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Subject: Re: [boost] [review][constrained_value] Review of Constrained Value Library begins today
From: Johan Råde (rade_at_[hidden])
Date: 2008-12-09 14:01:17

Kim Barrett wrote:
> At 2:23 PM +0100 12/6/08, Robert Kawulak wrote:
>> ... maybe the problem
>> could be somehow solved if we have a function float exact(float) that,
>> given a
>> floating point value (that may have greater precision because of
>> caching in a
>> register), returns a value that is truncated (has exactly the
>> precision of
>> float, not greater).
> I think that something along the lines of the following will likely work:
> inline double exact(double x) {
> struct { volatile double x; } xx = { x };
> return xx.x;
> }
> The idea is to force the value to make a round trip through a memory
> location of the "correct" size. The use of volatile should prevent
> the compiler from optimizing away the trip through memory.

The following should work:

inline double exact(double x)
     double y;
     memcpy(&y, &x, sizeof(double));
     return y;

But I'm not sure the library should do this at all.
It seems like forcing a policy upon the user.
And it may be inefficient.

The root of the problem is that Intel processors may store double values in 80-bit register.
These values may later be truncated to 64-bits.
However, you can force doubles to always be stored with 64-bit precision
by changing the processor floating point control word.
On Visual Studio, this can be done through the command

_controlfp(_PC_53, MCW_PC)

(53 = number of significand (mantissa) bits in the 64-bit format).

--Johan Råde

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