c++ - Objective-C 中的非规范化 float ？

Question

Stack Overflow 问题/答案的相关性是什么 Why does changing 0.1f to 0 slow down performance by 10x? 用于 Objective-C？如果有任何相关性，这应该如何改变我的编码习惯？有什么方法可以关闭 Mac OS X 上的非规范化 float 吗？

这似乎与 iOS 完全无关。对吗？

Answer 1

正如我在回复您的评论时所说:

it is more of a CPU than a language issue, so it probably has relevance for Objective-C on x86. (iPhone's ARMv7 doesn't seem to support denormalized floats, at least with the default runtime/build settings)

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 上则没有(默认build设置)。

正如预期的那样，在 iOS 模拟器(x86 上)上运行时，非规范化 float 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范化数字(在 iOS、Mac OS X 和 Linux 上)。奇怪的事情发生在使用

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 接触到它，它就被设置为零:

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反规范化可以是:

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

(“甚至没有任何目的适用性的默示保证”)

这是 ARM 对清零模式必须说的:http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0204h/Bcfheche.html

更新<<1

这是在 ARMv7 上禁用清零模式的方法:

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);

以下令人惊讶的结果。

• 第 1 列:一个 float ，每次迭代除以 2
• 第 2 列:此 float 的二进制表示
• 第 3 列:对这个 float 求和 1e7 次所花费的时间

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，它会以 10% 的减速为代价。)

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