diff mbox

PR54555: Use strict_low_part for loading a constant only if it is cheaper

Message ID mvm4mzk0ygc.fsf@hawking.suse.de
State New
Headers show

Commit Message

Andreas Schwab June 17, 2014, 7:47 a.m. UTC 
Postreload may transform (set (REGX) (CONST_INT A)) ... (set (REGX)
(CONST_INT B)) to (set (REGX) (CONST_INT A)) ... (set (STRICT_LOW_PART
(REGX)) (CONST_INT B)), but it should do that only if the latter is
cheaper.  On m68k, a full word load of a small constant with moveq is
cheaper than doing a byte load with move.b.

Tested on m68k-suse-linux and x86_64-suse-linux.  In both cases the size
of cc1* becomes smaller with this change.

Andreas.

	PR rtl-optimization/54555
	* postreload.c (move2add_use_add2_insn): Only substitute
	STRICT_LOW_PART if it is cheaper.
---
 gcc/postreload.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

Comments

Jeff Law June 17, 2014, 6:23 p.m. UTC | #1 
On 06/17/14 01:47, Andreas Schwab wrote:
> Postreload may transform (set (REGX) (CONST_INT A)) ... (set (REGX)
> (CONST_INT B)) to (set (REGX) (CONST_INT A)) ... (set (STRICT_LOW_PART
> (REGX)) (CONST_INT B)), but it should do that only if the latter is
> cheaper.  On m68k, a full word load of a small constant with moveq is
> cheaper than doing a byte load with move.b.
>
> Tested on m68k-suse-linux and x86_64-suse-linux.  In both cases the size
> of cc1* becomes smaller with this change.
>
> Andreas.
>
> 	PR rtl-optimization/54555
> 	* postreload.c (move2add_use_add2_insn): Only substitute
> 	STRICT_LOW_PART if it is cheaper.
Sadly, Kazu didn't add a testcase for the H8/300 cases which inspired 
his change, so we don't know if your patch hurts the H8/300 port or not.

Let's do better this time ;-)  Add a testcase for the m68k port which 
verifies we're getting the desired code.  I don't care if you test the 
assembly code or test the RTL dumps, just that we have a test for the 
case where STRICT_LOW_PART is not a win.

With a testcase, this is approved.

Thanks,

jeff
diff mbox

Patch

diff --git a/gcc/postreload.c b/gcc/postreload.c
index 9d71649..89f0c84 100644
--- a/gcc/postreload.c
+++ b/gcc/postreload.c
@@ -1805,10 +1805,14 @@  move2add_use_add2_insn (rtx reg, rtx sym, rtx off, rtx insn)
 				   gen_rtx_STRICT_LOW_PART (VOIDmode,
 							    narrow_reg),
 				   narrow_src);
-		  changed = validate_change (insn, &PATTERN (insn),
-					     new_set, 0);
-		  if (changed)
-		    break;
+		  get_full_set_rtx_cost (new_set, &newcst);
+		  if (costs_lt_p (&newcst, &oldcst, speed))
+		    {
+		      changed = validate_change (insn, &PATTERN (insn),
+						 new_set, 0);
+		      if (changed)
+			break;
+		    }
 		}
 	    }
 	}