From 262be13643cf5e3ff4f4e880b2dee601d4740fd8 Mon Sep 17 00:00:00 2001 From: Mike Hommey Date: Thu, 29 Mar 2018 09:34:39 +0900 Subject: Use f{32,64}::to_bits for is_zero test in vec::SpecFromElem vec::SpecFromElem provides an optimization to use calloc to fill a Vec when the element given to fill the Vec is represented by 0. For floats, the test for that currently used is `x == 0. && x.is_sign_positive()`. When compiled in a standalone function, rustc generates the following assembly: ``` xorps xmm1, xmm1 ucomisd xmm0, xmm1 setnp al sete cl and cl, al movq rax, xmm0 test rax, rax setns al and al, cl ret ``` A simpler test telling us whether the value is represented by 0, is `x.to_bits() == 0`, which rustc compiles to: ``` movq rax, xmm0 test rax, rax sete al ret ``` Not that the test is hot in any way, but it also makes it clearer what the intent in the rust code is. --- src/liballoc/vec.rs | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) (limited to 'src/liballoc') diff --git a/src/liballoc/vec.rs b/src/liballoc/vec.rs index 953f95876be..3c2f91d08e3 100644 --- a/src/liballoc/vec.rs +++ b/src/liballoc/vec.rs @@ -1574,8 +1574,8 @@ impl_spec_from_elem!(u64, |x| x == 0); impl_spec_from_elem!(u128, |x| x == 0); impl_spec_from_elem!(usize, |x| x == 0); -impl_spec_from_elem!(f32, |x: f32| x == 0. && x.is_sign_positive()); -impl_spec_from_elem!(f64, |x: f64| x == 0. && x.is_sign_positive()); +impl_spec_from_elem!(f32, |x: f32| x.to_bits() == 0); +impl_spec_from_elem!(f64, |x: f64| x.to_bits() == 0); //////////////////////////////////////////////////////////////////////////////// // Common trait implementations for Vec -- cgit 1.4.1-3-g733a5