Re: RFC: Generic Fixed-Point IIR Filter

[Patrick Noffke <patrick.noffke@gmail.com>]

On Thursday, March 26, 2015 at 3:34:23 PM UTC-5, Randy Brukardt wrote:
> "Patrick Noffke" wrote in message 
> ...
> > How is universal_fixed (likely) implemented on a 32-bit processor?
> > What happens if you multiply two fixed-point types that use all 32-bits
> > for range and precision?  What if one type uses all the bits for the range
> > and another uses all the bits for precision?
> 
> Note that this is wrong:
> 
> >It is therefore sometimes possible to avoid the 2nd type if all 
> >calculations
> >can be done in a single expression which yields a value that fits in the
> >smaller type, despite having intermediate results which do not.
> 
> A result of universal-fixed always has to be converted immediately to 
> another type. It's not legal to use it directly. Specifically, 4.5.5(19.1/2) 
> says: "The above two fixed-fixed multiplying operators shall not be used in 
> a context where the expected type for the result is itself universal_fixed 
> ".
> 

When I try to compute a 2nd order filter result as:

            Filter.Y (1) := (Filter.B (1) * Filter.X (1) +
                               Filter.B (2) * Filter.X (2) +
                               Filter.B (3) * Filter.X (3) -
                               Filter.A (2) * Filter.Y (2) -
                               Filter.A (3) * Filter.Y (3)) / Filter.A (1);

I get this error:

iir_filter.adb:46:60: ambiguous universal_fixed_expression
iir_filter.adb:46:60: possible interpretation as type "Standard.Duration"
iir_filter.adb:46:60: possible interpretation as type "Fixed_Type" defined at iir_filter.ads:3


I don't really understand the clause in 4.5.5(19.1/2).  What defines the context?  I think it's clear in my statement the "expected type for the result" is Fixed_Type, so why is Jeffrey's statement wrong?

And then how do I get past this error?  Is the problem with the + operator?  How do I remove the ambiguity?

> That's so the compiler can always know how many bits are needed for the 
> intermediate results (in order to avoid overflow). But you'll need to figure 
> those out yourself (and as you noted, you can't declare them inside of the 
> generic).
> 
> There's a reason that fixed point is rarely used!
> 

My processor doesn't have an FPU.

Patrick