我正在阅读this paper关于未定义的行为，示例“优化”之一看起来非常可疑:

if (arg2 == 0)
    ereport(ERROR, (errcode(ERRCODE_DIVISION_BY_ZERO),
                    errmsg("division by zero")));
/* No overflow is possible */
PG_RETURN_INT32((int32) arg1 / arg2);

Figure 2: An unexpected optimization voids the division-by-zero check, in src/backend/utils/adt/int8.c of PostgreSQL. The call to ereport(ERROR, :::) will raise an exception.

   84 /*----------
   85  * New-style error reporting API: to be used in this way:
   86  *      ereport(ERROR,
   87  *              (errcode(ERRCODE_UNDEFINED_CURSOR),
   88  *               errmsg("portal \"%s\" not found", stmt->portalname),
   89  *               ... other errxxx() fields as needed ...));
   90  *
   91  * The error level is required, and so is a primary error message (errmsg
   92  * or errmsg_internal).  All else is optional.  errcode() defaults to
   93  * ERRCODE_INTERNAL_ERROR if elevel is ERROR or more, ERRCODE_WARNING
   94  * if elevel is WARNING, or ERRCODE_SUCCESSFUL_COMPLETION if elevel is
   95  * NOTICE or below.
   96  *
   97  * ereport_domain() allows a message domain to be specified, for modules that
   98  * wish to use a different message catalog from the backend's.  To avoid having
   99  * one copy of the default text domain per .o file, we define it as NULL here
  100  * and have errstart insert the default text domain.  Modules can either use
  101  * ereport_domain() directly, or preferably they can override the TEXTDOMAIN
  102  * macro.
  103  *
  104  * If elevel >= ERROR, the call will not return; we try to inform the compiler
  105  * of that via pg_unreachable().  However, no useful optimization effect is
  106  * obtained unless the compiler sees elevel as a compile-time constant, else
  107  * we're just adding code bloat.  So, if __builtin_constant_p is available,
  108  * use that to cause the second if() to vanish completely for non-constant
  109  * cases.  We avoid using a local variable because it's not necessary and
  110  * prevents gcc from making the unreachability deduction at optlevel -O0.
  111  *----------

最佳答案

Is the compiler free to assume that a function will always return?

However, the programmer failed to inform the compiler that the call to ereport(ERROR, ::: ) does not return.

The gcc guys are full of it. The issue that is relevant here is the C standard's definition of sequence points, and in particular the requirement that visible side effects of a later statement cannot happen before the execution of an earlier function call. The last time I pestered them about this, I got some lame claim that a SIGFPE wasn't a side effect within the definitions of the spec. At that point useful discussion stopped, because it's impossible to negotiate with someone who's willing to claim that.

We found seven similar issues in PostgreSQL, which were noted as “GCC bugs” in source code comments

关于c++ - 是否假定C/C++中的所有函数都返回？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/20059532/