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Re: [patch 3/5] don't restrict bit range for -fstrict-volatile-bitfields

From: Sandra Loosemore <sandra at codesourcery dot com>
To: "Joseph S. Myers" <joseph at codesourcery dot com>
Cc: Julian Brown <julian at codesourcery dot com>, Richard Biener <richard dot guenther at gmail dot com>, Jakub Jelinek <jakub at redhat dot com>, GCC Patches <gcc-patches at gcc dot gnu dot org>
Date: Mon, 17 Jun 2013 18:02:42 -0600
Subject: Re: [patch 3/5] don't restrict bit range for -fstrict-volatile-bitfields
References: <51BE0D1C dot 9070404 at codesourcery dot com> <20130616192653 dot GP2336 at tucnak dot redhat dot com> <CAFiYyc0TVzU=QS1BabX6jtoR5Yjt_GKn-Gk5B_0GcowR7Dn9dw at mail dot gmail dot com> <20130617123120 dot 7451adb3 at octopus> <Pine dot LNX dot 4 dot 64 dot 1306171437130 dot 10096 at digraph dot polyomino dot org dot uk> <51BF3ED7 dot 6000206 at codesourcery dot com>

Earlier today I wrote:

On 06/17/2013 08:41 AM, Joseph S. Myers wrote:

On Mon, 17 Jun 2013, Julian Brown wrote:

IIUC, the incompatibility between the specified
-fstrict-volatile-bitfields behaviour and the C++ memory model is a
recognised deficiency in the ARM EABI. It might be an unpopular
suggestion, but how about disabling the option by default for C++ on
ARM [*]? Maybe even with a "sorry" if the user tries to turn it on
manually?


The C11 and C++11 memory models are the same, this is nothing to do with
C++ as opposed to C.


The problem with always choosing C++ memory model compliance over ABI
compliance, no matter what the setting of -fstrict-volatile-bitfields,
is that some users may have legacy code out there where they really want
the ABI-compliant behavior.  Perhaps we should not make
-fstrict-volatile-bitfields not be the default on any target any more,
but if you supply it explicitly it tells GCC that you really want the
ABI-compliant behavior to override the language-standard-compliant
behavior?

I had another thought: perhaps -fstrict-volatile-bitfields could remainthe default on targets where it currently is, but it can be overriddenby an appropriate -std= option. Perhaps also GCC could give an error if-fstrict-volatile-bitfields is given explicitly with an incompatible-std= option.


-Sandra

Follow-Ups:
- patch [6/5] check for conflict with -fstrict-volatile-bitfields and -std=
  - From: Sandra Loosemore

References:
- [patch 3/5] don't restrict bit range for -fstrict-volatile-bitfields
  - From: Sandra Loosemore
- Re: [patch 3/5] don't restrict bit range for -fstrict-volatile-bitfields
  - From: Jakub Jelinek
- Re: [patch 3/5] don't restrict bit range for -fstrict-volatile-bitfields
  - From: Richard Biener
- Re: [patch 3/5] don't restrict bit range for -fstrict-volatile-bitfields
  - From: Julian Brown
- Re: [patch 3/5] don't restrict bit range for -fstrict-volatile-bitfields
  - From: Joseph S. Myers
- Re: [patch 3/5] don't restrict bit range for -fstrict-volatile-bitfields
  - From: Sandra Loosemore

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