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long我试图了解使用或选择内置 64 位数据类型的要求long long。我无法理解longvs的类型等效性和对齐要求long long

long在 LP64/ILP64/ LLP64long long数据模型下选择 64 位类型时,最佳实践或标准是什么?


这里有一些相关的问题。它们完全涵盖了其他主题,例如sizeof(long) <= sizeof(long long),但它们并没有完全处理等价或对齐期望。它们还严重依赖非内置类型,例如uint64_t.


背景信息是类似于下面的代码来选择一个内置的 64 位类型来使用:

#if _LP64 || __LP64__ || _ILP64 || __ILP64__
typedef my_u64 unsigned long;
#else
typedef my_u64 unsigned long long;
#endif

i686 和 ARMv7 及以下版本可以使用typedef my_u64 unsigned long long.

第一个问题案例

在 x86_64 (AMD64) 机器上,如果 typedef my_u64 unsigned long有效(由于LP64),则会导致编译错误:

$ gcc -mrdrnd test.cc -o test.exe
test.cc: In function ‘int main(int, char**)’:
test.cc:18:22: error: invalid conversion from ‘my_u64* {aka long unsigned int*}’ to ‘long long unsigned int*’ [-fpermissive]
   _rdrand64_step(&val);
                      ^
In file included from /usr/lib/gcc/x86_64-linux-gnu/4.9/include/x86intrin.h:46:0,
                 from test.cc:2:
/usr/lib/gcc/x86_64-linux-gnu/4.9/include/immintrin.h:166:1: note: initializing argument 1 of ‘int _rdrand64_step(long long unsigned int*)’
 _rdrand64_step (unsigned long long *__P)
 ^

第二个问题案例

在 Aarch64 (ARM64) 机器上,如果 typedef my_u64 unsigned long long有效(修复 x86_64 错误),则会导致编译错误:

$ gcc test.cc -o test.exe
test.cc: In function ‘int main(int, char**)’:
test.cc:21:16: error: invalid conversion from ‘my_u64* {aka long long unsigned int*}’ to ‘const uint64_t* {aka const long unsigned int*}’ [-fpermissive]
   vld1q_u64(val);
                ^
In file included from test.cc:4:0:
/usr/lib/gcc/aarch64-linux-gnu/4.9/include/arm_neon.h:17003:1: note: initializing argument 1 of ‘uint64x2_t vld1q_u64(const uint64_t*)’
 vld1q_u64 (const uint64_t *a)
 ^

$ cat test.cc
#if __x86_64__
#include <x86intrin.h>
#elif __aarch64__
#include <arm_neon.h>
#include <arm_acle.h> 
#endif

#if _LP64 || __LP64__
typedef unsigned long my_u64;
#else
typedef unsigned long long my_u64;
#endif

int main(int argc, char* argv[])
{
#if __x86_64__
  my_u64 val;
  _rdrand64_step(&val);
#elif __aarch64__
  my_u64 val[2];
  vld1q_u64(val);
#endif

  return 0;
}
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