Here is exact cut and paste with zero editing, complete with a typo in the
second comment. The code is unit tested on MS Visual Studio -- hence the two
#ifdef's.
// Find First Set
static inline int Ffs64(unsigned long long valueToTest)
{
// returns index of first set bit. Uses UNIX convention
where bit 1 is LSB of word for compatibilit with z/OS ffs()
static const unsigned long long lswMask =
0x00000000ffffffff;
// note Windows provides a _BitScanForward64() but I did it
this way to make it more z/OS-like for test purposes
// if we ever needed Windows efficiency, or if IBM provides
an ffs64(), then this should be re-written to take advantage
if ( valueToTest == 0 ) return 0;
unsigned int testWord;
testWord = valueToTest & lswMask;
if ( testWord != 0 )
{
// _BitScanForward returns base 0
#ifdef WIN32
unsigned long index;
_BitScanForward(&index, testWord);
return index+1;
#else
return ffs(testWord);
#endif
}
else
{
testWord = valueToTest >> 32;
#ifdef WIN32
unsigned long index;
_BitScanForward(&index, testWord);
return index+33;
#else
return ffs(testWord) + 32;
#endif
}
}
I have strong -- but not utterly conclusive; you know what debugging a
complex program is like -- that for the value I had in the OP --
0x0034000000000000? -- the method returns 32, implying that the final ffs()
was called with testWord = 0.
Charles
-----Original Message-----
From: IBM Mainframe Discussion List [mailto:IBM-MAIN@LISTSERV.UA.EDU] On
Behalf Of David Crayford
Sent: Sunday, July 21, 2013 8:31 PM
To: IBM-MAIN@LISTSERV.UA.EDU
Subject: Re: Looking for help with an obscure C integer problem
I'm struggling to see what is wrong with testWord = valueToTest >> 32.
There are no side effects and the sequence point is at the end of the full
expression. Can anybody enlighten me?
Charles, is the code snippet you supplied the exact test cast that is
resulting in undefined behaviour? I cannot recreate the problem and I've
tried it on five different C++ compilers, including z/OS v1.13.
On 22/07/2013 2:33 AM, Charles Mills wrote:
> Right. There are two things here:
>
> 1. Resolving the immediate problem (and understanding exactly why it
> fails might be a good first step).
>
> 2. The quality of the code. You are right; it is poor code. I try to
> write pretty good code. I take no pride in avoiding the use of
> unnecessary parentheses. I confess, I (a.) failed to consider that
> testWord = valueToTest >> 32 would not reliably operate as intended;
> and (b.) I was completely satisfied when the function passed basic
> unit tests and though no more of it. Lesson learned, hopefully. Not
> certain exactly what the lesson is, but I will be more careful in the
> future. I have learned to be cautious about integer type conversions,
> but obviously not cautious enough. I guess the lesson is just like for
> sequences of logical operators: use parentheses to force what you expect.
>
> Charles
>
> -----Original Message-----
> From: IBM Mainframe Discussion List [mailto:IBM-MAIN@LISTSERV.UA.EDU]
> On Behalf Of Harry Wahl
> Sent: Sunday, July 21, 2013 11:21 AM
> To: IBM-MAIN@LISTSERV.UA.EDU
> Subject: Re: Looking for help with an obscure C integer problem
>
> Charles,
> Hi, here is my opinion (and this definitely falls under the category
> of "obscure C"):
> You are not considering the implications of "sequence points" in your
C/C++.
> "Sequence points" should not be confused with "operator precedence."
> Operator precedence is determinate, sequence points are not.
> I believe IBM XLC is at the C++11 level of C/C++. The C/C++ level is
> relevant here because there are sometimes subtle, and sometimes not so
> subtle, differences in how sequence points apply between various
> levels of
> C++.
> While C++11 (the most recent level of C/C++) seems to a have only
> tiny, mostly irrelevant and evolutionary changes from prior levels of
> C/C++; there are significant differences in how "sequence points" are
> defined and effect execution.
> Still, C++11 and the level of the C/C++ compiler that is compiling
> your program is only tangential to the situation you describe. Your
> code will execute with undefined behavior regardless of what compiler
> you use. But, knowing the level of the C/C++ compiler may be
> important if you wish to reconcile why it behaves one way sometimes and
other ways other times (e.g.
> on a different z/OS).
> To me, your failure to consider the subtle impact of sequence points
> renders your code ambiguous and subject to undefined behavior. This
> manifests itself, for example, by executing differently when
> optimized. It is at the compiler's and optimizer's discretion to
> decide the order of execution for code that the C++ standard does not
> specifically define. This includes overlapping execution.
> I think the C/C++ compiler and optimizer are working exactly as
> specified by applicable ISO/IEC standards.
> "The fault, dear Brutus, it not in our stars,But, in ourselves, that
> we are underlings"
> Cassius in Shakespeare's Julius Caesar
>
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