On Wed, Nov 30, 2016 at 6:59 PM, Jeff Law <l...@redhat.com> wrote: > On 11/30/2016 01:38 AM, Richard Biener wrote: >> >> On Tue, Nov 29, 2016 at 5:07 PM, Jeff Law <l...@redhat.com> wrote: >>> >>> On 11/29/2016 03:23 AM, Richard Biener wrote: >>>> >>>> >>>> On Mon, Nov 28, 2016 at 10:23 PM, Jeff Law <l...@redhat.com> wrote: >>>>> >>>>> >>>>> >>>>> >>>>> I was digging into issues around the patches for 78120 when I stumbled >>>>> upon >>>>> undesirable bb copying in bb-reorder.c on the m68k. >>>>> >>>>> The core issue is that the m68k does not define a length attribute and >>>>> therefore generic code assumes that the length of all insns is 0 bytes. >>>> >>>> >>>> >>>> What other targets behave like this? >>> >>> >>> ft32, nvptx, mmix, mn10300, m68k, c6x, rl78, vax, ia64, m32c >> >> >> Ok. >> >>> cris has a hack to define a length, even though no attempt is made to >>> make >>> it accurate. The hack specifically calls out that it's to make >>> bb-reorder >>> happy. >>> >>>> >>>>> That in turn makes bb-reorder think it is infinitely cheap to copy >>>>> basic >>>>> blocks. In the two codebases I looked at (GCC's runtime libraries and >>>>> newlib) this leads to a 10% and 15% undesirable increase in code size. >>>>> >>>>> I've taken a slight variant of this patch and bootstrapped/regression >>>>> tested >>>>> it on x86_64-linux-gnu to verify sanity as well as built the m68k >>>>> target >>>>> libraries noted above. >>>>> >>>>> OK for the trunk? >>>> >>>> >>>> >>>> I wonder if it isn't better to default to a length of 1 instead of zero >>>> when >>>> there is no length attribute. There are more users of the length >>>> attribute >>>> in bb-reorder.c (and elsewhere as well I suppose). >>> >>> >>> I pondered that as well, but felt it was riskier given we've had a >>> default >>> length of 0 for ports that don't define lengths since the early 90s. >>> It's >>> certainly easy enough to change that default if you'd prefer. I don't >>> have >>> a strong preference either way. >> >> >> Thinking about this again maybe targets w/o insn-length should simply >> always use the 'simple' algorithm instead of the STV one? At least that >> might be what your change effectively does in some way? > > From reading the comments I don't think STC will collapse down into the > simple algorithm if block copying is disabled. But Segher would know for > sure. > > WRT the choice of simple vs STC, I doubt it matters much for the processors > in question.
I guess STC doesn't make much sense if we can't say anything about BB sizes. Richard. > > JEff