Question

Stack Overflow问题/答案 Why does changing 0.1f to 0 slow down performance by 10x? 与Objective-C的相关性是什么？如果有任何相关性，这应该如何改变我的编码习惯？有没有办法在Mac OS X上关闭非规范化浮点？

这似乎与iOS完全无关。这是对的吗？

Answer 1

正如我在回答你的评论时所说：

它更多的是CPU而不是语言问题，所以它可能有与x86上的Objective-C相关。（iPhone的ARMv7似乎不支持非规范化浮点数，至少使用默认的运行时/构建设置）

更新

我刚刚测试过。在x86上的Mac OS X上，观察到减速，在ARMv7上的iOS上它不是（默认构建设置）。

正如预期的那样，在iOS模拟器（在x86上）上运行非规范化浮点数再次出现。

有趣的是，FLT_MIN和DBL_MIN分别被定义为最小的非规范化数字（在iOS，Mac OS X和Linux上）。使用

发生奇怪的事情

DBL_MIN/2.0

你的代码中的

;编译器愉快地设置了一个非规范化常量，但是一旦（arm）CPU接触到它，它就会被设置为零：

double test = DBL_MIN/2.0;
printf("test      == 0.0 %d\n",test==0.0);
printf("DBL_MIN/2 == 0.0 %d\n",DBL_MIN/2.0==0.0);

输出：

test      == 0.0 1  // computer says YES
DBL_MIN/2 == 0.0 0  // compiler says NO

因此，快速运行时检查是否支持非规范化可以是：

#define SUPPORT_DENORMALIZATION ({volatile double t=DBL_MIN/2.0;t!=0.0;})

（“甚至没有出于任何目的的适用性隐含保证”）

这就是ARM在刷新到零模式时所说的话：http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0204h/Bcfheche.html

<强>更新＆LT;＆。1

这是在ARMv7上禁用flush to zero模式的方法：

int x;
asm(
    "vmrs %[result],FPSCR \r\n"
    "bic %[result],%[result],#16777216 \r\n"
    "vmsr FPSCR,%[result]"
    :[result] "=r" (x) : :
);
printf("ARM FPSCR: %08x\n",x);

带来以下令人惊讶的结果。

第1列：浮动，每次迭代除以2
第2列：此浮动的二进制表示
第3列：将此浮点数加1e7次所需的时间

您可以清楚地看到非规范化的成本为零。（对于iPad 2.在iPhone 4上，它的成本降低了10％。）

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Objective-C中的非规范化浮点？

1 个答案: