On Thu, Jan 8, 2009 at 7:56 AM, H.J. Lu <hjl.to...@gmail.com> wrote: > 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. >
If we really want the maximum useful alignment for the target machine we are compiling for, I would prefer __attribute__ ((aligned (best))) and document that it may be changed by -mXXX. -- H.J.
