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? Richard. > Jeff
