Re: About feasibility of implementing an instruction

[Mohamed Shafi <shafitvm@gmail.com>]

2009/7/3 Ian Lance Taylor <iant@google.com>:
> Mohamed Shafi <shafitvm@gmail.com> writes:
>
>> I just want to know about the feasibility of implementing an
>> instruction for a port in gcc 4.4
>> The target has 40 bit register where the normal load/store/move
>> instructions will be able to access the 32 bits of the register. In
>> order to move data into the rest of the register [b32 to b39] the data
>> has to be stored into a 32bit memory location. The data should be
>> stored in such a way that if it is stored for 0-7 in memory the data
>> can be moved to b32-b39 of a even register and if the data in the
>> memory is stored in 16-23 of the memory word then it can be moved to
>> b32-b39 of a odd register. Hope i make myself clear.
>>
>> Will it be possible to implement this in the gcc back-end so that the
>> particular instruction is supported?
>
> In general, the gcc backend can do anything, so, yes, this can be
> supported.  It sounds like this is not a general purpose register, so I
> would probably do it using a builtin function.  If you need to treat it
> as a general purpose register (i.e., the register is managed by the
> register allocator) then you will need a secondary reload to handle
> this.
>

This is a general purpose register. All the 40 bits are used only for
fixed-point data types. When the register is used for fixed-point data
type all the operations except initialization, are done through
built-in functions. For initialization the immediate value should move
through a memory ..i.e there is no immediate load when the data is
40bit. So i am planning to control this using LEGITIMATE_CONSTANT
macro. But then i have a question. If all the operations are through
intrinsics will there be a need for spilling for the variables used in
the built-in functions? If so then depending on the register that get
spilled is even or odd [b32 to b39] of the register gets stored in the
memory to [b0 to b7] or [b16 tr b23] respectively. Will i be able to
keep track of the spilling so that i can reload into the proper
register?

Hope i am clear.

Regards
Shafi