[Ubuntu 15.10] gcc: Optimize swap removal in presence of floating-point vector convert/splat sequence

[bugproxy]

On Little Endian systems, GCC performs optimizations to remove unnecessary element reversals of vectors that are sometimes required to adjust for behavior of the lxvd2x, etc., instructions. In one not uncommon case, such optimizations are defeated when we perform a vector convert from double-precision to single-precision followed by a vector splat word. I have pushed patches upstream to fix this, implemented using the following revisions:

GCC trunk (6.0): 222351
GCC 5.1: 222394
GCC 4.9: 222420
GCC 4.8: 222427

I would like to see the backported patch applied to the gcc shipped with the Ubuntu 15.10 release. We should probably have a conversation about the next service pack for the 14.04 LTS release as well.

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[Steve Langasek (vorlon)]

It looks like this bug will be fixed as a matter of course when gcc 5.1 is included in the distro and becomes the default in Ubuntu 15.10. Assigning to Matthias for confirmation.

For 14.04, this appears to be addressed by the SRU of gcc-4.8 currently in trusty-proposed, which will bring us up to revision 222448.

[Hans Joachim Desserud (hjd)]

I think bug 1449115 might be a duplicate of this one, but I'm not quite sure how to be certain.

[Matthias Klose (doko)]

fixed in gcc-4.8 in trusty-updates
fixed in all version in 15.10.

[bugproxy (bugproxy)]

------- Comment From <email address hidden> 2015-08-27 13:16 EDT-------
Already available in 15.10 release with gcc version 5.2.1-15ubuntu2.
CLosing at IBM side.