Clang是否缺少针对ARMv8 / Aarch64的CRC32？

问题描述

我正在尝试为我们的Xcode交叉编译设置CI。交叉编译会同时测试ARMv7和ARMv8。事情看起来不错，除了需要链接到ARMv8：

  clang ++ -DNDEBUG -g2- O3 -fPIC -pipe -Wall -miphoneos-version-min = 7 -arch arm64 \ 
 -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS10.2 .sdk \ 
 -stdlib = libc ++ -c cryptlib.cpp 
 clang ++ -DNDEBUG -g2 -O3 -fPIC -pipe -Wall -miphoneos-version-min = 7 -arch arm64 \ 
 -isysroot /应用程序/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS10.2.sdk \ 
 -stdlib = libc ++ -c cpu.cpp 
 .. 。
 
 clang ++ -o cryptest.exe -DNDEBUG -g2 -O3 -fPIC -pipe -Wall -miphoneos-version-min = 7 -arch arm64 \ 
 -isysroot / Applications / Xcode .app / Contents / Developer / Platforms / iPhoneOS.platform / Developer / SDKs / iPhoneOS10.2.sdk \ 
 -stdlib = libc ++ test.o bench1.o bench2.o ... ./libcryptopp.a 
 
体系结构arm64的未定义符号：
 
 CryptoP P :: CRC32_Update_ARMV8（unsigned char const *，unsigned long，unsigned int&），引用自：$ crypt $：$ CRC $：$ CRC32 :: Update（unsigned char const *，unsigned long）在libcryptopp.a中（crc.o）
 
 CryptoPP :: CRC32C_Update_ARMV8（unsigned char const *，unsigned long，unsigned int&），引用自：
 
 CryptoPP :: CRC32C ::在libcryptopp.a（crc.o）
 
 ld中更新（unsigned char const *，unsigned long）
 
 ld：找不到体系结构arm64 
 
的符号clang：错误：链接器命令失败，退出代码为1（使用-v查看调用）
 
 make：*** [cryptest.exe]错误1 
  

我们显然没有运行输出工件 cryptest.exe 。我们只是编译并链接到测试对象。

该代码在LLVM Clang下进行了很好的测试。

全部ARMv8 / Aarch64机器具有CRC-32和CRC-32C;但是Crypto扩展名是可选的。该错误没有太大意义。

Clang是否缺少ARMv8 / Aarch64的CRC32？

---

下面是导致错误的代码。

 ＃如果已定义（__ARM_FEATURE_CRC32）
 
 void CRC32_Update_ARMV8（const uint8_t * s，size_t n，uint32_t& c）
 {
 for（;！IsAligned< uint32_t>（s）&& n> 0; s ++ ，n--）
c = __crc32b（c，* s）; 
 
 for（; n> 4; s + = 4，n- = 4）
 c = __crc32w（c，* s）; 
 
 for（; n> 0; s ++，n--）
 c = __crc32b（c，* s）; 
} 
 
 #endif 
  

解决方案

在Xcode 8.3.3下，我在__crc32 *（）处遇到了编译错误。然后我添加了命令行开关

  -march = armv8-a + crc 
  

位于链接，则代码可以很好地编译。我使用iphone7 + / iOS10.3.1进行了测试，并且可以正常工作。

根据（ARM®体系结构参考手册
ARMv8，适用于ARMv8-架构配置文件 DDI0487B_a_armv8_arm.pdf：第A1-58页），crc32指令对于v8是可选的，对于v8.1是必需的。当我在iphone6 + / iOS9.3.3上运行相同程序时，它在__crc32 *（）崩溃。我还使用内联汇编器对其进行了验证。因此，为避免崩溃，必须进行某种运行时检查。我还不完全了解，但是作为最后的手段，我们可以使用模型名称。

I'm attempting to set-up CI for our Xcode cross-compiles. The cross-compiles test both ARMv7 and ARMv8. Things look good except when it comes time to link for ARMv8:

clang++ -DNDEBUG -g2 -O3 -fPIC -pipe -Wall -miphoneos-version-min=7 -arch arm64 \
  -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS10.2.sdk \
  -stdlib=libc++ -c cryptlib.cpp
clang++ -DNDEBUG -g2 -O3 -fPIC -pipe -Wall -miphoneos-version-min=7 -arch arm64 \
  -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS10.2.sdk \
  -stdlib=libc++ -c cpu.cpp
...

clang++ -o cryptest.exe -DNDEBUG -g2 -O3 -fPIC -pipe -Wall -miphoneos-version-min=7 -arch arm64 \
  -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS10.2.sdk \
  -stdlib=libc++ test.o bench1.o bench2.o ... ./libcryptopp.a

Undefined symbols for architecture arm64:

  "CryptoPP::CRC32_Update_ARMV8(unsigned char const*, unsigned long, unsigned int&)", referenced from:

      CryptoPP::CRC32::Update(unsigned char const*, unsigned long) in libcryptopp.a(crc.o)

  "CryptoPP::CRC32C_Update_ARMV8(unsigned char const*, unsigned long, unsigned int&)", referenced from:

      CryptoPP::CRC32C::Update(unsigned char const*, unsigned long) in libcryptopp.a(crc.o)

ld: symbol(s) not found for architecture arm64

clang: error: linker command failed with exit code 1 (use -v to see invocation)

make: *** [cryptest.exe] Error 1

We obviously don't run the output artifact cryptest.exe. We just compile and link to test things.

The code has been testing fine under LLVM Clang.

All ARMv8/Aarch64 machines have CRC-32 and CRC-32C; but the Crypto extensions are optional. The error does not make much sense.

Does Clang lack CRC32 for ARMv8/Aarch64?

---

Below is the code that's causing the errors.

#if defined(__ARM_FEATURE_CRC32)

void CRC32_Update_ARMV8(const uint8_t *s, size_t n, uint32_t& c)
{
    for(; !IsAligned<uint32_t>(s) && n > 0; s++, n--)
        c = __crc32b(c, *s);

    for(; n > 4; s+=4, n-=4)
        c = __crc32w(c, *s);

    for(; n > 0; s++, n--)
        c = __crc32b(c, *s);
}

#endif

解决方案

Under Xcode 8.3.3, I came across the compile error at __crc32*(). Then I added the command line switch

-march=armv8-a+crc

as found at this link, then the code compiles well. I tested with iphone7+/iOS10.3.1 and it's working.

Note according to ARM's document ("ARM® Architecture Reference ManualARMv8, for ARMv8-A architecture profile" DDI0487B_a_armv8_arm.pdf: page A1-58), crc32 instructions are optional for v8 and mandatory for v8.1. When I ran the same program on iphone6+/iOS9.3.3, it crashed at __crc32*(). I also verified it with inline assembler. To avoid the crash, therefore, some kind of run-time check is necessary. I don't fully understand how, but as a last resort, we could use the model names.

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