Hello.

> I don't think you should have the unreachable "return false;" in is_even.
> The last "else if" can just be "else".

I don't think return false in is_even is unreachable. As per my
understanding, when one else if is true in the else if ladder, all the
subsequent "else ifs" including "else" are ignored. When REAL_EXP is
less than SIGNIFICAND_BITS, but the number is odd, the inner "if" for
even-odd will not return true in which case should return false. That
case will ignore next "else if" and will reach return false.

> Suppose REAL_EXP (r) > SIGNIFICAND_BITS.  Then the number is definitely
> even, so you should return true, not false.

for this condition, else if can be modified to just else and return true.
PATCH:

gcc/ChangeLog:

2019-06-12  Tejas Joshi  <tejasjoshi9673@gmail.com>

    * builtins.c (mathfn_built_in_2): Added CASE_MATHFN for ROUNDEVEN.
    * builtins.def: Added function definitions for roundeven function
    variants.
    * fold-const-call.c (fold_const_call_ss): Added case for function
    call and fold_const_conversion call for roundeven function.
    * fold-const.c (negate_mathfn_p): Added case for roundeven function.
    (tree_call_nonnegative_warnv_p): Added case for roundeven function.
    (integer_valued_real_call_p): Added case for roundeven function.
    * real.c (is_even): New function. Returns true if real number is
    even, otherwise returns false.
    (is_halfway_below): New function. Returns true if real number is
    halfway between two integers, else return false.
    (real_roundeven): New function. Round real number to nearest
    integer, rounding halfway cases towards even.
    * real.h (real_value): Added descriptive comments.
    Added function declaration for roundeven function.

gcc/testsuite/ChangeLog:

2019-06-12  Tejas Joshi  <tejasjoshi9673@gmail.com>

    * gcc.dg/torture/builtin-round-roundeven.c: New test.
    * gcc.dg/torture/builtin-round-roundevenf128.c: New test.



On Tue, 11 Jun 2019 at 01:56, Joseph Myers <joseph@codesourcery.com> wrote:
>
> On Sun, 9 Jun 2019, Tejas Joshi wrote:
>
> > Hello.
> > I have created another patch which addresses the above points,
> > attached herewith.
>
> I don't think you should have the unreachable "return false;" in is_even.
> The last "else if" can just be "else".
>
> > > a conditional with < not <=; if REAL_EXP (r) == SIGNIFICAND_BITS, the
> > > least significant bit has value 1 and the number must be an integer).
> >
> > The number is integer because of the whole word spaces is occupied by
> > integer part?
> > Also, why would the least significant bit will have value 1 if
> > REAL_EXP (r) == SIGNIFICAND_BITS, as it only concerns with 2^0th
> > position (even or odd)?
>
> My understanding is that the significand is, as per the comments in
> real.c, in the range [0.5, 1.0).  There are SIGNIFICAND_BITS bits.  The
> bit above the most significant one has value 2^REAL_EXP.  The most
> significant one has value 2^(REAL_EXP-1).  The least significant one has
> value 2^(REAL_EXP-SIGNIFICAND_BITS).  If REAL_EXP == SIGNIFICAND_BITS,
> that means the least significant bit has value 2^0 = 1, and there are no
> bits with value 0.5 or below, so the number is an integer.
>
> --
> Joseph S. Myers
> joseph@codesourcery.com