Re: Need help in deciding the instruction set for a new target.

[Ian Lance Taylor <iant at google dot com>]

Mohamed Shafi <shafitvm@gmail.com> writes:

> I want to know if it is good to have both sign and zero extension for
> 16bit immediate.

Hard to say.  It really depends on the kind of constants you expect your
programs to use.  It's generally a good idea to have an efficient way to
load small constants which many programs use, such as 0, 1, -1.  The
latter implies that you will want sign extending immediate operations.
Whether your programs will benefit from an efficient way to load 0xffff,
I don't know.

> Will it be of any use with a configuration where char == short == int == 32bit?

Yes, the size of supported immediate constants is really orthogonal to
the size of the data types.  Your programs are certainly going to refer
to numbers like 0, 1, and -1.  Efficient ways of using those constants
will generally pay off, though of course it is a tradeoff like
everything else in architecture design.

> Will I be able to support these kinds of instructions in a GCC port?

Yes.

> Or will it good to have a separate sign and zero extension
> instruction, which the current instruction set doesnât have.
> Do I need a separate sign and zero ext instructions along with the
> above instructions?

When char is 32 bits, you don't really need sign and zero extension of
unknown values.  So the question is whether you need them for immediate
constants.  As I mention above, it will probably pay off to have
instructions which support sign extending immediate constants.  Whether
that is done via operands to add, etc., or via a load-immediate
instruction, really depends on other characteristics of your
architecture and of the programs you expect to write.

Hope this helps.

Ian