Joe Buck <[EMAIL PROTECTED]> writes: >> > *hv = (HOST_WIDE_INT) -(unsigned HOST_WIDE_INT) h1; >> >> Can't that conversion overflow? > > Not on a two's complement machine,
Sure it can. Suppose we have a 64-bit two's complement machine with no padding, and h1 is - 2**63. Then (unsigned HOST_WIDE_INT) h1 is 2**63, -(unsigned HOST_WIDE_INT) h1 is also 2**63, and converting 2**63 to HOST_WIDE_INT overflows. In this case C99 says the overflow results in an implementation-defined value, or an implementation-defined signal. If the former, the most plausible value is - 2**63 but C99 allows other values. If the latter, GCC will crash. Either way, the code is "broken" -- unless you assume wrapv semantics of course. Even an expert like you can easily get this wrong, and this suggests how tricky this area of Standard C really is. Only a tiny fraction of C programmers know how to write this sort of code reliably and in conformance to minimal standard C. It's little wonder that most C programmers assume wrapv semantics in cases like this.
