk: f32;
+    let mut hi: f32;
+    let lo: f32;
+    let mut ix: u32;
+    let mut k: i32;
+
+    ix = ui;
+    k = 0;
+    if ix < 0x00800000 || (ix >> 31) > 0 {
+        /* x < 2**-126  */
+        if ix << 1 == 0 {
+            return -1. / (x * x); /* log(+-0)=-inf */
+        }
+        if (ix >> 31) > 0 {
+            return (x - x) / 0.0; /* log(-#) = NaN */
+        }
+        /* subnormal number, scale up x */
+        k -= 25;
+        x *= x1p25f;
+        ui = x.to_bits();
+        ix = ui;
+    } else if ix >= 0x7f800000 {
+        return x;
+    } else if ix == 0x3f800000 {
+        return 0.;
+    }
+
+    /* reduce x into [sqrt(2)/2, sqrt(2)] */
+    ix += 0x3f800000 - 0x3f3504f3;
+    k += (ix >> 23) as i32 - 0x7f;
+    ix = (ix & 0x007fffff) + 0x3f3504f3;
+    ui = ix;
+    x = f32::from_bits(ui);
+
+    f = x - 1.0;
+    s = f / (2.0 + f);
+    z = s * s;
+    w = z * z;
+    t1 = w * (LG2 + w * LG4);
+    t2 = z * (LG1 + w * LG3);
+    r = t2 + t1;
+    hfsq = 0.5 * f * f;
+
+    hi = f - hfsq;
+    ui = hi.to_bits();
+    ui &= 0xfffff000;
+    hi = f32::from_bits(ui);
+    lo = f - hi - hfsq + s * (hfsq + r);
+    dk = k as f32;
+    return dk * LOG10_2LO + (lo + hi) * IVLN10LO + lo * IVLN10HI + hi * IVLN10HI + dk * LOG10_2HI;
+}