Re: About GSOC.

[Joseph Myers <joseph@codesourcery.com>]

On Sat, 30 Mar 2019, Tejas Joshi wrote:

> Hello.
> I have developed a fairly working patch for roundeven, attaching herewith.
> The testcase function as follows :
> 
> double f()
> {
>   double x = 4.5;
>   double ret = __builtin_roundeven (x);
>   return ret;
> }

Tests need to be added to the testsuite, covering a range of inputs and 
automatically verifying that the test is optimized correctly.

"Round X to nearest even integer towards zero." is not correct.  The 
roundeven function does not round to an even integer.  It rounds to the 
nearest integer, whether even or odd - but, if two integers are equally 
close, the result is even (and for any input that is not halfway between 
two integers, it produces the same result as round (which rounds halfway 
cases away from zero) - so 2.501, 3 and 3.499 round to 3, but 2.5 rounds 
to 2 not 3, unlike round, and 3.5 rounds to 4, as with round).

The function can't rely on arguments being in the range of HOST_WIDE_INT, 
so it needs to examine the REAL_VALUE_TYPE representation directly to 
determine whether it's half way between two integers and which way to 
round in that case.

-- 
Joseph S. Myers
joseph@codesourcery.com