Aucbvax.6857
fa.info-vax
utzoo!decvax!ucbvax!info-vax
Thu Apr 22 09:30:57 1982
unix cc and float registers
>From dove@mit-dspg@mit-mc Thu Apr 22 09:22:29 1982
The following results were obtained from mit-vax (berkeley 4.1 I think).
The adb files were taken from a.out.  The only compiler switch was -O.

---junkc.c---
main()
{
	register double z=0, x=1;
	register int i=100000;

	while(i-->0)
	{
		z += x;
		z += x;
		z += x;
		z += x;
		z += x;
		z += x;
		z += x;
		z += x;
		z += x;
		z += x;
	}
}
---junkc.adb----
_main:
_main:		800
3e:		subl2	$10,sp
41:		movd	_environ+4,-8(fp)
49:		movd	_environ+c,-10(fp)
51:		movl	$186a0,r11
58:		brb	8c
5a:		addd2	-10(fp),-8(fp)
5f:		addd2	-10(fp),-8(fp)
64:		addd2	-10(fp),-8(fp)
69:		addd2	-10(fp),-8(fp)
6e:		addd2	-10(fp),-8(fp)
73:		addd2	-10(fp),-8(fp)
78:		addd2	-10(fp),-8(fp)
7d:		addd2	-10(fp),-8(fp)
82:		addd2	-10(fp),-8(fp)
87:		addd2	-10(fp),-8(fp)
8c:		movl	r11,r0
8f:		decl	r11
91:		tstl	r0
93:		bgtr	5a
95:		ret
96:		halt
97:		halt
_exit:

	
---junkp.p---
program junkp (input, output);
var	z, x :real; 
	i : integer;

begin	z := 0;
	x := 1;
	i := 100000;

	while i>0 do
	begin
		z := z+x;
		z := z+x;
		z := z+x;
		z := z+x;
		z := z+x;
		z := z+x;
		z := z+x;
		z := z+x;
		z := z+x;
		z := z+x;
		i := i-1
	end
end.

---junkp.adb---
_program:
_program:	0
66:		subl2	$10,sp
69:		moval	_UNWIND,(fp)
70:		moval	__disply+8,-4(fp)
78:		movq	__disply+8,-c(fp)
80:		movq	ap,__disply+8
87:		clrq	_z
8d:		cvtld	$1,_x
94:		movl	$186a0,_i
9f:		brb	_program+b2
a1:		addd2	_x,_z
ac:		addd2	_x,_z
b7:		addd2	_x,_z
c2:		addd2	_x,_z
cd:		addd2	_x,_z
d8:		addd2	_x,_z
e3:		addd2	_x,_z
ee:		addd2	_x,_z
f9:		addd2	_x,_z
_program+a0:	addd2	_x,_z
_program+ab:	subl2	$1,_i
_program+b2:	tstl	_i
_program+b8:	bgtr	a1
_program+ba:	movq	-c(fp),__disply+8
_program+c2:	ret
_program+c3:

	
As you can see, registers don't get used for floating point.
Here is an equivalent program with ints instead of doubles, and it does the
right thing:
---junkci.c---
main()
{
	register int z=0, x=1;
	register int i;

	for(i=0; i<100000; i++)
	{
		z += x;
		z += x;
		z += x;
		z += x;
		z += x;
		z += x;
		z += x;
		z += x;
		z += x;
		z += x;
	}
}

	
---junkci.adb---
_main:
_main:		e00
3e:		clrl	r11
40:		movl	$1,r10
43:		clrl	r9
45:		addl2	r10,r11
48:		addl2	r10,r11
4b:		addl2	r10,r11
4e:		addl2	r10,r11
51:		addl2	r10,r11
54:		addl2	r10,r11
57:		addl2	r10,r11
5a:		addl2	r10,r11
5d:		addl2	r10,r11
60:		addl2	r10,r11
63:		incl	r9
65:		cmpl	r9,$186a0
6c:		blss	45
6e:		ret
6f:		halt
_exit:

Comments?  (when will c understand floats!)