Re: [GCC 4.3] Strange -O2 optimization issue

[Mateusz Loskot <mateusz@loskot.net>]

Brian Dessent wrote:
> Mateusz Loskot wrote:
> 
>> Why the first value printed is different (136623933) in the 3rd
>> test case.
> 
> Your suspicion is correct, as this violates the ISO C aliasing rules:

Brian,

Good to hear I was close ;-)

>> static unsigned long HashDouble(double* pdfVal)
>> {
>>      unsigned int* pnValue = (unsigned int*)pdfVal;
> 
> You're accessing a variable of type double through a pointer of type
> unsigned int.  For the purposes of optimization the compiler is allowed
> to assume that values of type double will only be accessed through
> variables of type double, and thus it can assume that pdfVal and pnValue
> can't refer to the same thing.  It may seem nonsensical in this instance
> that it would assume that, but it's still legal for the compiler to do
> so.  
 > [...]

Thank you very much for in-depth explanation of the problem.
It was greatly helpful.

> If you want to rewrite your code in a conformant way you can use a union
> or memcpy; or you can disable strict aliasing with
> -fno-strict-aliasing.

I fixed my code with memcpy option.

> -Wstrict-aliasing is intended to warn about
> things like this, but since it's included in -Wall it obviously failed
> to warn in your case.  There are several levels of -Wstrict-aliasing
> though, so you may need to turn to -Wstrict-aliasing=2 to catch this
> case.  See the docs for details:
> <http://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#index-Wstrict_002daliasing-312>

-Wstrict-aliasing=2 does not warn about aliasing rules broken in that 
line with the case, but -Wstrict-aliasing=1 does. Yes, I've read the 
manual and understand it why.

> As to why only the first value printed differs, or why taking the
> address of pnValue changes the outcome, or why older versions of gcc
> worked fine: that is the general nature of undefined behavior.  It can
> take on any form whatsoever, from working perfectly, to failing
> spectacularly, or anywhere in between.  All rules are out the window. 

Right, I suspected UB from the beginning of my investigation.

> It is best not to try to understand the effects or outcome but rather to
> understand how to fix the code so that it is no longer undefined.

Very good point :-)

>> I'd be also very thankful for references in C/C++ standards
>> explaining this behavior of GCC.
> 
> See section 6.5.7 of the C99 standard.
> 
> Brian


Thank you very much again!

Best regards,
-- 
Mateusz Loskot, http://mateusz.loskot.net
Charter Member of OSGeo, http://osgeo.org