Speed up dec2flt fast path with additional tables.

src/etc/dec2flt_table.py

@@ -25,6 +25,7 @@
 """
 from __future__ import print_function
 import sys
+from math import ceil, log
 from fractions import Fraction
 from collections import namedtuple
 
@@ -33,7 +34,6 @@
 MIN_SIG = 2 ** (N - 1)
 MAX_SIG = (2 ** N) - 1
 
-
 # Hand-rolled fp representation without arithmetic or any other operations.
 # The significand is normalized and always N bit, but the exponent is
 # unrestricted in range.
@@ -92,7 +92,7 @@ def error(f, e, z):
     ulp_err = abs_err / Fraction(2) ** z.exp
     return float(ulp_err)
 
-LICENSE = """
+HEADER = """
 // Copyright 2015 The Rust Project Developers. See the COPYRIGHT
 // file at the top-level directory of this distribution and at
 // http://rust-lang.org/COPYRIGHT.
@@ -102,9 +102,23 @@ def error(f, e, z):
 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
+
+//! Tables of approximations of powers of ten.
+//! DO NOT MODIFY: Generated by `src/etc/dec2flt_table.py`
 """
 
+
 def main():
+    print(HEADER.strip())
+    print()
+    print_proper_powers()
+    print()
+    print_short_powers(32, 24)
+    print()
+    print_short_powers(64, 53)
+
+
+def print_proper_powers():
     MIN_E = -305
     MAX_E = 305
     e_range = range(MIN_E, MAX_E+1)
@@ -114,13 +128,10 @@ def main():
         err = error(1, e, z)
         assert err < 0.5
         powers.append(z)
-    typ = "([u64; {0}], [i16; {0}])".format(len(e_range))
-    print(LICENSE.strip())
-    print("// Table of approximations of powers of ten.")
-    print("// DO NOT MODIFY: Generated by a src/etc/dec2flt_table.py")
     print("pub const MIN_E: i16 = {};".format(MIN_E))
     print("pub const MAX_E: i16 = {};".format(MAX_E))
     print()
+    typ = "([u64; {0}], [i16; {0}])".format(len(powers))
     print("pub const POWERS: ", typ, " = ([", sep='')
     for z in powers:
         print("    0x{:x},".format(z.sig))
@@ -130,5 +141,17 @@ def main():
     print("]);")
 
 
+def print_short_powers(num_bits, significand_size):
+    max_sig = 2**significand_size - 1
+    # The fast path bails out for exponents >= ceil(log5(max_sig))
+    max_e = int(ceil(log(max_sig, 5)))
+    e_range = range(max_e)
+    typ = "[f{}; {}]".format(num_bits, len(e_range))
+    print("pub const F", num_bits, "_SHORT_POWERS: ", typ, " = [", sep='')
+    for e in e_range:
+        print("    1e{},".format(e))
+    print("];")
+
+
 if __name__ == '__main__':
     main()

src/libcore/num/dec2flt/algorithm.rs

@@ -60,17 +60,13 @@ pub fn fast_path<T: RawFloat>(integral: &[u8], fractional: &[u8], e: i64) -> Opt
     if f > T::max_sig() {
         return None;
     }
-    let e = e as i16; // Can't overflow because e.abs() <= LOG5_OF_EXP_N
     // The case e < 0 cannot be folded into the other branch. Negative powers result in
     // a repeating fractional part in binary, which are rounded, which causes real
     // (and occasioally quite significant!) errors in the final result.
-    // The case `e == 0`, however, is unnecessary for correctness. It's just measurably faster.
-    if e == 0 {
-        Some(T::from_int(f))
-    } else if e > 0 {
-        Some(T::from_int(f) * fp_to_float(power_of_ten(e)))
+    if e >= 0 {
+        Some(T::from_int(f) * T::short_fast_pow10(e as usize))
     } else {
-        Some(T::from_int(f) / fp_to_float(power_of_ten(-e)))
+        Some(T::from_int(f) / T::short_fast_pow10(e.abs() as usize))
     }
 }
 

src/libcore/num/dec2flt/rawfp.rs

@@ -37,6 +37,7 @@ use num::diy_float::Fp;
 use num::FpCategory::{Infinite, Zero, Subnormal, Normal, Nan};
 use num::Float;
 use num::dec2flt::num::{self, Big};
+use num::dec2flt::table;
 
 #[derive(Copy, Clone, Debug)]
 pub struct Unpacked {
@@ -73,6 +74,9 @@ pub trait RawFloat : Float + Copy + Debug + LowerExp
     /// represented, the other code in this module makes sure to never let that happen.
     fn from_int(x: u64) -> Self;
 
+    /// Get the value 10^e from a pre-computed table. Panics for e >= ceil_log5_of_max_sig().
+    fn short_fast_pow10(e: usize) -> Self;
+
     // FIXME Everything that follows should be associated constants, but taking the value of an
     // associated constant from a type parameter does not work (yet?)
     // A possible workaround is having a `FloatInfo` struct for all the constants, but so far
@@ -175,6 +179,10 @@ impl RawFloat for f32 {
         x as f32
     }
 
+    fn short_fast_pow10(e: usize) -> Self {
+        table::F32_SHORT_POWERS[e]
+    }
+
     fn max_normal_digits() -> usize {
         35
     }
@@ -222,6 +230,10 @@ impl RawFloat for f64 {
         x as f64
     }
 
+    fn short_fast_pow10(e: usize) -> Self {
+        table::F64_SHORT_POWERS[e]
+    }
+
     fn max_normal_digits() -> usize {
         305
     }

src/libcore/num/dec2flt/table.rs

@@ -7,8 +7,10 @@
 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
-// Table of approximations of powers of ten.
-// DO NOT MODIFY: Generated by a src/etc/dec2flt_table.py
+
+//! Tables of approximations of powers of ten.
+//! DO NOT MODIFY: Generated by `src/etc/dec2flt_table.py`
+
 pub const MIN_E: i16 = -305;
 pub const MAX_E: i16 = 305;
 
@@ -1237,3 +1239,43 @@ pub const POWERS: ([u64; 611], [i16; 611]) = ([
     946,
     950,
 ]);
+
+pub const F32_SHORT_POWERS: [f32; 11] = [
+    1e0,
+    1e1,
+    1e2,
+    1e3,
+    1e4,
+    1e5,
+    1e6,
+    1e7,
+    1e8,
+    1e9,
+    1e10,
+];
+
+pub const F64_SHORT_POWERS: [f64; 23] = [
+    1e0,
+    1e1,
+    1e2,
+    1e3,
+    1e4,
+    1e5,
+    1e6,
+    1e7,
+    1e8,
+    1e9,
+    1e10,
+    1e11,
+    1e12,
+    1e13,
+    1e14,
+    1e15,
+    1e16,
+    1e17,
+    1e18,
+    1e19,
+    1e20,
+    1e21,
+    1e22,
+];