RFR: 8346931: Replace divisions by zero in sharedRuntimeTrans.cpp [v5]

[Kim Barrett]

On Tue, 25 Mar 2025 08:32:03 GMT, Matthias Baesken <mbaesken at openjdk.org> wrote:

>> There are a few divisions by zero in sharedRuntimeTrans.cpp, used to "construct" NaN and -infinity. This should probably be replaced by using functionality from std::numeric_limits .
>
> Matthias Baesken has updated the pull request incrementally with one additional commit since the last revision:
> 
>   adjust check

Looks good.

src/hotspot/share/runtime/sharedRuntimeTrans.cpp line 518:

> 516:       z = ax;                   /*x is +-0,+-inf,+-1*/
> 517:       if(hy<0) {
> 518:         if (ix == 0) {

[Just a comment, as the level of change involved seems out of scope for the
issue at hand.  And probably not worth the effort, since x==0.0 is probably
not all that common.  Feel free to completely ignore this.]

I feel like there ought to be a way to restructure this to merge the `ix == 0`
here and the one a few lines earlier.  Something like (completely untested and
quite possibly wrong)

  if (ix == 0) {
    z = std::numeric_limits<double>::infinity();
    if (hx < 0 && yisint == 1) {
      z = -z;
    }
    return z;
  } else if (ix == 0x7ff00000 || ix == 0x3ff00000) { // ix==0 moved earlier
    ... ;
    return z;
  }

src/hotspot/share/runtime/sharedRuntimeTrans.cpp line 519:

> 517:       if(hy<0) {
> 518:         if (ix == 0) {
> 519:           z = std::numeric_limits<double>::infinity();

Good thing we have tests! I was looking at the comparison, and entirely missed that this needed to be
infinity rather than the quiet_nan in earlier commits.

-------------

Marked as reviewed by kbarrett (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/24136#pullrequestreview-2714333632
PR Review Comment: https://git.openjdk.org/jdk/pull/24136#discussion_r2012463092
PR Review Comment: https://git.openjdk.org/jdk/pull/24136#discussion_r2012435480