Re: [PATCH 1/2] simplify-rtx: Simplify trunc of and of shiftrt

[Segher Boessenkool <segher@kernel.crashing.org>]

On Tue, Nov 10, 2015 at 12:16:09PM +0100, Bernd Schmidt wrote:
> On 11/09/2015 08:33 AM, Segher Boessenkool wrote:
> >If we have
> >
> >	(truncate:M1 (and:M2 (lshiftrt:M2 (x:M2) C) C2))
> >
> >we can write it instead as
> >
> >	(and:M1 (lshiftrt:M1 (truncate:M1 (x:M2)) C) C2)
> >
> >
> >+  /* Likewise (truncate:QI (and:SI (lshiftrt:SI (x:SI) C) C2)) into
> >+     (and:QI (lshiftrt:QI (truncate:QI (x:SI)) C) C2) for suitable C
> >+     and C2.  */
> >+  if (GET_CODE (op) == AND
> >+      && (GET_CODE (XEXP (op, 0)) == LSHIFTRT
> >+	  || GET_CODE (XEXP (op, 0)) == ASHIFTRT)
> >+      && CONST_INT_P (XEXP (XEXP (op, 0), 1))
> >+      && CONST_INT_P (XEXP (op, 1))
> >+      && UINTVAL (XEXP (XEXP (op, 0), 1)) < precision
> >+      && ((GET_MODE_MASK (mode) >> UINTVAL (XEXP (XEXP (op, 0), 1)))
> >+	  & UINTVAL (XEXP (op, 1)))
> >+	 == ((GET_MODE_MASK (op_mode) >> UINTVAL (XEXP (XEXP (op, 0), 1)))
> >+	     & UINTVAL (XEXP (op, 1))))
> 
> In general this would be easier to read if there were intermediate 
> variables called shift_amount and mask.

Yes I know.  All the rest of the code around is it like this though.
Do you want this written in a saner way?

> I'm not entirely sure what the 
> last condition here is supposed to test.

It tests whether moving the truncate inside will give the same result.
It essentially looks if it works for an x with all bits set; if that
works, it works for any x.

> Is it related to...
> 
> >+	    return simplify_gen_binary (AND, mode, op0, XEXP (op, 1));
> 
> ... the fact that here I think you'd have to trunc_int_for_mode the AND 
> amount for the smaller mode?

Ugh yes, I still have to do that for it to be valid RTL in all cases.
Thanks for catching it.


Segher