SUBREG GIVs - Suggestion for a quick hack.

[Richard Henderson]

> Please comment on this, before I start out to try it (I would  
> especially be interested in how to change the second of the two  
> set's, i.e. how to `CONS' <some construct> into it ;-).

I've been trying to reproduce this in C, and I can't.  Correct me
if I'm wrong about this C translation:

  void daxpy(int *_n, double dx[], double dy[], double *_da)
  {
    double da = *_da;
    int i, n = *_n, in;
    for (i = 1, in = n-1; in >= 0; ++i, --in)
      dy[i-1] = dy[i-1] + da*dx[i-1];
  }

because with this, I get 

        ldt $f1,0($17)
        mult $f11,$f1,$f1
        ldt $f10,0($18)
        addt $f10,$f1,$f10
        subl $1,1,$1
        addq $17,8,$17
        stt $f10,0($18)
        addq $18,8,$18
        bge $1,$37

which, other than scheduling, is perfect.   The difference I see in 
the initial RTL is this:

	C				Fortran
	-----------			--------------
	r81 = i				r89 = i - 1
	r82 = r81 * 8			r90 = sign_extend(r89)
	r83 = r82 + dy			r91 = r90 
	r84 = r83 - 8			r92 = r91 * 8
					r93 = r92 + dy

I.e:	a = (dy + i*8) - 8		a = (i - 1) * 8 + y

I wonder how much effort it would be to get Fortran to emit its
accesses in this way.  If the answer is not too much, it would be 
a quick way to determine if this form is generally more recognizable
by the backend.


r~