Re: Storing 16bit values in upper part of 32bit registers

["H. Peter Anvin" <hpa@zytor.com>]

On 10/15/2009 08:56 AM, Richard Henderson wrote:
> On 10/15/2009 07:41 AM, Markus L wrote:
>> However the IS is designed so that it is beneficial to to store 16bit
>> values in the high part of the registers (rNh) and also the calling
>> conventions that we want follow require 16bit values to be passed and
>> returned in rNh.
>>
>> What would be the "proper way" make the compiler use the upper parts
>> of these registers for the 16bit operands?
> 
> This feature is going to be difficult, but not impossible, and unless 
> your ISA has some really odd features I won't vouch for the code quality.
> 
> You say you want to canonically represent HImode in the high-part of the 
> register.  Additionally, you'll have to represent QImode in the 
> high-part (if not further in the high byte).
> 
> You'll need to follow the mips port and define TRULY_NOOP_TRUNCATION and 
> the associated truncMN2 patterns.
> 
> If you do all this, you won't have to do anything with FUNCTION_VALUE 
> etc at all.
> 

Sorry for a *way* *late* reply to this, but wouldn't it also work to
model the register file as a set of 16-bit registers (since that's what
you really have -- individually addressable 16-bit registers) and
exclude SImode values from register pairs which are not aligned.  Then
one can simply prefer the high 16-bit registers to the low 16-bit
registers in the register priority sequence.

I'm assuming there is something wrong with this, but I'm kind of curious
as to what it would be.

	-hpa