From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugs-return-434791-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org>
Received: (qmail 23250 invoked by alias); 17 Nov 2013 11:41:45 -0000
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
Received: (qmail 23215 invoked by uid 48); 17 Nov 2013 11:41:39 -0000
From: "glisse at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/59159] New: Need opaque pass-through as optimization barrier
Date: Sun, 17 Nov 2013 11:41:00 -0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: c
X-Bugzilla-Version: 4.9.0
X-Bugzilla-Keywords:
X-Bugzilla-Severity: enhancement
X-Bugzilla-Who: glisse at gcc dot gnu.org
X-Bugzilla-Status: UNCONFIRMED
X-Bugzilla-Priority: P3
X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org
X-Bugzilla-Target-Milestone: ---
X-Bugzilla-Flags:
X-Bugzilla-Changed-Fields: bug_id short_desc product version bug_status bug_severity priority component assigned_to reporter
Message-ID: <bug-59159-4@http.gcc.gnu.org/bugzilla/>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 7bit
X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/
Auto-Submitted: auto-generated
MIME-Version: 1.0
X-SW-Source: 2013-11/txt/msg01568.txt.bz2

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59159

            Bug ID: 59159
           Summary: Need opaque pass-through as optimization barrier
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: glisse at gcc dot gnu.org

(category:c because I am asking for a built-in, but this is more about
middle/back-end)

Since gcc doesn't support FENV, I often need to insert optimization barriers in
my code that hide things from gcc optimizers, so it doesn't perform constant
propagation, or replace x*-y with -x*y, or move operations across fesetround,
etc. Note that speed is still very important, doing a store/read on a volatile
variable (the most portable optimization barrier) can make the whole
application 15% slower.

One common way to do that is to use an inline asm like:
asm volatile ("", "+g"(x));
(4.9 is the first version where this doesn't ICE all over the place on x86, use
"+m" earlier)
so no code is emitted but the value of x is hidden from the compiler. However,
the "g" constraint doesn't include FP registers. Depending if the code is
compiled for power, sparc, itanium, arm or x86_64, I need to add either "d",
"e", "f", "w", or "x" (yeah, there aren't 2 using the same letter). It looks
like there isn't even a letter for x87 (PR 59157). Using the more general "+X"
is tempting but ICEs (PR 59155).

There is at least one case this inline asm approach can never handle optimally:
constants. We need the variable as asm output or it won't have any effect. But
the constants are in a read-only location that is not suitable for an output
operand, so with "+m" gcc copies the constant to another memory location.

Ideally, there would be a __builtin_opaque built-in that does the right thing
and I wouldn't have to think about it...