[PATCH] combine: Omit redundant AND in change_zero_ext.

[Dominik Vogt]

This is another micro-optimisation in change_zero_ext.  If an

  (and (lshiftrt ... (N)) (M))

generated by change_zero_ext is equivalent to just

  (lshiftrt ... (N))

(because the AND constant selects the N rightmost bits of the
result), strip off the AND.

_But_ I'm still not completely convinced whether this is a good
idea.  It may become necessary to add md patterns to deal with
just the LSHIFTRT.  On the other hand it saves the need for
another special case in change_zero_ext, and a less obvious, very
specific risbg pattern on s390 

--

Bootstrapped and regression tested on s390x and s390.  (Targets
with risbg-like instructions (Power, others?) may need some
tuning.)

Ciao

Dominik ^_^  ^_^

-- 

Dominik Vogt
IBM Germany

gcc/ChangeLog-change_zero_ext-2

        * combine.c (change_zero_ext): Skip generation of redundant AND.

>From bbd2cfc122c74d1e50894222a7998915848b5ec6 Mon Sep 17 00:00:00 2001
From: Dominik Vogt <v...@linux.vnet.ibm.com>
Date: Tue, 13 Dec 2016 12:37:08 +0100
Subject: [PATCH] combine: Omit redundant AND in change_zero_ext.

In case (and (lshiftrt)) is equivalent to just (lshiftrt).
---
 gcc/combine.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/gcc/combine.c b/gcc/combine.c
index 19851a2..5ebf31c 100644
--- a/gcc/combine.c
+++ b/gcc/combine.c
@@ -11280,8 +11280,13 @@ change_zero_ext (rtx pat)
       else
        continue;
 
-      wide_int mask = wi::mask (size, false, GET_MODE_PRECISION (mode));
-      x = gen_rtx_AND (mode, x, immed_wide_int_const (mask, mode));
+      if (!(GET_CODE (x) == LSHIFTRT
+           && CONST_INT_P (XEXP (x, 1))
+           && size + INTVAL (XEXP (x, 1)) == GET_MODE_PRECISION (mode)))
+       {
+         wide_int mask = wi::mask (size, false, GET_MODE_PRECISION (mode));
+         x = gen_rtx_AND (mode, x, immed_wide_int_const (mask, mode));
+       }
 
       SUBST (**iter, x);
       changed = true;
-- 
2.3.0