On Thu, 12 Apr 2018, Jakub Jelinek wrote:

> On Thu, Apr 12, 2018 at 03:52:09PM +0200, Richard Biener wrote:
> > Not sure if I missed some important part of the discussion but
> > for the testcase we want to preserve the tailcall, right?  So
> > it would be enough to set avoid_libcall to
> > endp != 0 && CALL_EXPR_TAILCALL (exp) (and thus also handle
> > stpcpy)?
> For the testcase yes.  There the question is if some targets have so lame
> mempcpy that using a tailcall to mempcpy is slower over avoiding the
> tailcall (and on aarch64 it looked like maintainer's choice to have lame
> mempcpy and hope the compiler will avoid it at all costs).  On the other
> side, that change has been forced over to all targets, even when they don't
> have lame mempcpy.
> So, the tailcall is one issue, and we can either use mempcpy if endp
> and CALL_EXPR_TAILCALL, or only do that if -Os.
> And another issue is mempcpy uses in other contexts, here again I think x86
> has good enough mempcpy that if I use
> foo (mempcpy (x, y, z)) then it is better to use mempcpy over memcpy call,
> but not so on targets with lame mempcpy.
> My preference would be to have non-lame mempcpy etc. on all targets, but the
> aarch64 folks disagree.
> So, wonder e.g. about Martin's patch, which would use mempcpy if endp and
> either FAST_SPEED for mempcpy (regardless of the context), or not
> SLOW_SPEED and CALL_EXPR_TAILCALL.  That way, targets could signal they have
> so lame mempcpy that they never want to use it (return SLOW_SPEED), or ask
> for it to be used every time it makes sense from caller POV, and have the
> default something in between (only use it in tail calls).

Well, but that wouldn't be a fix for a regression and IMHO there's
no reason for a really lame mempcpy.  If targets disgree well,
then they get what they deserve.

I don't see any aarch64 specific mempcpy in glibc btw so hopefully
the default non-stupid one kicks in (it exactly looks like my C


>       Jakub

Richard Biener <rguent...@suse.de>
SUSE LINUX GmbH, GF: Felix Imendoerffer, Jane Smithard, Graham Norton, HRB 
21284 (AG Nuernberg)

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