On 12/02/2016 03:22 PM, Segher Boessenkool wrote:
On Fri, Dec 02, 2016 at 09:47:00AM +0100, Richard Biener wrote:
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.
I hope it special-cases backedges, that is, not make those fallthru.
It doesn't, and that is a good thing.
Firstly, what is classified as backedge doesn't make too much sense in
some cases; quite often the cfg isn't reducible.
Secondly, for simple loops it does not matter: the backedge has lower
frequency than the forward edges, so everything works out as you want.
Thirdly, consider this loop:
|
S<----
| |
A |
/ \ |
B C |
\ / |
D |
| |
E-----
|
F
where the backedge now has higher frequency than A->B and A->C.
With simple this ends up as:
S: ...; goto A
D: ...
E: ...; if (not again) goto F
S: ...
A: ...; if () goto C
B: ...; goto D
C: ...; goto D
and this is cheaper than the alternative:
S: ...
A: ...; if () goto C
B: ...
D: ...
E: ...; if (again) goto A
F:
C: ...; goto D
If a fraction f of the loop iterations does C (so 1-f does B), the
alternative has 1+2f jumps per iteration; the code simple makes has 1+f.
In fact, the latter can be particularly bad with small icaches. The
first sequence is far better. IIRC there were spec92 cases there
getting this kind of thing right made a measurable difference on
embedded targets.
Jeff
