From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugs-return-412233-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org>
Received: (qmail 31599 invoked by alias); 15 Jan 2013 14:31:31 -0000
Received: (qmail 30808 invoked by uid 48); 15 Jan 2013 14:31:06 -0000
From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/15184] [4.6/4.7/4.8 Regression] Direct access to byte inside word not working with -march=pentiumpro
Date: Tue, 15 Jan 2013 14:31:00 -0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: changed
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: target
X-Bugzilla-Keywords: missed-optimization
X-Bugzilla-Severity: minor
X-Bugzilla-Who: rguenth at gcc dot gnu.org
X-Bugzilla-Status: NEW
X-Bugzilla-Priority: P2
X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org
X-Bugzilla-Target-Milestone: 4.6.4
X-Bugzilla-Changed-Fields:
Message-ID: <bug-15184-4-GN73m0r997@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-15184-4@http.gcc.gnu.org/bugzilla/>
References: <bug-15184-4@http.gcc.gnu.org/bugzilla/>
X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/
Auto-Submitted: auto-generated
Content-Type: text/plain; charset="UTF-8"
MIME-Version: 1.0
Mailing-List: contact gcc-bugs-help@gcc.gnu.org; run by ezmlm
Precedence: bulk
List-Id: <gcc-bugs.gcc.gnu.org>
List-Archive: <http://gcc.gnu.org/ml/gcc-bugs/>
List-Post: <mailto:gcc-bugs@gcc.gnu.org>
List-Help: <mailto:gcc-bugs-help@gcc.gnu.org>
Sender: gcc-bugs-owner@gcc.gnu.org
X-SW-Source: 2013-01/txt/msg01352.txt.bz2


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=15184

--- Comment #24 from Richard Biener <rguenth at gcc dot gnu.org> 2013-01-15 14:31:02 UTC ---
We fail to fold

  (short unsigned int) ( X | (signed short) Y )

to

  (short unsigned int ) X | (short unsigned int) Y

and thus end up with needlessly many conversions on the tree level.

Index: gcc/convert.c
===================================================================
--- gcc/convert.c       (revision 195194)
+++ gcc/convert.c       (working copy)
@@ -750,7 +750,7 @@ convert_to_integer (tree type, tree expr
                                || ex_form == MULT_EXPR)))
                      typex = unsigned_type_for (typex);
                    else
-                     typex = signed_type_for (typex);
+                     ;
                    return convert (type,
                                    fold_build2 (ex_form, typex,
                                                 convert (typex, arg0),

"fixes" that (but has no effect on the resulting assembly).

Btw, even generic tuning assembler is bad which is IMHO the important fact.

I don't think combine is the right mechanism to deal with this as
partial register writes are certainly not wanted.  Instead it's necessary
to see that we can store into memory directly which requires us to combine
too much things.

This is rather to be dealt with on the tree level - the bswap pass
infrastructure should provide a starting point for value composition
like this.