On Wed, Jan 7, 2009 at 4:44 PM, Ian Lance Taylor <i...@google.com> wrote: > "Ye, Joey" <joey...@intel.com> writes: > >> From: Ian Lance Taylor [mailto:i...@google.com]: >>> Therefore, I propose that we do the following: >>> >>> 1) Introduce __attribute__ ((aligned (scalar))). This will be >>> documented as having a fixed value for each ABI. The value will be >>> guaranteed to be sufficient to hold any ordinary non-vector type. >>> The default will be BIGGEST_ALIGNMENT. The value for the >>> x86/x86_64 will be 128. >>> >>> 2) Introduce __attribute__ ((aligned (max))). This will be documented >>> as having the largest value available for any version of the >>> architecture, and thus in particular it may change if new versions >>> of the architecture are released. The value will not change based >>> on command line options which do not change the ABI; that is, if it >>> is possible to link together two files compiled with different set >>> of command line options and expect the result to work, then those >>> command line options must not change the value of this attribute. >>> The value will be guaranteed to be sufficient to hold any type, >>> including any vector type. The default will be BIGGEST_ALIGNMENT. >>> The value for the x86/x86_64 will (presumably) be 256. >> To me "new version of x86 architecture are released" usually means >> "change based on command line option". How about the default value >> grow to 512 or even higher in future? > > There is a command line option change, sure, but in general, as far as > I know, files compiled with -march=NEW can be linked with files > compiled with -march=OLD, and the result is expected to work. > > If that is not the case--if files compiled with -march=avx and > -march=i386 (or -march=nocona) can not be linked together and > work--then there is no problem here. > > Ian >
Given that __attribute__ ((aligned (max))) may change, I don't think it is very useful. A portable generic memory allocator should take an additional argument for alignment. -- H.J.
