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我编写了一个非常简单的代码来用一个常数值（1024）填充一个 32x32 16bpp 图像。图像缓冲区由std::vector. 我的图像的间距/步幅（即两个连续行之间的字节数）大到足以容纳一整行，但设置为奇数。这是我的代码：

#include <vector>
#include <stdint.h>

int main()
{
  int width = 32;
  int height = 32;
  int pitch = width * 2 + 1;

  std::vector<uint8_t> image(height * pitch);
  uint8_t* buffer = &image[0];

  for (int y = 0; y < height; y++)
  {
    uint16_t* p = reinterpret_cast<uint16_t*>(buffer + y * pitch);
    for (int x = 0; x < width; x++, p++)
    {
      *p = 1024;
    }
  }
}

我正在使用带有 gcc 4.6.1（Ubuntu 11.10）的 Linux x86_64。代码在-O0,-O1和-O2优化级别下运行良好。Valgrind 不报告任何访问冲突。但是，一旦我切换到-O3或使用-ftree-vectorizeauto-vectorization 选项，程序就会崩溃：

# g++ -g -O3 ./test.cpp -Wall -pedantic && ./a.out
Segmentation fault

# g++ -g -O2 -ftree-vectorize ./test.cpp -Wall -pedantic && ./a.out
Segmentation fault

gdb 和 valgrind 都没有提供任何有用的信息：

# valgrind ./a.out
==3840== Memcheck, a memory error detector
==3840== Copyright (C) 2002-2010, and GNU GPL'd, by Julian Seward et al.
==3840== Using Valgrind-3.6.1-Debian and LibVEX; rerun with -h for copyright info
==3840== Command: ./a.out
==3840== 
==3840== 
==3840== Process terminating with default action of signal 11 (SIGSEGV)
==3840==  General Protection Fault
==3840==    at 0x4005B3: main (test.cpp:18)
==3840== 
==3840== HEAP SUMMARY:
==3840==     in use at exit: 2,080 bytes in 1 blocks
==3840==   total heap usage: 1 allocs, 0 frees, 2,080 bytes allocated
==3840== 
==3840== LEAK SUMMARY:
==3840==    definitely lost: 2,080 bytes in 1 blocks
==3840==    indirectly lost: 0 bytes in 0 blocks
==3840==      possibly lost: 0 bytes in 0 blocks
==3840==    still reachable: 0 bytes in 0 blocks
==3840==         suppressed: 0 bytes in 0 blocks
==3840== Rerun with --leak-check=full to see details of leaked memory
==3840== 
==3840== For counts of detected and suppressed errors, rerun with: -v
==3840== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 4 from 4)
Segmentation fault

-m32当我使用gcc 标志切换到 32 位二进制文​​件时，不会发生崩溃。如果我使用均匀的音高（例如pitch = width * 2 + 2），它也不会发生。谁能帮我发现我在代码中犯的（当然是愚蠢的）错误？提前谢谢了！

更新：正如乔纳森所建议的，我刚刚向 GCC 开发人员报告了这个问题： http ://gcc.gnu.org/bugzilla/show_bug.cgi?id=56392

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1 回答 1

4

我的问题已由 Richard Blener 在gcc Bugzilla上回答：

“您正在取消引用指向 uint16_t 的指针，该指针对于该类型没有充分对齐。C 标准禁止这样做，导致未定义的行为。”

然而，在我看来，应该对这种未定义的行为产生警告。另请注意，@jmetcalfe 在对这篇文章的评论中也给出了这种解释。

于 2013-02-19T12:10:57.870 回答