问题描述

FJCVTZS 是浮动的"-point Javascript 转换为有符号定点，向零舍入".Arm v8.3-A 芯片及更高版本支持它.这很奇怪，因为您不希望看到 JavaScript 如此接近裸机.

FJCVTZS is "Floating-point Javascript Convert to Signed fixed-point, rounding toward Zero". It is supported in Arm v8.3-A chips and later. Which is odd, because you don't expect to see JavaScript so close to the bare metal.

我可以找到该指令作用的解释，但找不到它存在的原因.这个线程说它作为单个指令存在是因为 JS 缺少整数类型意味着某些用例经常因为没有好的算法原因而需要此操作.".这似乎是合理的，但我想要更详细的了解.

I can find explanations of what the instruction does, but not why it exists. This thread says "it exists as a single instruction is because JS's lack of an integer type means certain use cases need this operation obscenely often for no good algorithmic reason.". That's plausible but I would like a more detailed understanding.

推荐答案

是因为JS对数字使用了双精度，但是如果你想用位进行运算，任务就很不简单了，所以有一个具体的指令来转换JSdouble into integer 使事情变得更容易.

It is because JS uses double precision for the numbers, but if you want to perform operations with bits, the task is nontrivial, so a specific instruction to convert JS double into integer makes the thing easier.

这个 ARM 链接解释得很好:https://community.arm.com/processors/b/blog/posts/armv8-a-architecture-2016-additions

This ARM link explains it very well: https://community.arm.com/processors/b/blog/posts/armv8-a-architecture-2016-additions

为了补充更多关于fuz的注释信息，FCVTZS和FJCVTZS的区别(都是将浮点数转换为int)是在溢出的情况下，FJCVTZS 值将是 0x80000000 而不是溢出.此外，FJCVTZS 可以生成异常以指示转换的方式(即不精确).

In order to add more information regarding fuz's comment, the differences between FCVTZS and FJCVTZS (both of them convert floating point to int) are that in case of overflow, FJCVTZS value will be 0x80000000 instead of overflowing. Furthermore, FJCVTZS can generate an exception in order to indicate how the conversion was (i.e. inexact).

FJCVTZS : http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0801g/hko1477562192868.html

FCVTZS : http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0802a/FCVTZS_float_int.html

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