如何为浮点数指定舍入模式？

问题描述

我想把浮点数整数到最接近的整数，当有一个最接近的整数时，向正无穷大方向移动。

 使用std :: num :: Float; 
 
 $ main（）{
 assert_eq！（ -  0.0，（-0.5）.round（））; //失败！ 
 
 $ / code> 

但是， round 说：



我还没有看到任何让我改变舍入模式的东西，但是有一些方法，对吧？


解决方案

似乎执行 Float :: round ，至少对于 f32 和 f64 ，转发给 roundf32 / roundf64 instrinsics ，它们本身是使用LLVM函数 llvm.round.f32 和 llvm.round.f64 来实现的。 llvm.round。* 特定的四舍五入模式，或者说它是未定义的。

我不能找到任何关于此的可靠信息。从2011年起，讨论特定于x86的内部函数，以及针对Native Client问题的，似乎在谈论一个假设的内在的，它将如何很难做到可移植。

盲目地刺它：我会尝试写一个小C库，它只是链接到它。它似乎不直接受LLVM支持。

I'd like to round floating point numbers to the nearest integer, going towards positive infinity when there is a tie for "nearest integer".

use std::num::Float;

fn main() {
    assert_eq!(-0.0, (-0.5).round()); // fails!
}

However, the docs for round say:

I haven't seen anything that would allow me to change the rounding mode, but there's got to be some way, right?

解决方案

It appears that the implementation of Float::round, at least for f32 and f64, forward to the roundf32/roundf64 instrinsics, which themselves are implemented using the LLVM functions llvm.round.f32 and llvm.round.f64. The documentation for llvm.round.* doesn't say anything about how to control the rounding mode, sadly. There doesn't appear to be anything else in the LLVM reference about it, either. The other functions I could find that even mentioned rounding modes either specified one particular rounding mode, or said it was undefined.

I couldn't find any solid information about this. There was a post on the LLVM mailing list from 2011 that talks about x86-specific intrinsics, and a 2013 post to the Native Client issue tracker that appears to talk about a hypothetical intrinsic and how it would be hard to do portably.

Taking a blind stab at it: I'd try writing a little C library that does it and just link to that. It doesn't appear to be directly supported in LLVM.

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