On Fri, Jun 14, 2013 at 08:12:02PM +0930, Alan Modra wrote: > > As for the > > typedef int vec_align __attribute__ ((vector_size(16), aligned(32))); > > > > vec_align x = { 0, 0, 0, 0 }; > > > > changes, that is ABI changing bugfix, so the question is, are you fine with > > breaking the ABI (between 4.8 and 4.9, or if you wanted to backport it to > > 4.8 too (I certainly plan to backport the non-ppc DATA_ABI_ALIGNMENT changes > > to 4.8.2, already am using it in our compilers))? The other option is > > to fix the ABI, but keep things backwards ABI compatible. That would be > > done by decreasing the alignment as it used to do before in > > DATA_ABI_ALIGNMENT, > > and increasing it to the desirable level only in DATA_ALIGNMENT. That has > > the effect that when emitting the decls into assembly e.g. the above will > > now be correctly 32 byte aligned, but accesses to such decl in compiler > > generated code will only assume that alignment if > > decl_binds_to_current_def_p, otherwise they will keep assuming the old > > (broken) lowered alignment. At least for 4.8 backport IMHO that would be a > > better idea (but of course would need big comment explaning it). > > I see your point, but for there to be a real problem we'd need > a) A library exporting such a type with (supposed) increased > alignment, and, > b) gcc would need to make use of the increased alignment. > > (a) must be rare or non-existent or you'd think we would have had a > bug report about lack of user alignment in vector typedefs. The code > has been like this since 2001-11-07, so users have had a long time to > discover it. (Of course, this is an argument for just ignoring the > bug too.)
It doesn't have to be an exported symbol from a library, it is enough to compile some objects using one compiler and other objects using another compiler, then link into the same library. > (b) doesn't happen in the rs6000 backend as far as I'm aware. Do you > know whether there is some optimisation based on alignment in generic > parts of gcc? A quick test like Tons of them, the DECL_ALIGN value is used say by get_pointer_alignment, vectorizer assumptions, is added to MEM_ATTRS, so anything looking at alignment in RTL can optimize too. > typedef int vec_align __attribute__ ((vector_size(16), aligned(32))); > vec_align x = { 0, 0, 0, 0 }; > > long f1 (void) > { > return (long) &x & -32; > } Try (long) &x & 31; ? That &x & -32 not being optimized into &x is guess a missed optimization. Consider if you put: typedef int vec_align __attribute__ ((vector_size(16), aligned(32))); vec_align x = { 0, 0, 0, 0 }; into one TU and compile with gcc 4.8.1, then typedef int vec_align __attribute__ ((vector_size(16), aligned(32))); extern vec_align x; long f1 (void) { return (long) &x & 31; } in another TU and compile with gcc trunk after your patch. I bet it will be optimized into return 0; by the trunk + your patch compiler, while the alignment will be actually just 16 byte. Jakub