[Patch, AVR]: Fix PR46779

[Denis Chertykov]

2011/6/9 Georg-Johann Lay <avr@gjlay.de>:
> This is a tentative patch to fix PR46779 and hopefully also related
> issues like PR45291.
>
-  /* Disallow QImode in stack pointer regs.  */
-  if ((regno == REG_SP || regno == (REG_SP + 1)) && mode == QImode)
+  /* Don't allocate data to non-GENERAL_REGS registers.  */
+
+  if (regno >= 32)
     return 0;

I think that we not need in this code.
GCC core must bother about this.

+
+  if (GET_MODE_SIZE (mode) == 1)
     return 1;

I'm agree with this.

+
+  /* Disallow big registers that overlap the frame pointer.
+     This will hopefully reduce the number of spill failures.  */
+
+  if (GET_MODE_SIZE (mode) > 2
+      && regno <= REG_Y
+      && regno + GET_MODE_SIZE (mode) >= REG_Y + 1)
+    {
+      return 0;
+    }

Fragment from GCC info:
--------------------------------------
HARD_REGNO_MODE_OK (regno, mode)A C expression that is nonzero if it
is permissible to store a value of mode mode in hard register number
regno (or in several registers starting with that one). For a machine
where all registers are equivalent, a suitable definition is

#define HARD_REGNO_MODE_OK(REGNO, MODE) 1

You need not include code to check for the numbers of fixed registers,
because the allocation mechanism considers them to be always occupied.
-----------------------------------------
Again, GCC core must bother about this.

-  /* Otherwise disallow all regno/mode combinations that span r28:r29.  */
-  if (regno <= (REG_Y + 1) && (regno + GET_MODE_SIZE (mode)) >= (REG_Y + 1))
-    return 0;
-
-  if (mode == QImode)
-    return 1;
-
-  /* Modes larger than QImode occupy consecutive registers.  */
-  if (regno + GET_MODE_SIZE (mode) > FIRST_PSEUDO_REGISTER)
-    return 0;
-

This is a right change.

Denis.