Objective-C中的非规范化浮点？

Question

堆栈溢出问题/答案Why does changing 0.1f to 0 slow down performance by 10x? 与 Objective-C 有什么相关性？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 上的非规范化浮点吗？

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的那样：

这更像是 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好像不支持 非规范化的浮点数，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 上则没有（默认构建设置）。

不出所料，在 iOS 模拟器（在 x86 上）上运行的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

会发生奇怪的事情

DBL_MIN/2.0

在您的代码中；编译器很高兴地设置了一个非规范化的常量，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

所以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;})

(“即使没有任何适用性的默示保证”)

这是 ARM 在清零模式下所说的：http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0204h/Bcfheche.html

更新

这是在 ARMv7 上禁用刷新为零模式的方法：

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);

得到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：将此浮点数求和 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，它的代价是降低了 10%。）

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