Reload bug

[Eric Botcazou <ebotcazou at libertysurf dot fr>]

Hello Jan,

I've narrowed down the bug reported as PR optimization/10233, a regression 
from GCC 3.2 present on the 3.2 branch since 3.2.1 (but latent on all active 
branches), to this patch

Thu Oct 17 17:14:07 CEST 2002  Jan Hubicka  <jh at suse dot cz>

	PR opt/7630
	* reload.c (reload_inner_reg_of_subreg): New argument output;
	(push_reload): Update call.


We are trying to reload

	(subreg:SI (reg/v:DI 29 rmm0 [68]) 4)

as the 'in' argument of push_reload.

Before your patch, the compiler correctly detected that the whole register 
needs to be reloaded because we can't access the high part of MMX regs.

But now reload_inner_reg_of_subreg returns 0 for this subreg because its 
return value depends upon the context: if we pass it an 'in' argument, it 
will not check the following condition

  /* If the outer part is a word or smaller, INNER larger than a
     word and the number of regs for INNER is not the same as the
     number of words in INNER, then INNER will need reloading.  */

which happens to apply to this subreg.

Then the subsequent logic is fooled and we end up reloading

	(reg/v:SI 29 emm0)


The rationale for the patch is there:
http://gcc.gnu.org/ml/gcc-patches/2002-10/msg00628.html

The last sentence: "However it is completely irrelevant for input operand in 
all cases - we always can move just the small part of register and use it 
directly, so I've added new operand to bypass this logic in the input 
operand case."

Did you intend to disable the condition for low parts only? (there appears to 
be no testcase associated with the patch so it's hard to tell). If so, does 
the following patch fit your needs?

-- 
Eric Botcazou


	* reload.c (reload_inner_reg_of_subreg): Require reloading for
	non-low parts of input operands.

Index: reload.c
===================================================================
RCS file: /cvs/gcc/gcc/gcc/reload.c,v
retrieving revision 1.178.2.4.2.6
diff -u -p -r1.178.2.4.2.6 reload.c
--- reload.c	29 Mar 2003 19:30:39 -0000	1.178.2.4.2.6
+++ reload.c	8 Apr 2003 17:40:34 -0000
@@ -826,10 +826,12 @@ reload_inner_reg_of_subreg (x, mode, out
      word and the number of regs for INNER is not the same as the
      number of words in INNER, then INNER will need reloading.  */
   return (GET_MODE_SIZE (mode) <= UNITS_PER_WORD
-	  && output
 	  && GET_MODE_SIZE (GET_MODE (inner)) > UNITS_PER_WORD
 	  && ((GET_MODE_SIZE (GET_MODE (inner)) / UNITS_PER_WORD)
-	      != HARD_REGNO_NREGS (REGNO (inner), GET_MODE (inner))));
+	      != HARD_REGNO_NREGS (REGNO (inner), GET_MODE (inner)))
+	  /* This is not necessary for low parts of input operands
+	     because we can always access them directly.  */
+	  && (output || ! subreg_lowpart_p (x)));
 }
 
 /* Record one reload that needs to be performed.