On Tue, 2014-01-07 at 21:51 +0000, Joseph S. Myers wrote: > On Tue, 7 Jan 2014, Bill Schmidt wrote: > > > +@item -maltivec=be > > +@opindex maltivec=be > > +Generate Altivec instructions using big-endian element order, > > +regardless of whether the target is big- or little-endian. > > + > > +@item -maltivec=le > > +@opindex maltivec=le > > +Generate Altivec instructions using little-endian element order, > > +regardless of whether the target is big- or little-endian. This > > +option is currently ignored for big-endian targets, but may be enabled > > +in the future. > > I don't understand what "using big-endian element order" or "using > little-endian element order" means. Is this about the interpretation of > element numbers in AltiVec intrinsics? (In GNU C vector indexing with > array notation, GENERIC, GIMPLE and RTL, vector element numbers are always > in memory order like in arrays, so a command-line option like this > *shouldn't* affect anything other than intrinsics. Cf. the ARM NEON code > that deals with flipping element order conventions for big endian so that > intrinsics can follow the ARM convention while GNU C, GENERIC, GIMPLE and > RTL follow the convention that the architecture-independent compiler > expects.) >
Yes, sorry for not being more clear. This is indeed for interpretation of element numbers in Altivec intrinsics such as vec_splat, vec_extract, vec_insert, and so forth. By default these will match array element order for the target endianness. But with -maltivec=be for a little endian target, we will force use of big-endian element order (matching the behavior of the underlying hardware instructions). There are certain VSX load and store instructions that assume big-endian ordering of vector elements in a register regardless of target endianness. Applications developed for Power BE that use these instructions via intrinsics will be easier to port to Power LE with -maltivec=be.
