On Fri, Dec 21, 2018 at 07:39:45PM +0000, Joseph Myers wrote:
> On Fri, 21 Dec 2018, Steve Kargl wrote:
>
> > scalbln(double x, long n)
> > {
> >
> > return (scalbn(x, (n > NMAX) ? NMAX : (n < NMIN) ? NMIN : (int)n));
> > }
> >
> > A search for glibc's libm locates https://tinyurl.com/ybcy8w4t
> > which is a bit-twiddling routine. Not sure it's worth the
> > effort. Joseph Myers might have an opinion.
>
> Such comparisons are needed in the scalbn / scalbln implementations anyway
> to deal with large exponents. I suppose where there's a suitable scalbln
> implementation, and you don't know if the arguments are within the range
> of int, calling scalbln at least saves code size in the caller and avoids
> duplicating those range checks.
>
I was thinking along the lines of -ffast-math and whether
__builtin_scalbn and __builtin_scalbln are then inlined.
The comparisons may inhibit inlining __builtin_scalbn;
while, if gfortran used __builtin_scalbln, inlining would
occur.
As it is, for
function foo(x,i)
use ieee_arithmetic
real(8) foo, c
integer(8) i
foo = ieee_scalb(c, i)
end function foo
the options -ffast-math -O3 -fdump-tree-optimized give
<bb 2> [local count: 1073741824]:
_gfortran_ieee_procedure_entry (&fpstate.0);
_8 = *i_7(D);
_1 = MIN_EXPR <_8, 2147483647>;
_2 = MAX_EXPR <_1, -2147483647>;
_3 = (integer(kind=4)) _2;
_4 = __builtin_scalbn (c_9(D), _3);
_gfortran_ieee_procedure_exit (&fpstate.0);
fpstate.0 ={v} {CLOBBER};
return _4;
It seems this could be
<bb 2> [local count: 1073741824]:
_gfortran_ieee_procedure_entry (&fpstate.0);
_3 = (integer(kind=4)) *i_7(D);
_4 = __builtin_scalbn (c_9(D), _3);
_gfortran_ieee_procedure_exit (&fpstate.0);
fpstate.0 ={v} {CLOBBER};
--
Steve
