On 12/01/2016 05:04 AM, Segher Boessenkool wrote:
On Thu, Dec 01, 2016 at 10:19:42AM +0100, Richard Biener wrote:
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.

STC tries to make as large as possible consecutive "traces", mainly to
help with instruction cache utilization and hit rate etc.  It cannot do
a very good job if it isn't allowed to copy blocks.

"simple" tries to (dynamically) have as many fall-throughs as possible,
i.e. as few jumps as possible.  It never copies code; if that means it
has to jump every second insn, so be it.  It provably is within a factor
three of optimal (optimal is NP-hard), under a really weak assumption
within a factor two, and it does better than that in practice.

STC without block copying makes longer traces which is not a good idea
for most architectures, only for those that have a short jump that is
much shorter than longer jumps (and thus, cannot cover many jump
targets).

I do not know how STC behaves when it does not know the insn lengths.
mn103 & m68k are definitely sensitive to jump distances, the former more so than the latter. Some of the others probably are as well.

I think we've probably discussed this more than is really necessary. We just need to pick an alternative and go with it, I think either alternative is reasonable (avoid copying when STC has no length information or fall back to simple when there is no length information).



jeff

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