path: root/src/math/exp10l.c
diff options
authorRich Felker <>2012-04-30 03:26:53 -0400
committerRich Felker <>2012-04-30 03:26:53 -0400
commitf6819755779a084bf2f82cb90175a4d9a018de73 (patch)
tree94048c26dd47a1f82507421df1d8815e14675197 /src/math/exp10l.c
parent63374ee233c44891db80e6600d7a5a8c82e4ccca (diff)
first try at writing an efficient and "correct" exp10
this is a nonstandard function so it's not clear what conditions it should satisfy. my intent is that it be fast and exact for positive integral exponents when the result fits in the destination type, and fast and correctly rounded for small negative integral exponents. otherwise we aim for at most 1ulp error; it seems to differ from pow by at most 1ulp and it's often 2-5 times faster than pow.
Diffstat (limited to 'src/math/exp10l.c')
1 files changed, 19 insertions, 0 deletions
diff --git a/src/math/exp10l.c b/src/math/exp10l.c
new file mode 100644
index 00000000..4d0c5a01
--- /dev/null
+++ b/src/math/exp10l.c
@@ -0,0 +1,19 @@
+#define _GNU_SOURCE
+#include <math.h>
+long double exp10l(long double x)
+ static const long double p10[] = {
+ 1e-15, 1e-14, 1e-13, 1e-12, 1e-11, 1e-10,
+ 1e-9, 1e-8, 1e-7, 1e-6, 1e-5, 1e-4, 1e-3, 1e-2, 1e-1,
+ 1, 1e1, 1e2, 1e3, 1e4, 1e5, 1e6, 1e7, 1e8, 1e9,
+ 1e10, 1e11, 1e12, 1e13, 1e14, 1e15
+ };
+ long double n, y = modfl(x, &n);
+ if (fabsl(n) < 16) {
+ if (!y) return p10[(int)n+15];
+ y = exp2l(3.32192809488736234787031942948939L * y);
+ return y * p10[(int)n+15];
+ }
+ return powl(10.0, x);