Question

So, I’m building a custom backend for GCC for a processor. This processor has 4 address spaces: local, global, mmm, and mmr. I want to make it such that when writing c code, you can do this:

int global x = 5;

which would cause the compiler to spit out an instruction like this:

ldi.g %reg, 5

I know that certain processors like blackfin and MeP do something similar to this, so I figure its possible to do, however I have no idea how to do it. The technique that should allow me to do this is a variable attribute.

Any suggestions on how I could go about doing this?

Answer 1

You can add target-specific attributes by registering a struct attribute_spec table using TARGET_ATTRIBUTE_TABLE, as described in the GCC internals documentation. The details of struct attribute_spec can be found in the source (gcc/tree.h).

This handler doesn’t need to do anything beyond returning NULL_TREE, although typically it will at least do some error checking. (Read the comments in gcc/tree.h, and look at examples in other targets.)

Later, you can obtain the list of attributes for a declaration tree node with DECL_ATTRIBUTES() (see the internals docs again), and use lookup_attribute() (see gcc/tree.h again) to see if a given attribute in the list.

You want to references to a symbol to generate different assembly based on your new attributes, so you probably want to use the TARGET_ENCODE_SECTION_INFO hook (“Define this hook if references to a symbol or a constant must be treated differently depending on something about the variable or function named by the symbol”) to set a flag on the symbol_ref (as the docs suggest). You can define a predicate for testing this flag in the .md .