Objective-C中的非规范化浮点

Denormalized floating point in Objective-C?

本文关键字：规范化 非规范 Objective-C      更新时间：2023-10-16

Stack的相关性是什么；溢出问题/答案为什么将0.1f更改为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？有没有办法关闭Mac上的非规范化浮点；OS；十、

这似乎与iOS完全无关。这是正确的吗？

正如我在回应您的评论时所说：

与其说是语言问题，不如说是CPU问题，所以它可能与x86上Objective-C的相关性。（iPhone的ARMv7似乎不支持非规范化浮点，至少使用默认运行时/构建设置）

更新

我刚测试过。在x86上的Mac OS X上可以观察到速度减慢，而在ARMv7上的iOS上则不然（默认构建设置）。

不出所料，在iOS模拟器（x86上）上运行的非规范化浮点再次出现。

有趣的是，FLT_MIN和DBL_MIN分别被定义为最小的非规范化数（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；编译器很乐意设置一个非规范化常数，但（arm）CPU一接触它，它就被设置为零：

double test = DBL_MIN/2.0;
printf("test      == 0.0 %dn",test==0.0);
printf("DBL_MIN/2 == 0.0 %dn",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，运行时快速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;})

（"在没有任何用途的适用性暗示保证的情况下提供"）

这是ARM在"清零"模式下要说的话：http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0204h/Bcfheche.html

更新<lt；1

这就是在ARMv7:上禁用刷新为零模式的方法

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

结果令人吃惊。

  • 第1列：浮点值，每次迭代除以2
  • 第2列：此浮点的二进制表示
  • 第3列：将此浮点值相加1e7次所花费的时间

你可以清楚地看到，非规范化是零成本的。（对于iPad2。在iPhone 4上，它只需要降低10%的速度。）

0.000000000000000000000000000000000100000004670110: 10111100001101110010000011100000 110 ms
0.000000000000000000000000000000000050000002335055: 10111100001101110010000101100000 110 ms
0.000000000000000000000000000000000025000001167528: 10111100001101110010000001100000 110 ms
0.000000000000000000000000000000000012500000583764: 10111100001101110010000110100000 110 ms
0.000000000000000000000000000000000006250000291882: 10111100001101110010000010100000 111 ms
0.000000000000000000000000000000000003125000145941: 10111100001101110010000100100000 110 ms
0.000000000000000000000000000000000001562500072970: 10111100001101110010000000100000 110 ms
0.000000000000000000000000000000000000781250036485: 10111100001101110010000111000000 110 ms
0.000000000000000000000000000000000000390625018243: 10111100001101110010000011000000 110 ms
0.000000000000000000000000000000000000195312509121: 10111100001101110010000101000000 110 ms
0.000000000000000000000000000000000000097656254561: 10111100001101110010000001000000 110 ms
0.000000000000000000000000000000000000048828127280: 10111100001101110010000110000000 110 ms
0.000000000000000000000000000000000000024414063640: 10111100001101110010000010000000 110 ms
0.000000000000000000000000000000000000012207031820: 10111100001101110010000100000000 111 ms
0.000000000000000000000000000000000000006103515209: 01111000011011100100001000000000 110 ms
0.000000000000000000000000000000000000003051757605: 11110000110111001000010000000000 110 ms
0.000000000000000000000000000000000000001525879503: 00010001101110010000100000000000 110 ms
0.000000000000000000000000000000000000000762939751: 00100011011100100001000000000000 110 ms
0.000000000000000000000000000000000000000381469876: 01000110111001000010000000000000 112 ms
0.000000000000000000000000000000000000000190734938: 10001101110010000100000000000000 110 ms
0.000000000000000000000000000000000000000095366768: 00011011100100001000000000000000 110 ms
0.000000000000000000000000000000000000000047683384: 00110111001000010000000000000000 110 ms
0.000000000000000000000000000000000000000023841692: 01101110010000100000000000000000 111 ms
0.000000000000000000000000000000000000000011920846: 11011100100001000000000000000000 110 ms
0.000000000000000000000000000000000000000005961124: 01111001000010000000000000000000 110 ms
0.000000000000000000000000000000000000000002980562: 11110010000100000000000000000000 110 ms
0.000000000000000000000000000000000000000001490982: 00010100001000000000000000000000 110 ms
0.000000000000000000000000000000000000000000745491: 00101000010000000000000000000000 110 ms
0.000000000000000000000000000000000000000000372745: 01010000100000000000000000000000 110 ms
0.000000000000000000000000000000000000000000186373: 10100001000000000000000000000000 110 ms
0.000000000000000000000000000000000000000000092486: 01000010000000000000000000000000 110 ms
0.000000000000000000000000000000000000000000046243: 10000100000000000000000000000000 111 ms
0.000000000000000000000000000000000000000000022421: 00001000000000000000000000000000 110 ms
0.000000000000000000000000000000000000000000011210: 00010000000000000000000000000000 110 ms
0.000000000000000000000000000000000000000000005605: 00100000000000000000000000000000 111 ms
0.000000000000000000000000000000000000000000002803: 01000000000000000000000000000000 110 ms
0.000000000000000000000000000000000000000000001401: 10000000000000000000000000000000 110 ms
0.000000000000000000000000000000000000000000000000: 00000000000000000000000000000000 110 ms
0.000000000000000000000000000000000000000000000000: 00000000000000000000000000000000 110 ms
0.000000000000000000000000000000000000000000000000: 00000000000000000000000000000000 110 ms
相关文章：