快速上浮到INT的转换和浮点precision基于ARM（iPhone 3GS的/ 4） [英] Fast float to int conversion and floating point precision on ARM (iPhone 3GS/4)

问题描述

我读（ http://www.stereopsis.com/FPU.html ）中（<一提到href=\"http://stackoverflow.com/questions/78619/what-is-the-fastest-way-to-convert-float-to-int-on-x86\">http://stackoverflow.com/questions/78619/what-is-the-fastest-way-to-convert-float-to-int-on-x86).有谁知道，如果慢投简单（见下面的代码段）不适用于ARM架构，也？

I read (http://www.stereopsis.com/FPU.html) mentioned in (http://stackoverflow.com/questions/78619/what-is-the-fastest-way-to-convert-float-to-int-on-x86). Does anyone know if the slow simple cast (see snippet below) does apply to ARM architecture, too?

inline int Convert(float x)
{
  int i = (int) x;
  return i;
}

要申请，你必须设置浮点运算的precision FPU的文章中提到的一些技巧。我该怎么做，关于ARM？

To apply some tricks mentioned in the FPU article you have to set the precision for floating point operations. How do I do that on ARM?

什么是最快的浮子INT转换ARM架构？

What is the fastest float to int conversion on ARM architecture?

谢谢！

推荐答案

短版，不。

这文章是古代甚至不适用于现代的x86系统，更不用说ARM。一个简单的强制转换为整数相当快上的ARMv7（iPhone 3GS的/ 4），虽然有一个不起眼的摊位从VFP / NEON移动数据寄存器的通用寄存器。然而，鉴于你的浮动数据可能是从VFP / NEON寄存器完成的计算来了，你将不得不支付这一举动不管你怎么做转换。

That article is ancient and doesn't even apply to modern x86 systems, let alone ARM. A simple cast to integer is reasonably fast on ARMv7 (iPhone 3GS/4), though there is a modest stall moving data from the VFP/NEON registers to the general purpose registers. However, given that your float data is probably coming from a computation done in VFP/NEON registers, you will have to pay for that move no matter how you do the conversion.

我不认为这是一个最优化路径获利，除非你有痕迹显示，这是你的程序的一大瓶颈。即使这样，最快的转换是你不做转换;你几乎总是会更好发现算法的方式从你的程序消除转换。

I don't think that this is a profitable path for optimization unless you have traces showing that this is a major bottleneck for your program. Even then, the fastest conversion is the conversion you don't do; you will almost always be better off finding algorithmic ways to eliminate conversions from your program.

如果你的真正的需要优化的转换，寻找到 vcvt.i32.f32 指令，它转换的两个或四个矢量浮点数为两个或四个整数的的载体，而不的移动数据出来的NEON寄存器（因此，不会引起我提到的失速）。当然，你需要做的NEON单元对你以后的整数计算为这是一个有利可图的优化。

If you do genuinely need to optimize conversions, look into the vcvt.i32.f32 instruction, which converts a vector of two or four floating point numbers to a vector of two or four integers without moving the data out of the NEON registers (and therefore, without incurring the stall that I mentioned). Of course, you will need to do your subsequent integer computations on the NEON unit for this to be a profitable optimization.

问：什么是你的真正的意欲何为？为什么你认为你需要一个更快的引脚悬空> INT转换？

Question: What are you really trying to do? Why do you think you need a faster float->int conversion?

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