On Tue, Apr 10, 2018 at 12:45 PM, Szabolcs Nagy <szabolcs.n...@arm.com> wrote:
> i had a query earlier about libmvec vector functions in fortran: > https://gcc.gnu.org/ml/gcc/2017-11/msg00007.html > > but there were no simple solutions to make math functions vectorizable > in fortran, because it's hard to make libc headers with simd attributes > visible to the fortran front end. > > i think a possible workaround is to have a dummy libmvec implementation > in libgcc.a (or more likely as a separate libgccmvec.a) that just calls > scalar functions from libm like > > vdouble _ZGVbN2v_sin(vdouble x) > { > return (vdouble){sin(x[0]), sin(x[1])}; > } > > and similarly for all relevant single and double precision functions > for all vector lengths and other supported variants. > > then gcc knows that there is an implementation for these functions > available and with the right link order a better implementation from > libmvec can override these dummy implementations. (the cost model > cannot assume a faster vector algorithm than the scalar one though) > > - this allows vectorizing loops with math functions even in fortran, > - and on targets without a libmvec implementation (but with a vector abi), > - and allows users to provide their own vector math implementation > more easily without hacking around glibc math.h (which may not support > vector math or only enable it for a small subset of math functions). > > gcc needs a new cflag and ldflag to enable this. > (maybe -mveclibabi= already present in x86 and ppc can be used for this) > > As I mentioned previously in that thread you linked to, the fortran frontend never generates a direct call to libm sin(), or for that matter ZGVbN2v_sin(). Instead it generates a "call" to __builtin_sin(). And similarly for other libm functions that have gcc builtins. The middle-end optimizers are then free to do whatever optimizations they like on that __builtin_sin call, such as constant folding, and at least as far as the fortran frontend is concerned, vectorizing if -mveclibabi= or such is in effect. -- Janne Blomqvist
