Thanks. How would I solve this with a cast? I can force it to be wrong LOL
but can I force it to be right?
It seems to me like testWord = static_cast<unsigned long long>(valueToTest
>> 32) might not solve the problem because that cast seems to me to imply
that the expression inside the parentheses is *not* 64 bits.
Frankly, I am now leaning toward
union {
unsigned long long ll;
struct {unsigned int hi; unsigned int lo} s; } u;
u.ll = valueToTest;
and then using u.s.hi where I now use testWord.
I generally avoid unions because they can be a tad problematic. I think the
above actually violates the C standard which says you can't assign to member
x and then read member y (which pretty much negates the whole purpose of
unions other than, as Stroustrup suggests, saving space in memory).
Charles
-----Original Message-----
From: IBM Mainframe Discussion List [mailto:IBM-MAIN@LISTSERV.UA.EDU] On
Behalf Of David Crayford
Sent: Saturday, July 20, 2013 2:28 PM
To: IBM-MAIN@LISTSERV.UA.EDU
Subject: Re: Looking for help with an obscure C integer problem
As a general ROT I always use explicit casts.
On 21/07/2013, at 4:24 AM, Charles Mills <charl...@mcn.org> wrote:
> Cross-posted to IBM-MAIN and MVS-OE.
>
> I have the following code fragment in an inline function, compiled by
> the IBM XLC compiler as C++:
>
> unsigned long long valueToTest;
> unsigned int testWord;
> testWord = valueToTest >> 32;
>
> It *appears* to me (from somewhat circumstantial evidence in a much
> more complex big picture) when valueToTest has a value of
> 0x0034000000000000 then
>
> - If I compile Opt(0),NoInline then testWord gets the value I expect,
> 0x00340000; but
> - If I compile Opt(2),Inline then testWord gets a value of 0.
>
> Questions:
>
> 1. Does that seem plausible? That the code would work as intended
> Opt(0),NoInline but that with Opt(2),Inline the compiler would (I am
> guessing here) first cast valueToTest to an int of 0, then shift it
> right 32, and then assign it to testWord?
>
> 2. What should I code to avoid that? I guess I could shift valueToTest
> first (I don't need it again) and then in a separate statement assign
> it to testWord. Is that the "proper" coding technique?
>
> It's fairly involved to test the whole thing so I took the liberty of
> imposing on you folks rather than just trying stuff. Thanks much.
>
> Charles
>
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