gcc - 什么是隔离错误路径优化

Question

什么是 GCC 中的隔离错误路径优化？怎么运行的 ？

隔离去年添加到 gcc 的错误路径优化:http://gcc.1065356.n5.nabble.com/RFA-PATCH-Isolate-erroneous-paths-optimization-td980376.html

Answer 1

添加的两个标志的文档应该有助于您的理解:

-fisolate-erroneous-paths-dereference Detect paths which trigger erroneous or undefined behaviour due to dereferencing a NULL pointer. Isolate those paths from the main control flow and turn the statement with erroneous or undefined behaviour into a trap.

考虑这个人为的代码块:

char
foo (int a)
  {
    char *x = "hello world";
    if (a)      /* (1).  */
      x = NULL; 
    else
      x = x;
    return *x;  /* (2).  */
 }

在 a != 0 的情况下，我们采用标记为 (1) 的路径。这会导致 (2) 处出现未定义的行为，其中 x 等于 NULL 指针。

这种优化允许我们将上面的内容重写为:

char
foo (int a)
  {
    char *x = "hello world";
    if (a)
      abort ();
    else
      x = x;
    return *x;
 }

好处是更好的持续传播，我们不再需要考虑会触发未定义行为的值。

考虑到恒定传播现在允许我们将示例重写为:

char
foo (int a)
  {
    if (a)
      abort ();
    return 'h';
 }

作为引用，此优化的另一半适用于受属性限制返回 null/非 null 的函数。

-fisolate-erroneous-paths-attribute Detect paths which trigger erroneous or undefined behaviour due a NULL value being used in a way which is forbidden by a returns_nonnull or nonnull attribute. Isolate those paths from the main control flow and turn the statement with erroneous or undefined behaviour into a trap. This is not currently enabled, but may be enabled by -O2 in the future.

可以在 http://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html#Optimize-Options 找到文档