On Thu, Jan 8, 2009 at 1:36 PM, Richard Guenther <richard.guent...@gmail.com> wrote: > On Thu, Jan 8, 2009 at 10:17 PM, H.J. Lu <hjl.to...@gmail.com> wrote: >> On Thu, Jan 8, 2009 at 12:40 PM, Ian Lance Taylor <i...@google.com> wrote: >>> "H.J. Lu" <hjl.to...@gmail.com> writes: >>> >>>> We can solve it with >>>> >>>> 1. A target should define MALLOC_ABI_ALIGNMENT properly. >>>> 2. g++ should issue an error when the default new operator >>>> is used on a type whose alignment is greater than >>>> MALLOC_ABI_ALIGNMENT. >>>> 3. It is user's responsibility to provide a new operator to return >>>> a properly aligned memory. >>>> 4. g++ can assume memory return by the user-provided new >>>> operator is properly aligned. >>> >>> Sounds good as far as it goes. How do we get there from here? What >>> should the behaviour of __attribute__ ((aligned)) be? Should we add >> >> For 4.4, we can apply my patch: >> >> http://gcc.gnu.org/ml/gcc-patches/2009-01/msg00367.html >> >> and update document with >> >> As in the preceding examples, you can explicitly specify the alignment >> (in bytes) that you wish the compiler to use for a given variable or >> structure field. Alternatively, you can leave out the alignment factor >> and just ask the compiler to align a variable or field to the maximum >> useful alignment for the target ABI you are compiling for. For >> example, you could write: >> >> @smallexample >> short array[3] __attribute__ ((aligned)); >> @end smallexample >> >> Whenever you leave out the alignment factor in an @code{aligned} attribute >> specification, the compiler automatically sets the alignment for the declared >> variable or field to the largest alignment for the target ABI you are >> compiling for. Doing this can often make copy operations more efficient, >> because the compiler can use whatever instructions copy the biggest >> chunks of memory when performing copies to or from the variables or >> fields that you have aligned this way. > > I don't think we should encourage more uses of __attribute__((aligned)). >
Then we can leave minimum documentation for __attribute__((aligned)) and remove example. -- H.J.
