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Teach Clang to provide ::max_align_t in C11 and C++11 modes.

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Committed
chandlercFeb 19 2014, 2:35 PM
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rL201728: GC now unused variable.
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Description

Teach Clang to provide ::max_align_t in C11 and C++11 modes.

This definition is not chosen idly. There is an unfortunate reality with
max_align_t -- the specific nature of its definition leaks into the ABI
almost immediately. Because it is part of C11 and C++11 it becomes
essential for it to match with other systems on that ABI. There is an
effort to discourage any further use of this construct as a consequence

  • using max_align_t introduces an immediate ABI problem. We can never

update it to have larger alignment even as the microarchitecture changes
to necessitate higher alignment. =/

The particular definition here exactly matches the ABI of GCC's chosen
::max_align_t definition, for better or worse. This was written with the
help of Richard Smith who was decoding the exact ABI implications of the
selected definition in GCC. Notably, in-register arguments are impacted
by the particular definition chosen. =/

No one is under the illusion that this is a "good" or "useful"
definition of max_align_t, and we are working with the standards
committee to specify a more useful interface to address this need.

rL201729

cfe/trunk/lib/Headers/stddef.h

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cfe/trunk/test/Headers/c11.c

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