Re: How to support 40bit GP register

[Richard Henderson <rth at redhat dot com>]

On 10/21/2009 07:25 AM, Mohamed Shafi wrote:
> For accessing a->b GCC generates the following code:
>
>         move.l  (sp-16), d3
>         lsrr.l  #<16, d3
>         move.l  (sp-12),d2
>         asll    #<16,d2
>         or      d3,d2
>         cmpeq.w #<2,d2
>         jf      _L2
>
> Because data registers are 40 bit for 'asll' operation the shift count
> should be 16+8 or there should be sign extension from 32bit to 40 bits
> after the 'or' operation. The target has instruction to sign extend
> from 32bit to 40 bit.
>
> Similarly there are other operation that requires sign/zero extension.
> So is there any way to tell GCC that the data registers are 40bit and
> there by expect it to generate sign/zero extension accordingly ?

Define a machine mode for your 40-bit type in cpu-modes.def.  Depending 
on how your 40-bit type is stored in memory, you'll use either

  INT_MODE (RI, 5)		  // load-store uses exactly 5 bytes
  FRACTIONAL_INT_MODE (RI, 40, 8) // load-store uses 8 bytes

Where I've arbitrarily chosen "RImode" as a mnemonic for Register 
Integral Mode.  Now you define arithmetic operations, as needed, on
RImode.  You define the "extendsiri" pattern to be that sign-extend from 
32-to-40-bit instruction.  You define your comparison patterns on 
RImode, and not on SImode, since your comparison instruction works on 
the entire 40 bits.

You'll wind up with a selection of patterns in your machine description 
that have a sign-extension pattern built in, depending on the exact 
behaviour of your ISA.  There are plenty of examples on x86_64, mips64, 
and Alpha (to name a few) that have similar properties with SI and 
DImodes.  Examine the -fdump-rtl-combine-details dump for exemplars of 
the canonical forms that the combiner creates when it tries to merge 
sign-extension instructions into preceeding patterns.


r~