Re: My egcs patch for Alpha

[Richard Henderson <rth at cygnus dot com>]

On Tue, Mar 17, 1998 at 11:34:31AM -0500, Richard Gorton wrote:
>       * fold-const.c (operand_equal_for_comparison_p): locally relax
>       width comparison to permit better (faster) unsigned vs. signed
>       comparisons.
>       * expmed.c (extract_bit_field): return subreg when target machine
>       has extzv patterns involving widened modes.

These two were taken care of by Kenner's follow-on patches, correct?
They seem to be, but just making sure.

> 	* alpha.md: modify extzv pattern and add a helper pattern to
> 	perform better byte extract/insert operations

I've modified the extzv pattern to not reject register operands,
though surely you didn't intend to kill my unaligned load bits.  ;-)

As for the helper pattern,

  (define_insn ""
    [(set (match_operand:DI 0 "register_operand" "=r")
        (ashift:DI (subreg:DI (zero_extend:SI (subreg:QI
 			(match_operand:SI 1 "register_operand" "r") 0)) 0)
                   (match_operand:DI 2 "mul8_operand" "I")))]
    "INTVAL (operands[2]) != 0"
    "insbl %1,%s2,%0"
    [(set_attr "type" "shift")])

this is quite ugly, and would also call for many other similar patterns
for inswl etc, and possibly other cases.

Rather than add something like this to the md file, better is to have
combine recognize this and have it simplified.  In this case, all that
is required to match the existing insbl pattern

  [(set (match_operand:DI 0 "register_operand" "=r")
        (ashift:DI (zero_extend:DI (match_operand:QI 1 "register_operand" "r"))
                   (match_operand:DI 2 "mul8_operand" "I")))]

is to realize that (subreg:DI (zero_extend:SI X) 0) can be simpified
to (zero_extend:DI X).

The following patch accomplishes this.

As an aside, can someone tell me why the test immediately above mine
tests to see if the new code is the same as the original pattern's
code?  It seems to me that if the substitution is valid now at all
it shouldn't matter what the previous code was, so there's no point
in testing that.

Along the same lines, is there any point in my test for paradoxical
subreg?  It now occurs to me that the high zeros of

  (subreg:SI (zero_extend:DI X))

is rather pointless.  We might even be able to get rid of the extension
as well.


r~


Wed Mar 18 16:26:06 1998  Richard Henderson  <rth@cygnus.com>

	* combine.c (make_compound_operation): Widen extensions through
	paradoxical subregs.

Index: combine.c
===================================================================
RCS file: /egcs/carton/cvsfiles/egcs/gcc/combine.c,v
retrieving revision 1.25
diff -c -p -d -r1.25 combine.c
*** combine.c	1998/03/18 07:17:51	1.25
--- combine.c	1998/03/19 00:23:48
*************** make_compound_operation (x, in_code)
*** 6038,6043 ****
--- 6038,6054 ----
  
  	  return newer;
  	}
+ 
+       /* If this is a paradoxical subreg, and the new code is a sign or
+ 	 zero extension, omit the subreg and widen the extension.  */
+       if (GET_MODE_SIZE (mode) > GET_MODE_SIZE (GET_MODE (tem))
+ 	  && (GET_CODE (tem) == SIGN_EXTEND
+ 	      || GET_CODE (tem) == ZERO_EXTEND)
+ 	  && subreg_lowpart_p (x))
+ 	{
+ 	  PUT_MODE (tem, mode);
+ 	  return tem;
+ 	}
        break;
        
      default: