[Proj] Problems with /Op option used to compile proj in Microsoft Visual Studio

Calogero Mauceri mauceri at actgate.com
Thu Mar 15 03:05:35 EST 2012

Hi Janne,

>> Disabling that option, the result returned in the version of proj
>> compiled with Microsoft Visual C++ 2003 is more consistent.
>> Is there any reason why the /Op option is used to compile the proj?
> I think that the /Op option only works with the 2003 version and what it does is as follows
> (text from MSDN):
> "This option improves the consistency of floating-point tests for equality and inequality by disabling optimizations that could change the precision of floating-point calculations.
> By default, the compiler uses the coprocessor's 80-bit registers to hold the intermediate results of floating-point calculations. This increases program speed and decreases program size. However, because the calculation involves floating-point data types that are represented in memory by less than 80 bits, carrying the extra bits of precision (80 bits minus the number of bits in a smaller floating-point type) through a lengthy calculation can produce inconsistent results."
> http://msdn.microsoft.com/en-us/library/aa984742(v=vs.71).aspx
> So it is rather obvious that MS compilers use the full 80 bit FPP registers in PC processors by default and in most cases and the reason for using that switch in the first place must be the idea to have similar performance with other ANSI compiler environments? I am not sure if this answered the question?
> regards: Janne.

Ok, the /Op option seems to be used to produce consistent and repeatable 
The problem is that the cost of tha//t repeatability is that the 
intermidiate results in computations *lose accuracy*.
That explains the problem proj (compiled with Visual Studio 2003) is 
having in the conversion I stated before.

It is well explained here (text from MSDN)


"Many C++ compilers offer a "consistency" floating-point model, (through 
a /Op or /fltconsistency switch) which enables a developer to create 
programs compliant with strict floating-point semantics. When engaged, 
this model prevents the compiler from using most optimizations on 
floating-point computations [...]. The consistency model, however, has a 
dark-side. In order to return predictable results on different FPU 
architectures, nearly all implementations of /Op round intermediate 
expressions to the user specified precision; for example, consider the 
following expression:

float a, b, c, d, e;
. . .
a = b*c + d*e;

In order to produce consistent and repeatable results under /Op, this expression gets evaluated as if it were implemented as follows:

float x = b*c;
float y = d*e;
a = x+y;

The final result now suffers from single-precision rounding errors  /at each step in evaluating the expression/. Although this interpretation doesn't strictly break any C++ semantics rules, it's almost never the best way to evaluate floating-point expressions. It is generally more desirable to/  compute the intermediate results in as high as precision as is practical/. For instance, it would be better to compute the expression a=b*c+d*e in a higher precision as in,

double x = b*c;
double y = d*e;
double z = x+y;
a = (float)z;

or better yet

long double x = b*c;
long double y = d*e
long double z = x+y;
a = (float)z;

When computing the intermediate results in a higher precision, the final 
result is *significantly more accurate*. Ironically, by adopting a 
consistency model, the likelihood of error is increased precisely when 
the user is trying to reduce error by disabling unsafe optimizations. 
Thus the consistency model can seriously reduce efficiency while 
simultaneously providing no guarantee of increased accuracy. To serious 
numerical programmers, this doesn't seem like a very good tradeoff and 
is the primary reason that the model is not generally well received.

 From Visual C++ 2005 they are suggesting to use other kind of options 
fp:precise, fp:fast or fp:strict.

"Under*fp:precise*, only safe optimizations are performed on 
floating-point code and, unlike /Op, intermediate computations are 
consistently performed at the highest/practical/precision."

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