Re: Question on cse_not_expected in explow.c:memory_address_addr_space()

[Paolo Bonzini <bonzini at gnu dot org>]

On 09/28/2011 02:14 PM, Georg-Johann Lay wrote:
> This leads to unpleasant code. The machine can access all RAM locations by
> direct addressing. However, the resulting code is:
>
> foo:
> 	ldi r24,lo8(-86)	 ;  6	*movqi/2	[length = 1]
> 	ldi r30,lo8(-64)	 ;  34	*movhi/5	[length = 2]
> 	ldi r31,lo8(10)
> 	std Z+3,r24	 ;  7	*movqi/3	[length = 1]
> .L2:
> 	lds r24,2754	 ;  10	*movqi/4	[length = 2]
> 	sbrs r24,7	 ;  43	*sbrx_branchhi	[length = 2]
> 	rjmp .L2
> 	ldi r24,lo8(-69)	 ;  16	*movqi/2	[length = 1]
> 	ldi r30,lo8(-64)	 ;  33	*movhi/5	[length = 2]
> 	ldi r31,lo8(10)
> 	std Z+3,r24	 ;  17	*movqi/3	[length = 1]
> .L3:
> 	lds r24,2754	 ;  20	*movqi/4	[length = 2]
> 	sbrs r24,7	 ;  42	*sbrx_branchhi	[length = 2]
> 	rjmp .L3
> 	ret	 ;  39	return	[length = 1]
>
> Insn 34 loads 2752 (0xAC0) to r30/r31 (Z) and does an indirect access (*(Z+3),
> i.e. *2755) in insn 7.  The same happens in insn 33 (load 2752) and access
> (insn 17).
>
> Is there a way to avoid this? I tried -f[no-]rerun-cse-after-loop but without
> effect, same for -Os/-O2 and trying to patch rtx_costs. cse_not_expected is
> overridden in some places in the middle-end.

fwprop should take care of propagating the address.  Have you tried 
patching address_costs?  Might be as simple as this (untested):

Index: avr.c
===================================================================
--- avr.c	(revision 177688)
+++ avr.c	(working copy)
@@ -5986,8 +5986,8 @@ avr_address_cost (rtx x, bool speed ATTR
     return 18;
   if (CONSTANT_ADDRESS_P (x))
     {
-      if (optimize > 0 && io_address_operand (x, QImode))
-	return 2;
+      if (optimize > 0)
+	return io_address_operand (x, QImode) ? 2 : 3;
       return 4;
     }
   return 4;

Paolo