<div dir="ltr"><div class="gmail_extra"><br><div class="gmail_quote">On Wed, Jan 7, 2015 at 5:19 PM, Philip Reames <span dir="ltr"><<a href="mailto:listmail@philipreames.com" target="_blank">listmail@philipreames.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div bgcolor="#FFFFFF" text="#000000">I've been playing with approaches to getting better optimization of
loops which contain infrequently executed slow paths. I've gotten
as far as throwing together a proof of concept implementation of a
profile guided optimization to separate a single loop with multiple
latches into a loop nest, but I want to get feedback from interested
parties before investing much more effort. <br>
<br>
The primary case I'm concerned about looks something like this C
example:<br>
while( some_condition )<br>
//do lots of work here<br>
if (!some_rare_condition) { <-- This is loop variant<br>
helper();<br>
}<br>
}<br>
<br>
The key point here is that the 'do lots of work' section contains
things we could lift out of the loop, but our knowledge of memory
gets completely destoryed by the 'helper' call which doesn't get
inlined. This ends up crippling LICM in particular, and GVN to a
lessor degree. <br>
<br>
The approach I've been playing with would create this loop
structure:<br>
goto inner<br>
while (true) {<br>
outer:<br>
helper();<br>
inner:<br>
while( some_condition )<br>
//do lots of work here<br>
if (!some_rare_condition) { <-- This is loop variant<br>
goto outer;<br>
}<br>
}<br>
}<br>
<br>
(Excuse the psuedo-C. Hopefully the idea is clear.)<br></div></blockquote><div><br></div><div>Yep.</div><div><br></div><div>I've not thought about this a lot, but I have two initial questions that maybe you can answer:</div><div><br></div><div>How does this compare with classical approaches of loop peeling, partitioning, fission, or whatever you might call it? Is there any literature behind this approach or some literature it should be compared with? (I genuinely don't know this area that well, so I'm of little help here...)</div><div> </div><div><br></div><div>Some of your points I have quick feedback on:</div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div bgcolor="#FFFFFF" text="#000000"><div><div><p>Points for discussion:</p>
<ul>
<li>Is using profile information
for this purpose even a reasonable thing to do?</li></ul></div></div></div></blockquote><div>Yes!</div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div bgcolor="#FFFFFF" text="#000000"><div><div><ul>
<li>I chose to implement this
without relying on the existing block frequency analysis. My
reasoning was that a) this is a rarely taken case and adding
an expensive analysis dependency probably wasn't worthwhile
and b) that block frequency analysis was more
expensive/precise than I really needed. Is this reasonable?</li></ul></div></div></div></blockquote><div>I think we should always use the analyses. Either BlockFrequency or BranchProbability. I think probably both in the common joint usage (frequency of the loop header combined with probability of the cold region).</div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div bgcolor="#FFFFFF" text="#000000"><div><div><ul>
<li>If so, is the notion of
'rareness' of a loop block something that's worth extracting
out on it's own and reusing? Are there other similar uses
anyone can think of?</li>
<li>Currently, I'm only supporting
a fairly small set of controlling conditions. Are there
important cases I'm not considering?</li></ul></div></div></div></blockquote><div>To both of these, I think the general combination to use is to identify the set of blocks dominated by a block which is in the loop body of a hot loop, and is cold relative to the other successors of its predecessor(s). These form cold "regions" as I think of them without requiring the complexity of the region analysis.</div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div bgcolor="#FFFFFF" text="#000000"><div><div><ul>
<li>Since the rarest latch is often
deep in a loop - with other "if (X) continue;" (i.e.
latches) before it - this tends to create loops with
multiple exiting blocks. Some of the existing passes might
not deal with this well, is that a major concern?
Suggestions for how to analysis and validate?</li></ul></div></div></div></blockquote><div>I'm somewhat concerned about this, but want to think more about the fundamental transformation.</div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div bgcolor="#FFFFFF" text="#000000"><div><div><ul>
<li>Currently, I've structured this
as pulling off the rarest latch as an outer iteration. I
could also pull off the most popular latch as an inner
iteration. This might give different tradeoffs; thoughts?</li>
</ul>
<p>Generally, any thoughts anyone have on the problem or
approach are welcome. I'm not particular attached to the
particular approach laid out here and if there's a more
advantageous approach, all the better.</p></div></div></div></blockquote></div>Thanks for pushing on this! ;] Now I need to go and ponder a lot so i can reply more deeply on the actual transform.<br><br></div></div>