Commits on Jan 7, 2019

1. Optimise floating point is_finite (2x) and is_infinite (1.6x). …

   These can both rely on IEEE754 semantics to be made faster, by folding
   away the sign with an abs (left private for now), and then comparing
   to infinity, letting the NaN semantics of a direct float comparison
   handle NaN input properly.
   
   The `abs` bit-fiddling is simple (a single and), and so these new
   forms compile down to a few instructions, without branches, e.g. for
   f32:
   
   ```asm
   is_infinite:
           andps   xmm0, xmmword ptr [rip + .LCPI2_0] ; 0x7FFF_FFFF
           ucomiss xmm0, dword ptr [rip + .LCPI2_1]   ; 0x7F80_0000
           setae   al
           ret
   
   is_finite:
           andps   xmm0, xmmword ptr [rip + .LCPI1_0] ; 0x7FFF_FFFF
           movss   xmm1, dword ptr [rip + .LCPI1_1]   ; 0x7F80_0000
           ucomiss xmm1, xmm0
           seta    al
           ret
   ```
   
   When used in loops/repeatedly, they get even better: the memory
   operations (loading the mask 0x7FFFFFFF for abs, and infinity
   0x7F80_0000) are likely to be hoisted out of the individual calls, to
   be shared, and the `seta`/`setae` are likely to be collapsed into
   conditional jumps or moves (or similar).
   
   The old `is_infinite` did two comparisons, and the old `is_finite` did
   three (with a branch), and both of them had to check the flags after
   every one of those comparison. These functions have had that old
   implementation since they were added in
   rust-lang@6284190
   7 years ago.
   
   Benchmark (`abs` is the new form, `std` is the old):
   
   ```
   test f32_is_finite_abs            ... bench:          55 ns/iter (+/- 10)
   test f32_is_finite_std            ... bench:         118 ns/iter (+/- 5)
   
   test f32_is_infinite_abs          ... bench:          53 ns/iter (+/- 1)
   test f32_is_infinite_std          ... bench:          84 ns/iter (+/- 6)
   
   test f64_is_finite_abs            ... bench:          52 ns/iter (+/- 12)
   test f64_is_finite_std            ... bench:         128 ns/iter (+/- 25)
   
   test f64_is_infinite_abs          ... bench:          54 ns/iter (+/- 5)
   test f64_is_infinite_std          ... bench:          93 ns/iter (+/- 23)
   ```
   
   ```rust
    #![feature(test)]
   extern crate test;
   
   use std::{f32, f64};
   use test::Bencher;
   
   const VALUES_F32: &[f32] = &[0.910, 0.135, 0.735, -0.874, 0.518, 0.150, -0.527, -0.418, 0.449, -0.158, -0.064, -0.144, -0.948, -0.103, 0.225, -0.104, -0.795, 0.435, 0.860, 0.027, 0.625, -0.848, -0.454, 0.359, -0.930, 0.067, 0.642, 0.976, -0.682, -0.035, 0.750, 0.005, -0.825, 0.731, -0.850, -0.740, -0.118, -0.972, 0.888, -0.958, 0.086, 0.237, -0.580, 0.488, 0.028, -0.552, 0.302, 0.058, -0.229, -0.166, -0.248, -0.430, 0.789, -0.122, 0.120, -0.934, -0.911, -0.976, 0.882, -0.410, 0.311, -0.611, -0.758, 0.786, -0.711, 0.378, 0.803, -0.068, 0.932, 0.483, 0.085, 0.247, -0.128, -0.839, -0.737, -0.605, 0.637, -0.230, -0.502, 0.231, -0.694, -0.400, -0.441, 0.142, 0.174, 0.681, -0.763, -0.608, 0.848, -0.550, 0.883, -0.212, 0.876, 0.186, -0.909, 0.401, -0.533, -0.961, 0.539, -0.298, -0.448, 0.223, -0.307, -0.594, 0.629, -0.534, 0.959, 0.349, -0.926, -0.523, -0.895, -0.157, -0.074, -0.060, 0.513, -0.647, -0.649, 0.428, 0.401, 0.391, 0.426, 0.700, 0.880, -0.101, 0.862, 0.493, 0.819, -0.597];
   
    #[bench]
   fn f32_is_infinite_std(b: &mut Bencher) {
       b.iter(|| test::black_box(VALUES_F32).iter().any(|x| x.is_infinite()));
   }
    #[bench]
   fn f32_is_infinite_abs(b: &mut Bencher) {
       b.iter(|| test::black_box(VALUES_F32).iter().any(|x| x.abs()== f32::INFINITY));
   }
    #[bench]
   fn f32_is_finite_std(b: &mut Bencher) {
       b.iter(|| test::black_box(VALUES_F32).iter().all(|x| x.is_finite()));
   }
    #[bench]
   fn f32_is_finite_abs(b: &mut Bencher) {
       b.iter(|| test::black_box(VALUES_F32).iter().all(|x| x.abs() < f32::INFINITY));
   }
   
   const VALUES_F64: &[f64] = &[0.910, 0.135, 0.735, -0.874, 0.518, 0.150, -0.527, -0.418, 0.449, -0.158, -0.064, -0.144, -0.948, -0.103, 0.225, -0.104, -0.795, 0.435, 0.860, 0.027, 0.625, -0.848, -0.454, 0.359, -0.930, 0.067, 0.642, 0.976, -0.682, -0.035, 0.750, 0.005, -0.825, 0.731, -0.850, -0.740, -0.118, -0.972, 0.888, -0.958, 0.086, 0.237, -0.580, 0.488, 0.028, -0.552, 0.302, 0.058, -0.229, -0.166, -0.248, -0.430, 0.789, -0.122, 0.120, -0.934, -0.911, -0.976, 0.882, -0.410, 0.311, -0.611, -0.758, 0.786, -0.711, 0.378, 0.803, -0.068, 0.932, 0.483, 0.085, 0.247, -0.128, -0.839, -0.737, -0.605, 0.637, -0.230, -0.502, 0.231, -0.694, -0.400, -0.441, 0.142, 0.174, 0.681, -0.763, -0.608, 0.848, -0.550, 0.883, -0.212, 0.876, 0.186, -0.909, 0.401, -0.533, -0.961, 0.539, -0.298, -0.448, 0.223, -0.307, -0.594, 0.629, -0.534, 0.959, 0.349, -0.926, -0.523, -0.895, -0.157, -0.074, -0.060, 0.513, -0.647, -0.649, 0.428, 0.401, 0.391, 0.426, 0.700, 0.880, -0.101, 0.862, 0.493, 0.819, -0.597];
   
    #[bench]
   fn f64_is_infinite_std(b: &mut Bencher) {
       b.iter(|| test::black_box(VALUES_F64).iter().any(|x| x.is_infinite()));
   }
    #[bench]
   fn f64_is_infinite_abs(b: &mut Bencher) {
       b.iter(|| test::black_box(VALUES_F64).iter().any(|x| x.abs() == f64::INFINITY));
   }
    #[bench]
   fn f64_is_finite_std(b: &mut Bencher) {
       b.iter(|| test::black_box(VALUES_F64).iter().all(|x| x.is_finite()));
   }
    #[bench]
   fn f64_is_finite_abs(b: &mut Bencher) {
       b.iter(|| test::black_box(VALUES_F64).iter().all(|x| x.abs() < f64::INFINITY));
   }
   ```

   

   huonw committed Jan 7, 2019
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