[LLVMdev] Heads up! Planning to remove old vector shuffle lowering this week...

[Simon Pilgrim]

On 4 Jan 2015, at 23:30, Chandler Carruth <chandlerc at gmail.com> wrote:

> On Sun, Jan 4, 2015 at 3:20 PM, Simon Pilgrim <llvm-dev at redking.me.uk> wrote:
> On 24 Nov 2014, at 17:53, Chandler Carruth <chandlerc at gmail.com> wrote:
> 
> > I'll be skimming the PRs to see if there are any really critical regressions, but so far it looks pretty good.
> >
> > If you are actively disabling the new vector shuffling and have some PR that blocks you, please reply here. Later this week, the flag will go away unless I hear strenuous objections. There is a really staggering amount of cleanup and tidying that needs to take place and can't until we remove the old code paths.
> 
> It doesn’t look like there has been any changes for this yet - is the plan for the old shuffle code to be removed before the branch for 3.6? If so do you have this in hand or do you want assistance to get it done in time?
> 
> I just got distracted by other things. I should be able to take care of it now that folks are back from the holidays.
> 
> One question -- do you see any regressions that need fixing first? I don't see any, but I'm curious about others. The silence on this thread didn't inspire confidence, but perhaps its just that nothing is broken with the new stuff?

No notable regressions, I’m seeing different code but mostly for the better - although there are a number of vec256 shuffles (mostly lower/upper crossings) that are rather poor (I think Quentin raised bugs on a couple of these) - but the old system could be a lot worse. I think the few cases that remain can easily be dealt with by individual bug reports.

The amount of domain crossing is much lower now - but there are a number of float shuffles that now use double shuffles instead - fine from a domain point of view but rather unexpected. IIRC this often appeared in matrix transpose code - movlhps / movhlps being replaced by unpcklpd / unpckhpd is the one I seem to remember.

Overall - a massive improvement - thank you!
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