Re: Excluding parts of the instruction set during compilation

[Kyrill Tkachov <kyrylo dot tkachov at arm dot com>]

On 13/02/14 10:21, Fabian Oboril wrote:
> Hello everybody,
>
> I am currently working on a project for which I need to compile C-code
> using only a small subset of the available instruction set. So for
> example, instead of using add, addi, addiu and addu, I would like that
> the compiler is using just one of them. Is there a way, that this can be
> handled with gcc, for example by giving a list of not-to-use
> instructions at compile time?
>
> Can anyone help?

Hi Fabian,

It's not really possible to give gcc a list of instructions not to use. What if 
some operations in your source code can only be performed by those instructions 
that you excluded? Getting the compiler to synthesise any operation using any 
arbitrary subset of instructions is not realistic.
Some targets, for example arm, can generate code for multiple revisions of the 
architecture and newer revisions might allow instructions that are not present 
in previous ones. In that case gcc will know not to generate those instructions 
if asked to compile for the old instruction by having -march set appropriately, 
but the list of instructions available for use is defined by the architecture, 
not the user.

HTH,
Kyrill