Hi,
The following procedure Unbiased_Rounding for Float works as expected.

function Unbiased_Rounding (X : Float) return Float is
Y : Float;
begin
Asm ("vrintn.f32 %0,%1",
Outputs => Float'asm_output ("=t", Y),
Inputs => Float'asm_input ("t", X));
return Y;
end Unbiased_Rounding;

according to https://gcc.gnu.org/onlinedocs/gcc/Machine-Constraints.html
the constraint t means "VFP floating-point registers s0-s31. Used for 32 bit values” and the constraint w means "VFP floating-point registers d0-d31 and the appropriate subset d0-d15 based on command line options. Used for 64 bit values only”

therefore we wrote our long_float version as

function Unbiased_Rounding (X : Long_Float) return Long_Float is
Y : Long_Float;
begin
Asm ("vrintn.f64 %0,%1",
Outputs => Long_Float'asm_output ("=w", Y),
Inputs => Long_Float'asm_input ("w", X));
return Y;
end Unbiased_Rounding;

however this fails to compile.
GNAT 11.2/0-4 (Alire) complains
Error: invalid instruction shape -- `vrintn.f64 s14,s14’

presumably because the operands are S registers rather than double precisions D registers.
Is this a bug or have we misunderstood something?

Best wishes,
Ahlan

On Sunday, November 19, 2023 at 1:22:22 PM UTC+1, Ahlan Marriott wrote:
> Hi,
> The following procedure Unbiased_Rounding for Float works as expected.
>
> function Unbiased_Rounding (X : Float) return Float is
> Y : Float;
> begin
> Asm ("vrintn.f32 %0,%1",
> Outputs => Float'asm_output ("=t", Y),
> Inputs => Float'asm_input ("t", X));
> return Y;
> end Unbiased_Rounding;
>
> according to https://gcc.gnu.org/onlinedocs/gcc/Machine-Constraints.html
> the constraint t means "VFP floating-point registers s0-s31. Used for 32 bit values” and the constraint w means "VFP floating-point registers d0-d31 and the appropriate subset d0-d15 based on command line options. Used for 64 bit values only”
>
> therefore we wrote our long_float version as
>
> function Unbiased_Rounding (X : Long_Float) return Long_Float is
> Y : Long_Float;
> begin
> Asm ("vrintn.f64 %0,%1",
> Outputs => Long_Float'asm_output ("=w", Y),
> Inputs => Long_Float'asm_input ("w", X));
> return Y;
> end Unbiased_Rounding;
>
> however this fails to compile.
> GNAT 11.2/0-4 (Alire) complains
> Error: invalid instruction shape -- `vrintn.f64 s14,s14’
>
> presumably because the operands are S registers rather than double precisions D registers.
> Is this a bug or have we misunderstood something?
>
> Best wishes,
> Ahlan

The solution is to use %P to access the parameters constrained using “w”
Try as I might I can’t find this wonderful secret documented anywhere.
I stumbled on the solution in the NXP forum where jingpan replied to a question on how to use the ARM VSQRT instruction for double.
When using the inline assembler from C and using named parameters you need to access parameters constrained by “w”, ie D registers using %P[name] rather than %[name] as everywhere else.
Using positional parameters one needs to use %Pn rather than %n
And yes it must be a capital P
I fail to understand why one needs to do this because surely the assembler already knows that the parameter has been constrained to a D register - but I guess this is just an additional quirk to an already very quirky assembler.
My GNAT Ada code to implement the Unbiased_Rounding attribute efficiently using the VFLOATN instruction is therefore

subtype T is Long_Float;

function Unbiased_Rounding (X : T) return T is
Y : T;
begin
Asm (“vrintn.f64 %P0,%P1”,
Outputs => T’asm_output (“=w”, Y),
Inputs => T’asm_input (“w”, X));
return Y;
end Unbiased_Rounding;

Of course we wouldn’t have to resort to assembler at all had there been a built-in intrinsic for VFLOATN as there is for all the other VFLOAT instructions. But I guess that is hoping for too much.

Best wishes and happy programming,
Ahlan

On 2023-11-24 10:09, Ahlan Marriott wrote:
>
> subtype T is Long_Float;
>
> function Unbiased_Rounding (X : T) return T is
> Y : T;
> begin
> Asm (“vrintn.f64 %P0,%P1”,
> Outputs => T’asm_output (“=w”, Y),
> Inputs => T’asm_input (“w”, X));
> return Y;
> end Unbiased_Rounding;

What do you get from T'Unbiased_Rounding?

--
Jeff Carter
"In the frozen land of Nador they were forced to
eat Robin's minstrels, and there was much rejoicing."
Monty Python & the Holy Grail
70