Re: C code .vs. Assembly code for Microcontrollers/DSPs ?

In article 96-03-044, max@gac.edu (Max Hailperin) wrote:


> An even bigger problem, in my experience is that C/C++ are built on
> the strange notion that when you operate on two n-bit numbers, you
> get an n-bit result, even when multiplying. This doesn't make a
> whole lot of mathematical sense, and moreover the processor
> architects have in my experience always gotten it right -- they have
> instructions for multplying two 16-bit numbers and getting a 32-bit
> product, or two 32-bit numbers and getting a 64-bit product, or
> whatever. The C compiler "hides" these from you, which can cause a
> major slowdown. In multiple-precision multiplication, which is
> generally implemented roughly as the normal grammar-school
> multiplication algorithm but with each "digit" being n bits wide
> (i.e., radix 2^n), you typically wind up with having to use half as
> large a value of n, hence twice as many "digits" in each of the
> multiplier and multiplicand, hence a four-fold slowdown (2 squared).


I agree with you here. I've been fighting this one for years. It's
utterly amazing that a language like C that has been used for so many
'embedded' type programs has never standardized a way to use
'efficient' multiple precision arithmetic.


The compiler technology has now matured to the point where even a
relatively kludgey interface involving passing C++ references to
results can be open coded to produce excellent machine-level
instructions.


I'd say that the time has now come to standardize access to carry
information on +/- and high-order bits of *.


Perhaps the answer can be found in the previous thread about sincos,
divrem, etc.


--- Proposal (the names of the functions probably need some work):


sum(x,y,c)/difference(x,y,c) produce a structure consisting of a
low-order value and a 'carry'. These can be extracted by 'sumlo' and
'carryhi' -- e.g., sumlo(sum(x,y,c)) produces the low-order 'sum' bits
of x+y+c and carryhi(sum(x,y,c)) produces the hi-order 'carry' bit.


similarly for
prodhi(times(x,y,c)) is the high order bits of x*y+c and
prodlo(times(x,y,c)) is the low order bits of x*y+c, where
x,y,c are single-precision numbers.


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