[Bug other/54408] New: sqrt for vector types

[Bug other/54408] New: sqrt for vector types

[glisse at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54408

             Bug #: 54408
           Summary: sqrt for vector types
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: other
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: glisse@gcc.gnu.org


Hello,

it would be nice to add sqrt to the gcc vector extensions. There are already
target specific builtins, but a generic one would fit better (and get lowered
to elementwise sqrt when the target doesn't support vector sqrt). I don't know
if __builtin_sqrt should be extended, or maybe a different __builtin_vec_sqrt.

Component "other" because fixing this might involve front, middle and
back-ends.

[Bug c/54408] sqrt for vector types

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54408

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2012-09-03
                 CC|                            |jsm28 at gcc dot gnu.org
          Component|other                       |c
     Ever Confirmed|0                           |1

--- Comment #1 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-09-03 10:17:11 UTC ---
Hmm, I suppose the extension would be to the C type-generic math piece.  Of
course that would require library support for most routines (and an ABI for
them) or GCC, if no such routine is specified, expanding to a call for each
vector component.  Just special-casing sqrt may be convenient but odd.

Joseph, any input?

[Bug c/54408] sqrt for vector types

[joseph at codesourcery dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54408

--- Comment #2 from joseph at codesourcery dot com <joseph at codesourcery dot com> 2012-09-03 15:47:38 UTC ---
Built-in mathematical functions are generally type-generic where they 
correspond to type-generic math.h macros (e.g. isgreater, isnan), not 
otherwise.  (__builtin_signbit should be changed to type-generic: bug 
36757.)

I think __builtin_sqrt should be expected to correspond directly to the 
sqrt (double) library function rather than being type-generic - thus, I 
think vector versions of library functions (where those functions are not 
type-generic) should be separate from the normal built-in versions of 
those functions (e.g. define __builtin_vec_sqrt - which could itself be 
type-generic over different vector types).

Remember to define, both at the language level and at the ABI level, 
things such as how such a built-in function handles errors / exceptional 
values (and whether or not this depends at the language level on the 
various -f options that affect this for ordinary C language arithmetic and 
library calls).

[Bug c/54408] sqrt for vector types

[glisse at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54408

--- Comment #3 from Marc Glisse <glisse at gcc dot gnu.org> 2012-09-06 15:04:53 UTC ---
Thanks for the explanations.

One goal would be, in C++, to be able to write a single function template:

template<class T> T f(T x){return x+sqrt(x);}

which would work for float, double, long double, float vectors and double
vectors. (how exactly sqrt is spelled, possibly __builtin_math_sqrt, isn't that
important)

Currently, std::sqrt is overloaded on arithmetic types. If we had
__builtin_vec_sqrt, I guess we could add an extra overload to std::sqrt for
vector types (sfinae-constrained in some way). Not quite as convenient as a
completely generic __builtin_sqrt, but good enough I think (assuming the
libstdc++ maintainers are ok with it). Otherwise, a more generic
__builtin_math_sqrt which works on both scalars and vectors would be better.