Re: MODES_TIEABLE_P yes or no

[Björn Haase <bjoern dot m dot haase at web dot de>]

Hi,

thank you for having a look at these aspects of the AVR back-end! If I 
understand the meaning of MODES_TIEABLE_P correctly, I agree with you. I have 
also done a preliminary test of your suggestion and found out that with your 
suggested change the code is *much* more efficient for all cases I have 
looked at so far.

Concerning the AVR convention for mode alignment:
IIUC, the *only* reason for aligning all modes of size HI and upward is, that 
this way it is easier to make use of the move-word (movw) instruction. This 
instruction can move 2 bytes simultaneously, given that the first register 
has an even number. By forcing all "larger" modes to start in even registers, 
there is no need to check whether a variable of this mode starts in an even 
register. Otherwise it would be cumbersome for the back-end implementation.
I, therefore, do not expect any problem concerning alignment, when changing 
the definition of MODES_TIEABLE_P.

The main question in my opinion is, whether it may happen that paradoxial 
subreg expressions show up. IIUC, when trying to perform a SI operation on a 
HI register, gcc allways generates appropriate SIGN_EXTEND or ZERO_EXTEND 
patterns. If this is allways true, I do not expect any problem. However, in 
order to get a really reliable answer, one probably would need to ask Marek 
Michalkiewicz or Denis Chertykov.

I have done a couple of course tests concerning your suggestion:
Indeed, when changing the macro definition, gcc generates considerably better 
code! I have compiled the following example with a gcc-4.0.0 snapshot dating 
from 12/12/2004 (head presently is broken again) after using of a patch 
concerning the reload problem. Indeed the four superfluous mov instructions 
have vanished.

Yours, 

Björn

/// Sample program
#include <inttypes.h>
int8_t 
foo  (int16_t a, int32_t b)
{ int8_t c;
  c = a - b;
  if (c)
   { return 1;
   }
  else
  { return 2;
  }
}

Compilation result with MODES_TIEABLE_P  defined to be 0
foo:
/* prologue: frame size=0 */
/* prologue end (size=0) */
	 ;  basic block 0
	mov r18,r20
	mov r19,r21
	mov r20,r22
	mov r21,r23
	cp r24,r18
	breq .L2
	 ;  basic block 1
	ldi r24,lo8(1)
	ldi r25,hi8(1)
	ret
.L2:
	 ;  basic block 2
	ldi r24,lo8(2)
	ldi r25,hi8(2)
	ret
/* epilogue: frame size=0 */


Compilation result with MODES_TIEABLE_P  defined to be 1
foo:
/* prologue: frame size=0 */
/* prologue end (size=0) */
	 ;  basic block 0
	cp r24,r20
	breq .L2
	 ;  basic block 1
	ldi r24,lo8(1)
	ldi r25,hi8(1)
	ret
.L2:
	 ;  basic block 2
	ldi r24,lo8(2)
	ldi r25,hi8(2)
	ret
/* epilogue: frame size=0 */