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
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.
Jeff
