检测ARM NEON可用性在preprocessor？ [英] Detect ARM NEON availability in the preprocessor?

问题描述

按照 ARM ARM ，当霓虹灯SIMD指令可 __ __ ARM_NEON 定义。我遇到了麻烦GCC提供它。

氖这​​个 BananaPi专业版开发运行Debian 8.2板可供选择：

  $执行cat / proc内/ cpuinfo | grep的霓虹灯
特点：SWP一半拇指fastmult VFP EDSP霓虹灯VFPv3的TLS vfpv4 idiva idivt
 

我使用GCC 4.9：

  $ GCC --version
海湾合作委员会（Debian的4.9.2-10）4.9.2
 

尝试GCC和 -march =本地：

  $ G ++ -march =本地-dM -E  - ＆LT;的/ dev / null的| grep的-i霓虹灯
＃定义__ARM_NEON_FP 4
 

OK，什么尝试谷歌建立了霓虹灯时，使用Android的：

  $ G ++ -march =的ARMv7-A -mfpu =的VFPv3-D16 -mfloat-ABI = softfp -dM -E  - ＆LT;的/ dev / null的| grep的-i霓虹灯
＃定义__ARM_NEON_FP 4
 

也许的ARMv7-A用硬浮动：

  $ G ++ -march =的ARMv7-A -mfloat-ABI =硬-dM -E  - ＆LT;的/ dev / null的| grep的-i霓虹灯
＃定义__ARM_NEON_FP 4
 

我的问题是：

• 为什么我没有看到 __ __ ARM_NEON ？


• 我怎样检测在preprocessor霓虹灯可用性？

也许

• 我应该用什么GCC开关，使霓虹灯SIMD指令？

---

相关，在 LeMaker HiKey ，它是运行Linaro的海湾合作​​委员会AARCH64 / ARM64 4.9.2，这里是从preprocessor输出：

  $ CPP -dM＆LT;的/ dev / null的| grep的-i霓虹灯
＃定义__ARM_NEON 1
 

据ARM，这款主板确实有高级SIMD指令，即使：

  $执行cat / proc内/ cpuinfo
处理器：AArch64处理器转3（aarch64）
...
特点：FP asimd evtstrm AES pmull SHA1 SHA2 CRC32
 

解决方案

有一些藏在这里的问题，我会尝试提取它们反过来...

  

据ARM的ARM，当霓虹灯SIMD指令可 __ __ ARM_NEON 定义。我遇到了麻烦GCC提供它。

这是编译器文档[老版] ARM编译器，而不是ARM Architceture参考手册。一个更好的宏观检查的高级SIMD指令的presence将 __ ARM_NEON ，这是在的 ARM C语言扩展。

  

尝试GCC和 -march =本地：

如您可能已经发现。 GCC针对ARM目标分离出来 -march （对于结构调整为其GCC应该产生code）， -mfpu （对于浮点/高级SIMD单元可用）和 -mfloat-ABI （对于点如何浮动参数应该被传递，并为presence或没有一个浮点单元的）。最后是 -mtune （它要求GCC尝试优化为特定的处理器）和 -mcpu （充当作为组合 -mtune 和 -march ）。

通过要求 -march =本地你问GCC产生code适合于在其上运行的处理器的体系结构检测。这对 -mfpu 设置没有任何影响，所以不必启用高级SIMD指令的生成。

请注意，上述只适用于一个编译器定位AArch32。该AArch64 GCC不支持 -mfpu 并通过 -march =原生检测的高级SIMD支持presence

  

OK，什么尝试谷歌建立了霓虹灯时，使用Android的：

  
  

$ G ++ -march =的ARMv7-A -mfpu =的VFPv3-D16 -mfloat-ABI = softfp -dM -E

本编译标志不足以使对高级SIMD指令的支持，您的笔记可能是不完整的。在 -mfpu 标志由GCC 4.9.2支持的我期望的任何的：

氖，氖-FP16 ，氖vfpv4 ，氖-FP-armv8 ，加密霓虹灯-FP-armv8

要给你你想要的东西。

  

据ARM，这款主板确实有高级SIMD指令，即使：

看起来就像你在一个AArch64内核，它通过 asimd 功能公开支持高级SIMD运行 - 在您的示例输出

According to the ARM ARM, __ARM_NEON__ is defined when Neon SIMD instructions are available. I'm having trouble getting GCC to provide it.

Neon available on this BananaPi Pro dev board running Debian 8.2:

$ cat /proc/cpuinfo | grep neon
Features    : swp half thumb fastmult vfp edsp neon vfpv3 tls vfpv4 idiva idivt 

I'm using GCC 4.9:

$ gcc --version
gcc (Debian 4.9.2-10) 4.9.2

Try GCC and -march=native:

$ g++ -march=native -dM -E - </dev/null | grep -i neon
#define __ARM_NEON_FP 4

OK, try what Google uses for Android when building for Neon:

$ g++ -march=armv7-a -mfpu=vfpv3-d16 -mfloat-abi=softfp -dM -E - </dev/null | grep -i neon
#define __ARM_NEON_FP 4

Maybe a ARMv7-a with a hard float:

$ g++ -march=armv7-a -mfloat-abi=hard -dM -E - </dev/null | grep -i neon
#define __ARM_NEON_FP 4

My questions are:

• why am I not seeing __ARM_NEON__?
• how do I detect Neon availability in the preprocessor?

And maybe:

• what GCC switches should I use to enable Neon SIMD instructions?

---

Related, on a LeMaker HiKey, which is AARCH64/ARM64 running Linaro with GCC 4.9.2, here's the output from the preprocessor:

$ cpp -dM </dev/null | grep -i neon
#define __ARM_NEON 1

According to ARM, this board does have Advanced SIMD instructions even though:

$ cat /proc/cpuinfo 
Processor   : AArch64 Processor rev 3 (aarch64)
...
Features    : fp asimd evtstrm aes pmull sha1 sha2 crc32

解决方案

There are a number of questions hidden in here, I'll try to extract them in turn...

According to the ARM ARM, __ARM_NEON__ is defined when Neon SIMD instructions are available. I'm having trouble getting GCC to provide it.

That is compiler documentation for [an old version of] the ARM Compiler rather than the ARM Architceture Reference Manual. A better macro to check for the presence of the Advanced SIMD instructions would be __ARM_NEON, which is defined in the ARM C Language Extensions.

Try GCC and -march=native:

As you may have found. GCC for the ARM target separates out -march (For the architecture revision for which GCC should generate code), -mfpu (For the floating point/Advanced SIMD unit available) and -mfloat-abi (For how floating point arguments should be passed, and for the presence or absence of a floating point unit). Finally there is -mtune (Which asks GCC to try to optimise for a particular processor) and -mcpu (which acts as a combination of -mtune and -march).

By asking for -march=native You're asking GCC to generate code appropriate for the detected architecture of the processor on which you are running. This has no impact on the -mfpu setting, and so does not necessarily enable Advanced SIMD instruction generation.

Note that the above only applies to a compiler targeting AArch32. The AArch64 GCC does not support -mfpu and will detect presence of Advanced SIMD support through -march=native.

OK, try what Google uses for Android when building for Neon:

$ g++ -march=armv7-a -mfpu=vfpv3-d16 -mfloat-abi=softfp -dM -E

These build flags are not sufficient to enable support for Advanced SIMD instructions, your notes may be incomplete. Of the -mfpu flags supported by GCC 4.9.2 I'd expect any of:

neon, neon-fp16, neon-vfpv4, neon-fp-armv8, crypto-neon-fp-armv8

To give you what you want.

According to ARM, this board does have Advanced SIMD instructions even though:

Looks like you're running on an AArch64 kernel, which exposes support for Advanced SIMD through the asimd feature - as in your example output.

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