diff mbox

[combine] Don't create LSHIFTRT of zero bits in change_zero_ext

Message ID 566EA5E6.7070704@foss.arm.com
State New
Headers show

Commit Message

Kyrill Tkachov Dec. 14, 2015, 11:20 a.m. UTC 
On 11/12/15 01:26, Segher Boessenkool wrote:
> On Thu, Dec 10, 2015 at 05:05:12PM +0100, Bernd Schmidt wrote:
>> On 12/10/2015 03:36 PM, Kyrill Tkachov wrote:
>>> I'm okay with delaying this for next stage 1 if people prefer, though I
>>> think it's
>>> pretty low risk.
>> I think this is something we should fix now.
> I agree.
>
>>> +	  x = XEXP (x, 0);
>>> +	  if (start > 0)
>>> +	    x = gen_rtx_LSHIFTRT (mode, x, GEN_INT (start));
>> I think this should just use simplify_shift_const. gen_rtx_FOO should be
>> avoided.
> A lot of combine does that, it really is stuck in the 80's.  I wouldn't
> use simplify_shift_const here, but simply simplify_gen_binary.

In change_zero_ext it also creates the final AND with gen_rtx_AND.
Perhaps that should also be changed to simplify_gen_binary.
But I haven't seen any cases where it causes trouble yet, so we
could fix it up separately.


> The patch is okay with or without that change.

Thanks for the suggestions.
I'll go with simplify_gen_binary.
Here's what I'll be committing.

Thanks,
Kyrill



2015-12-10  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>

     * combine.c (change_zero_ext): Do not create a shift of zero length.

>
> Segher
>
diff mbox

Patch

diff --git a/gcc/combine.c b/gcc/combine.c
index 9465e5927145e768f1a5fc43ce7c3621033d8aef..8601d8983ce345e2129dd047b3520d98c0582842 100644
--- a/gcc/combine.c
+++ b/gcc/combine.c
@@ -11037,7 +11037,8 @@  change_zero_ext (rtx *src)
 	  if (BITS_BIG_ENDIAN)
 	    start = GET_MODE_PRECISION (mode) - size - start;
 
-	  x = gen_rtx_LSHIFTRT (mode, XEXP (x, 0), GEN_INT (start));
+	  x = simplify_gen_binary (LSHIFTRT, mode,
+				   XEXP (x, 0), GEN_INT (start));
 	}
       else if (GET_CODE (x) == ZERO_EXTEND
 	       && GET_CODE (XEXP (x, 0)) == SUBREG