Re: c/3752: gcc 3.0: __builtin_constant_p(ptr==ptr), regression

[Richard Henderson <rth at redhat dot com>]

On Sun, Jul 22, 2001 at 10:58:54PM -0700, Richard Henderson wrote:
> I guess this worked before by having CSE simplify the code appropriately.
> If so, this means we've got a CSE regression somewhere too.  I guess I'll
> look for that...

CSE isn't very good at simplifying

	(set (reg:SI X) (const_int 0))

	[...]

	(set (strict_low_part (subreg:QI (reg:SI X) 0))
	     (eq:QI ...))

It's probably best if we avoid that construct early on,
and make that sort of transformation later if we can.

Tested on i686 linux; applied mainline and branch.


r~


        * config/i386/i386.c (ix86_expand_setcc): Don't use method 0
        before CSE.
        * config/i386/i386.md: New setcc+movzbl peephole2.

Index: i386.c
===================================================================
RCS file: /cvs/gcc/gcc/gcc/config/i386/i386.c,v
retrieving revision 1.288
diff -c -p -d -r1.288 i386.c
*** i386.c	2001/07/22 21:42:35	1.288
--- i386.c	2001/07/23 21:13:21
*************** ix86_expand_setcc (code, dest)
*** 6357,6365 ****
            emit subreg setcc, zero extend.
       2 -- destination is in QImode:
            emit setcc only.
-   */
  
!   type = 0;
  
    if (GET_MODE (dest) == QImode)
      type = 2;
--- 6357,6368 ----
            emit subreg setcc, zero extend.
       2 -- destination is in QImode:
            emit setcc only.
  
!      We don't use mode 0 early in compilation because it confuses CSE.
!      There are peepholes to turn mode 1 into mode 0 if things work out
!      nicely after reload.  */
! 
!   type = cse_not_expected ? 0 : 1;
  
    if (GET_MODE (dest) == QImode)
      type = 2;
Index: i386.md
===================================================================
RCS file: /cvs/gcc/gcc/gcc/config/i386/i386.md,v
retrieving revision 1.288
diff -c -p -d -r1.288 i386.md
*** i386.md	2001/07/20 12:15:36	1.288
--- i386.md	2001/07/23 21:13:21
***************
*** 13133,13138 ****
--- 13133,13157 ----
  			   (match_dup 0)
  			   (pc)))]
    "")
+ 
+ ;; Convert setcc + movzbl to xor + setcc if operands don't overlap.
+ 
+ (define_peephole2
+   [(set (reg 17) (match_operand 0 "" ""))
+    (set (match_operand:QI 1 "register_operand" "")
+ 	(match_operator:QI 2 "ix86_comparison_operator"
+ 	  [(reg 17) (const_int 0)]))
+    (set (match_operand 3 "q_regs_operand" "")
+ 	(zero_extend (match_dup 1)))]
+   "peep2_reg_dead_p (3, operands[1])
+    && ! reg_overlap_mentioned_p (operands[3], operands[0])"
+   [(parallel [(set (match_dup 3) (const_int 0))
+ 	      (clobber (reg:CC 17))])
+    (set (match_dup 4) (match_dup 0))
+    (set (strict_low_part (match_dup 5))
+ 	(match_dup 2))]
+   "operands[4] = gen_rtx_REG (GET_MODE (operands[0]), 17);
+    operands[5] = gen_rtx_REG (QImode, REGNO (operands[3]));")
  
  ;; Call instructions.