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