Re: Using Log_Float in inline assembler for ARM

[Ahlan Marriott <ahlan@marriott.org>]

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