Re: Register constraints + and =

[Jan Hubicka <hubicka at ucw dot cz>]

> 
> I can accept the issue as a matter of documentation, but I don't
> understand the rest.  Remember that all the patterns are executed in
> parallel.  I don't see how adding a USE in parallel could affect
> anything about how the operand is used.

> >> >> (define_insn "*rep_movqi"
> >> >>    [(set (match_operand:P 2 "register_operand" "=c") (const_int 0))
> >> >>     (set (match_operand:P 0 "register_operand" "=D")
> >> >>          (plus:P (match_operand:P 3 "register_operand" "0")
> >> >> 		(match_operand:P 5 "register_operand" "2")))
> >> >>     (set (match_operand:P 1 "register_operand" "=S")
> >> >>          (plus:P (match_operand:P 4 "register_operand" "1") (match_dup 5)))
> >> >>     (set (mem:BLK (match_dup 3))
> >> >> 	(mem:BLK (match_dup 4)))
> >> >>     (use (match_dup 5))]

Without the extra use the patterns says that the only use of operand 5 (CX) is
to add it to DX and SX.

So seeing something like

memcpy(DX,SX,10)
DX+=10
SX+=10

is IMO foldable to

memcpy(DX,SX,20)

Not that current forwprop/combine would do that. But in theory I think it is valid
transform.

The extra use says that one can not change 10 to 20 because the value is used otherwise, 
not only as addent to DX and SX.

At least that is what I was shooting for back then ;)
Honza