From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <jwakely@redhat.com>
Received: from us-smtp-delivery-124.mimecast.com
 (us-smtp-delivery-124.mimecast.com [216.205.24.124])
 by sourceware.org (Postfix) with ESMTP id 8CAF4385840B
 for <libstdc++@gcc.gnu.org>; Tue,  5 Oct 2021 17:05:34 +0000 (GMT)
DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org 8CAF4385840B
Received: from mail-vs1-f70.google.com (mail-vs1-f70.google.com
 [209.85.217.70]) (Using TLS) by relay.mimecast.com with ESMTP id
 us-mta-337-MzT0JM3CPxeuu8jfRZk_sg-1; Tue, 05 Oct 2021 13:05:32 -0400
X-MC-Unique: MzT0JM3CPxeuu8jfRZk_sg-1
Received: by mail-vs1-f70.google.com with SMTP id
 p7-20020a056102274700b002d863e54bcaso1077225vsu.14
 for <libstdc++@gcc.gnu.org>; Tue, 05 Oct 2021 10:05:31 -0700 (PDT)
X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;
 d=1e100.net; s=20210112;
 h=x-gm-message-state:mime-version:references:in-reply-to:from:date
 :message-id:subject:to:cc:content-transfer-encoding;
 bh=VCt8u1fNmAPGJGfRHPceGGutFjFJz5Fsm1VHo5BTeNM=;
 b=huIOo3OboOILJt9SbI7VOyI52OT/G7n91ILB36RrzSPycrUpCwfdSVuGJH0cY/CE0G
 h2+JSw5JsiwLIKxheuqH9L7CEL/otHdBuxk0vCjXuysz0gHoLmoXLyCPP+RB3AQYcJfh
 AgPTjVnjY/Esvo+CllpfhbznUqJpDe/NR4Uklp5ojI+ffz6DSyeE/HMCF938jYPixuwN
 HqZQCfuueREl4BR993epcKHxpVRh+YFoGk1Vr38pa+bjHNZmzxmuAb+sD209Ilzi30XI
 W9Os5y10h29I+6JYbZAQd9DG+5PfP+h56Lsn1uSK4Zx/42Ubw+FMuVtaZAD03lCZ/CZd
 X10w==
X-Gm-Message-State: AOAM530wEBocSkBfjpcBnJ11lw0tH/1ppjJhwll1pqsPXUROzwrcwD5q
 Mv1g2Xm5Kfjm1qYxZujPeE+tp3m090qI71TrmfV0uOzc4lbYwtxtQ6jDmQ61Z2bvXOM/kugA+DK
 h++c2HxfjDmRT+kXMCX2tMNC+aH8wfCU=
X-Received: by 2002:ab0:7e85:: with SMTP id j5mr13208784uax.2.1633453531538;
 Tue, 05 Oct 2021 10:05:31 -0700 (PDT)
X-Google-Smtp-Source: ABdhPJyCYIXJiGcxCHuQmdrvyx/dJ9hE++iX3UVFBf/BImurZSpAfuXwhoaK/X6NLCFjApMcLvU1nE1xyaIrqyvSpyE=
X-Received: by 2002:ab0:7e85:: with SMTP id j5mr13208765uax.2.1633453531353;
 Tue, 05 Oct 2021 10:05:31 -0700 (PDT)
MIME-Version: 1.0
References: <DM6PR05MB4697451DE3651462E6FE004ED6AF9@DM6PR05MB4697.namprd05.prod.outlook.com>
In-Reply-To: <DM6PR05MB4697451DE3651462E6FE004ED6AF9@DM6PR05MB4697.namprd05.prod.outlook.com>
From: Jonathan Wakely <jwakely@redhat.com>
Date: Tue, 5 Oct 2021 18:05:20 +0100
Message-ID: <CACb0b4mj9VRtSQRr0nzqJvmWZjgRLfGLknEqwZL5WjUP1+vq+Q@mail.gmail.com>
Subject: Re: An interesting thought on exception_defines.h just for fun
To: unlvsur unlvsur <unlvsur@live.com>
Cc: "unlvsur unlvsur via Libstdc++" <libstdc++@gcc.gnu.org>
X-Mimecast-Spam-Score: 0
X-Mimecast-Originator: redhat.com
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
X-Spam-Status: No, score=-6.1 required=5.0 tests=BAYES_00, DKIMWL_WL_HIGH,
 DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, RCVD_IN_DNSWL_LOW,
 RCVD_IN_MSPIKE_H2, SPF_HELO_NONE, SPF_NONE,
 TXREP autolearn=ham autolearn_force=no version=3.4.4
X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on
 server2.sourceware.org
X-BeenThere: libstdc++@gcc.gnu.org
X-Mailman-Version: 2.1.29
Precedence: list
List-Id: Libstdc++ mailing list <libstdc++.gcc.gnu.org>
List-Unsubscribe: <https://gcc.gnu.org/mailman/options/libstdc++>,
 <mailto:libstdc++-request@gcc.gnu.org?subject=unsubscribe>
List-Archive: <https://gcc.gnu.org/pipermail/libstdc++/>
List-Post: <mailto:libstdc++@gcc.gnu.org>
List-Help: <mailto:libstdc++-request@gcc.gnu.org?subject=help>
List-Subscribe: <https://gcc.gnu.org/mailman/listinfo/libstdc++>,
 <mailto:libstdc++-request@gcc.gnu.org?subject=subscribe>
X-List-Received-Date: Tue, 05 Oct 2021 17:05:36 -0000

On Tue, 5 Oct 2021 at 17:57, unlvsur unlvsur via Libstdc++
<libstdc++@gcc.gnu.org> wrote:
>
> Is that possible to use if constexpr(true) for __try and if constexpr(fal=
se) for __catch when we compile it with C++17 and C++20 instead of If(true)=
 and if(false) when people disable exceptions?
>
> gcc/exception_defines.h at 16e2427f50c208dfe07d07f18009969502c25dc8 =C2=
=B7 gcc-mirror/gcc =C2=B7 GitHub<https://github.com/gcc-mirror/gcc/blob/16e=
2427f50c208dfe07d07f18009969502c25dc8/libstdc%2B%2B-v3/libsupc%2B%2B/except=
ion_defines.h>

Yes, that should work OK, but I don't expect it to make much
difference in practice. It might prevent some template instantiations
if something is only odr-used in a catch handler, but that wouldn't be
very common, and those instantiations would be discarded by the
compiler anyway, because the if (false) branch is never taken.

--- a/libstdc++-v3/libsupc++/exception_defines.h
+++ b/libstdc++-v3/libsupc++/exception_defines.h
@@ -32,8 +32,13 @@

#if ! __cpp_exceptions
// Iff -fno-exceptions, transform error handling code to work without it.
-# define __try      if (true)
-# define __catch(X) if (false)
+# if __cplusplus >=3D 201703L
+#  define __try      if constexpr (true)
+#  define __catch(X) if constexpr (false)
+# else
+#  define __try      if (true)
+#  define __catch(X) if (false)
+# endif
# define __throw_exception_again
#else
// Else proceed normally.