Objective-C中的非规范化浮点？

Question

Stack Overflow问题/答案的相关性为什么将0.1f改为0会使性能降低10倍？对于Objective-C？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上关闭非规范化的浮点数？

这似乎与iOS完全无关.那是对的吗？

Answer 1

正如我在回答你的评论时所说:

它更像是CPU而不是语言问题,所以它可能与x86上的Objective-C有关.(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非规格化数(iOS上的Mac OS X和Linux).发生奇怪的事情

DBL_MIN/2.0

在你的代码中; 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;})

("甚至没有任何暗示的适用性保证")

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

更新<< 1

这是在ARMv7上禁用刷新到零模式的方法:

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);

具有以下令人惊讶的结果.

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

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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