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Subject: Re: [boost] Win64 and abi_prefix.hpp/abi_suffix.hpp warnings
From: John Maddock (john_at_[hidden])
Date: 2008-09-26 04:40:32

Chris Newbold wrote:
>>> I'm unclear on the rationale for forcing 8-byte alignment in
>>> config/abi/msvc_(pre|suf)fix.hpp: 8-byte alignment is the compiler
>>> default. Is there concern over Boost headers being included in a
>>> context where the alignment has been otherwise artificially altered?

Yes: it's to ensure that the included header is binary compatible with the
boost binaries, which are all built with default alignment settings.
Obviously for header only libraries, including the abi_prefix/suffix headers
is unnecessary.

>> I'm starting to question the wisdom of unconditionally forcing
>> 8-byte alignment in the MSVC ABI headers, particularly given that
>> this happens on two architectures (x86 and x86_64) that have
>> different pointer widths.
>> I can see some merit to enforcing a uniform alignment for Boost to
>> avoid ABI issues due to users' project settings, etc., but I think
>> that we need to account for the fact that different architectures
>> have different default alignments. On x86_64, the default alignment
>> for MSVC is 16 bytes.
>> However, changing msvc_prefix.hpp to look like...
>> #if defined(_M_X64)
>> # pragma pack(push,16)
>> #else
>> # pragma pack(push,8)
>> #endif
>> ... is only an incomplete solution. It suppresses the warnings in
>> the default case where the user has not already changed the
>> alignment. If, for whatever reason, the user has changed the default
>> alignment they're going to start getting the warnings again.

Sigh, yes indeed.

But we should definitely make that change IMO.

>> I think the only solution is to combine this change (so that Boost
>> uses the "true" default alignment on x86_64) with the big-hammer
>> disable-the-warning-for-good change.

I'm less keen on that as it may suppress warnings about genuine bugs, but
don't know what else to suggest :-(

Unless there are any violent objections, I'm going to commit the x64 fix
soon, but what do folks feel about suppressing the warnings globally as

Thanks for raising this, John.

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