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Re: [PATCH 1/2, combine] Try REG_EQUAL for nonzero_bits

On Tue, Feb 10, 2015 at 11:03:57PM -0700, Jeff Law wrote:
> On 02/09/15 19:19, Thomas Preud'homme wrote:
> >>From: Andrew Pinski []
> >>Sent: Tuesday, February 10, 2015 9:57 AM
> >
> >>>+/* If MODE has a precision lower than PREC and SRC is a non-negative
> >>constant
> >>>+   that would appear negative in MODE, sign-extend SRC for use in
> >>nonzero_bits
> >>>+   because some machines (maybe most) will actually do the sign-
> >>extension and
> >>>+   this is the conservative approach.
> >>>+
> >>>+   ??? For 2.5, try to tighten up the MD files in this regard instead of
> >>this
> >>>+   kludge.  */
> >>
> >>I don't know if this has been mentioned and even though you are just
> >>copying a comment from below but would it make sense to look fixing
> >>what the comment says we should look at after GCC 2.5 (which was over
> >>20 years ago)? Or maybe just remove the comment if it no longer
> >>applies.
> >
> >Actually this bit seems unnecessary as there is already some logic in
> >nonzero_bits1 for the CONST_INT case. So I guess the code can be
> >removed and the comment be moved there at the very least but
> >I'd prefer people from one of the affected target to test it.

I can tell you that the following doesn't trigger on an
--enable-targets=all,go powerpc64-linux bootstrap.  (Ada not built due
to lack of gnat on the machine I used.)  So for powerpc it looks like
the combine SHORT_IMMEDIATES_SIGN_EXTEND code can disappear.  The
rtlanal.c occurrence *is* executed.

Index: gcc/combine.c
--- gcc/combine.c	(revision 220611)
+++ gcc/combine.c	(working copy)
@@ -1742,7 +1742,7 @@ set_nonzero_bits_and_sign_copies (rtx x, const_rtx
 	      && CONST_INT_P (src)
 	      && INTVAL (src) > 0
 	      && val_signbit_known_set_p (GET_MODE (x), INTVAL (src)))
-	    src = GEN_INT (INTVAL (src) | ~GET_MODE_MASK (GET_MODE (x)));
+	    gcc_unreachable ();
 	  /* Don't call nonzero_bits if it cannot change anything.  */
@@ -9802,7 +9802,7 @@ reg_nonzero_bits_for_combine (const_rtx x, machine
 	  && CONST_INT_P (tem)
 	  && INTVAL (tem) > 0
 	  && val_signbit_known_set_p (GET_MODE (x), INTVAL (tem)))
-	tem = GEN_INT (INTVAL (tem) | ~GET_MODE_MASK (GET_MODE (x)));
+	gcc_unreachable ();
       return tem;

Alan Modra
Australia Development Lab, IBM

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