On Tue, Nov 10, 2015 at 12:41:07AM +0000, Joseph Myers wrote: > I don't see any conversions between KFmode and TImode (in either > direction, signed or unsigned) here - I suppose there are no instructions > for that?
No in power9 there is no instruction that converts 128-bit integer to IEEE 128-bit floating point or vice versa. > If so, I would guess (without having tested it) that it is more efficient > to use the libgcc2 implementations of those functions (whether copied, or > with some logic to build selected libgcc2.c functions for KFmode), which > implement them using a few hardware operations on DImode [note that where > libgcc2.c has e.g. __floatditf, that gets mapped to __floattitf for 64-bit > systems], than to use the soft-fp implementations doing everything with > integer arithmetic. (There are IEEE exceptions issues with the libgcc2.c > conversions from double-word integers to floating-point - see bug 59412 - > but since that's a preexisting issue for all architectures using this > code, it's clearly not your problem to fix.) > > Ideally, I'd think that for optimal efficiency if objects built for power8 > are linked with libgcc built for power9, or if an executable using shared > libgcc that was built for power8 gets run with shared libgcc for power9, > you'd want power9 libgcc to contain t-hardfp versions of all the functions > that can be expanded inline for power9, and libgcc2 versions of those > (such as TImode comparisons) that aren't expanded inline, but not to > contain soft-fp versions of any of those KFmode functions. Cf. how > config.host ensures various 32-bit powerpc variants use the right mixture > of hardfp and soft-fp functions. It's a bit fiddly to make sure you get > the preferred implementation of every function and that the ABI doesn't > change depending on the configured processor, but not that hard. Yep, that is my thinking. > Since none of the libgcc pieces for KFmode support are yet in, and the > proposed changes are optimizations rather than a matter of correctness, > none of the above should directly affect this patch in any way - it simply > indicates desirable followup once both the libgcc soft-fp KFmode support, > and this patch, are in. -- Michael Meissner, IBM IBM, M/S 2506R, 550 King Street, Littleton, MA 01460-6245, USA email: meiss...@linux.vnet.ibm.com, phone: +1 (978) 899-4797
