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Re: [patch 4/4] -fstrict-volatile-bitfields cleanup v3: remove from defaults on all targets
- From: Richard Biener <richard dot guenther at gmail dot com>
- To: "Joseph S. Myers" <joseph at codesourcery dot com>
- Cc: Richard Earnshaw <rearnsha at arm dot com>, Bernd Edlinger <bernd dot edlinger at hotmail dot de>, "gcc-patches at gcc dot gnu dot org" <gcc-patches at gcc dot gnu dot org>, "sandra at codesourcery dot com" <sandra at codesourcery dot com>, "dj at redhat dot com" <dj at redhat dot com>
- Date: Tue, 3 Sep 2013 10:30:22 +0200
- Subject: Re: [patch 4/4] -fstrict-volatile-bitfields cleanup v3: remove from defaults on all targets
- Authentication-results: sourceware.org; auth=none
- References: <DUB122-W35943644A5D83EC2CAFDB7E4370 at phx dot gbl> <52245D01 dot 1030702 at arm dot com> <Pine dot LNX dot 4 dot 64 dot 1309021600390 dot 17654 at digraph dot polyomino dot org dot uk>
On Mon, Sep 2, 2013 at 6:05 PM, Joseph S. Myers <firstname.lastname@example.org> wrote:
> On Mon, 2 Sep 2013, Richard Earnshaw wrote:
>> On 01/09/13 14:10, Bernd Edlinger wrote:
>> > IMHO the AAPCS forbids packed structures. Therefore we need not
>> > interfere with the C++ memory model if we have unaligned data.
>> The AAPCS neither forbids nor requires packed structures. They're a GNU
>> extension and as such not part of standard C++. Thus the semantics of
>> such an operation are irrelavant to the AAPCS: you get to chose what the
>> behaviour is in this case...
> The trouble is that AAPCS semantics are incompatible with the default GNU
> semantics for non-packed structures as well - AAPCS
> strict-volatile-bitfields is only compatible with --param
> allow-store-data-races=1, which is not the default for any language
> variant accepted by GCC (and I say that the default language semantics
> here should not depend on the target architecture).
As I said it should be easy to fulfil AAPCS requirements if they do not violate
language constrains during code generation and thus warn about accesses
that are emitted in a way not conforming to AAPCS (a warning at struct
declaration time would be nicer, but I guess requires more coding and thought,
though at the point we compute DECL_BIT_FIELD_REPRESENTATIVE in
stor-layout.c would be a suitable place).
> Joseph S. Myers