Re: Calculation of displacement of a variable.

["C Jaiprakash, Noida" <cjaiprakash at noida dot hcltech dot com>]

Rama Singh wrote:

> --- C Jaiprakash, Noida
> <cjaiprakash@noida.hcltech.com> wrote: > 
>  
>
> > Hi,
> >   I am working on a gcc port, i have a question
> > about how displacements
> > are calculated for variables in gcc. Consider for ex
> > following code
> >
> > void fun()
> > {
> >    char a,b,c,d;
> >    int    x,y,z;
> >    long  i,j,k;
> >
> > }
> > Assuming following sizes for 
> > char = 8 bit
> > int   = 16 bit
> > long  = 32 bit
> > following will the dislacements for above variables
> >
> > a = Base Reg + 1
> > b = Base Reg + 2
> > c = Base Reg + 3
> > d = Base Reg + 4
> >
> > x = Base Reg + 6
> > y = Base Reg + 8
> > z = Base Reg + 10
> >
> > i = Base Reg + 12
> > j = Base Reg + 16
> > k = Base Reg + 20
> >
> > But the problem is h/w accesses memory as nth byte
> > ot nth word so ( Base
> > Reg
> > + 1 ) if accessed in byte access mode will read 1st
> > byte.
> > For ex byte at 0001h if BASE REG = 0000h
> > and it will access 1st word if memory is accessed in
> > word mode. i.e a
> > word from memory location 0002h.
> >
> > Due to this behaviour of h/w the actual
> > displacements should have been
> > generated as below
> >
> > a = Base Reg + 1
> > b = Base Reg + 2
> > c = Base Reg + 3
> > d = Base Reg + 4
> >
> > x = Base Reg + 3
> > y = Base Reg + 4
> > z = Base Reg + 5
> >
> > i = Base Reg + 6
> > j = Base Reg + 8
> > k = Base Reg + 10
> >
> > Is it possible to generate such displacements in
> > gcc? Basically
> > definitions of BITS_PER_UNIT and UNITS_PER_WORD here
> > are dependent on
> > mode, i.e for QImode : BITS_PER_UNIT = 8,
> > UNITS_PER_WORD = 2 HImode :
> > BITS_PER_UNIT = 16, UNITS_PER_WORD = 1 SImode :
> > BITS_PER_UNIT = 16,
> > UNITS_PER_WORD = 1
> >
> > Can this behaviour be implemented in gcc? any
> > pointers in this direction
> > are appreciated.
> >
> >    
> I am not sure about your hardware but whatever is
> clear from your mail is that you can have two memory
> addresses both deferring on the basis of _Modes_. Now
> if you are switching Memory_Access_mode through some
> instruction then you must be emitting that
> memory_switch_insn. Following that just have the
> BAse_Reg + Displacement/2 otherwise keep the default
> setting of Base_Reg + Displacement. 
Yes abosulte address can be divided by 2 for higher modes but this will 
be a costly thing to do.

> This is a little
> tricky part but the displacement can be modified in
> your target.c file. 
Again same as above, for non constant offset the cost is very high. So 
this idea of dividing or modifying displacement can not be applied.

C Jaiprakash