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<p>Hi Quentin,</p>
<p>thanks for picking up the conversation!<br>
</p>
<div>
<p>> I think we should step back and check what we want before
investing any time in some rewrite.</p>
<p>That is a very fair point and I might have been getting ahead
of myself in my last email.<br>
What I would like to see from RegBankSelect is to produce the
mapping with the overall lowest cost. Keeping track of all
different combinations of mappings will certainly be non-trivial
however, so I wonder if there is a smart way to do this without
spending too much compilation time. <br>
</p>
<p>Ideally for instructions with no operands (like G_CONSTANT) it
could also check whether a cross-bank copy is actually worth it
or if it would be more beneficial to simply rematerialize the
instruction on the required bank. For such instructions this
information should already be available as part of the
cost-modelling in RegBankSelect: we could simply compare the
cost of a mapping on the required bank vs. the cost of a
cross-bank copy.<br>
</p>
<p>Would you see this as a valid direction for RegBankSelect?</p>
<p>Best regards,</p>
<p>Dominik<br>
</p>
</div>
<div><br>
</div>
<div class="moz-cite-prefix">Am 07.10.20 um 19:47 schrieb Quentin
Colombet:<br>
</div>
<blockquote type="cite"
cite="mid:6932924D-B430-4259-9176-6A4BE77D64B0@apple.com">
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
Hi Dominik,
<div class=""><br class="">
</div>
<div class="">Thanks for sending this!<br class="">
<div><br class="">
<blockquote type="cite" class="">
<div class="">On Oct 7, 2020, at 5:21 AM, Dominik Montada
via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org"
class="" moz-do-not-send="true">llvm-dev@lists.llvm.org</a>>
wrote:</div>
<br class="Apple-interchange-newline">
<div class="">
<div class="">Hi all,<br class="">
<br class="">
this is the second email for the round table follow-up,
this time regarding the issues around the greedy
RegBankSelect and alternative mappings.<br class="">
<br class="">
The issue I brought up was that because RegBankSelect
goes top-down, it never looks at all available mappings
for the operands when considering which of the mappings
to apply to the current instruction. In our architecture
we have one register bank dedicated to pointers and
another one for anything else. We often see code where
we have a G_PTR_ADD with a constant. Since the constant
is not a pointer, we put it on the other register bank.
We could put it on the address regbank and do provide
alternative mappings for that, but since the greedy
algorithm doesn't actually check the usage of the
constant, it is always put on the other bank.<br
class="">
</div>
</div>
</blockquote>
<div><br class="">
</div>
<div>The intent behind the greedy algorithm was for the
mapping to look at X instructions ahead, depending on the
optimization level, when assigning one instruction.</div>
<div>Right now the window is simply 1 instruction and the code
is not structured to allow to have more than one.</div>
<div><br class="">
</div>
<div>As we gain more insights on what we want RegBankSelect to
do, it makes sense to redesign it. </div>
<br class="">
<blockquote type="cite" class="">
<div class="">
<div class=""><br class="">
When RegBankSelect then sees the G_PTR_ADD and sees that
one of its inputs is on the other register bank already,
it then inserts a costly cross-bank copy instead of
checking if that operand has any alternative mappings
which would make the overall mapping for the current
instruction cheaper.<br class="">
</div>
</div>
</blockquote>
<div><br class="">
</div>
<div>The idea is when a mapping is done, that means it was the
best one at the time of the decision (greedy), thus we don’t
challenge that.</div>
<div>“Reverting” an already set mapping is not that simple
because yes for this particular use we are inserting costly
cross copies, but there are no guarantees that replacing the
mapping of the definition will not insert even more costly
copies for the other uses.</div>
<div><br class="">
</div>
<div>E.g., consider:</div>
<div>```</div>
<div><font class="" face="Menlo">A = def <— RBS starts here</font></div>
<div><font class="" face="Menlo">= useFP A</font></div>
<div><font class="" face="Menlo">= useInt A</font></div>
<div><font class="" face="Menlo">= useInt A</font></div>
<div>```</div>
<div><br class="">
</div>
<div>Let’s assume that greedy works the way it is intended.
I.e., it assigns A to the int register bank because there
are 2 such uses vs. only 1 fp bank use:</div>
<div>
<div>```</div>
<div><font class="" face="Menlo">A<int> = def</font></div>
<div><font class="" face="Menlo">= useFP A<int> <—
Next, RBS looks at this one</font></div>
<div><font class="" face="Menlo">= useInt A<int></font></div>
<div><font class="" face="Menlo">= useInt A<int></font></div>
<div class="">```</div>
</div>
<div><br class="">
</div>
Now the regbank for useFP is not right and has to be repaired.
Right now, we will insert the costly cross copy for that use:</div>
<div>
<div>
<div>```</div>
<div><font class="" face="Menlo">A<int> = def</font></div>
<div><font class="" face="Menlo">AFP<fp> = cross_copy
A<int></font></div>
<div><font class="" face="Menlo">= useFP AFP<fp></font></div>
<div><font class="" face="Menlo">= useInt A<int></font></div>
<div><font class="" face="Menlo">= useInt A<int></font></div>
<div class="">```</div>
<div class=""><br class="">
</div>
<div class="">Now, if we were to change the definition of A
to avoid this copy we would create two costly copy for the
useInt. Actually, another question is what would we do
when we look at the first useInt? Is this use allowed to
change the definition of the instruction again? Do we
duplicate the definition? Etc..</div>
<div class="">
<div>```</div>
<div><font class="" face="Menlo">A<fp> = def <—
reassign</font></div>
<div><span style="font-family: Menlo;" class="">= useFP
AFP<fp></span></div>
<div><font class="" face="Menlo">AInt1<int> =
cross_copy A<fp></font></div>
<div><font class="" face="Menlo">= useInt AInt1<int></font></div>
<div><font class="" face="Menlo">AInt2<int> =
cross_copy A<fp></font></div>
<div><font class="" face="Menlo">= useInt AInt2<int></font></div>
<div class="">```</div>
</div>
<div class=""><br class="">
</div>
<div class="">Bottom line, if we allow to modify the
assignments of the definition, the decision making is not
local anymore and in particular may require to add
repairing code all over the place. As a result the cost
model becomes much more complicated.</div>
<div class=""><br class="">
</div>
</div>
<blockquote type="cite" class="">
<div class="">
<div class=""><br class="">
Matt suggested that RegBankSelect should probably go
bottom-up instead and I agree with him. I don't think
there is a particular reason why RegBankSelect
necessarily has to go top-down.<br class="">
</div>
</div>
</blockquote>
<div><br class="">
</div>
<div>The rationale for going top-down is that when you reach
an instruction, all the operands are assigned so you know
what would be the cost of repairing.</div>
<div>You could said that the problem is the same for
definitions when going bottom-up. However, this is likely to
be more problematic because usually you have fewer
definitions than arguments on each individual instruction,
therefore there is more guess work going on (e.g., top-down,
you assume a cost for 1 definition, going bottom-up you have
to assume a cost for 2 arguments or more precisely you would
have to track a window of X instructions for 2 arguments
instead of 1 definition.)</div>
<br class="">
<blockquote type="cite" class="">
<div class="">
<div class=""><br class="">
I'm not too familiar with the implementation of
RegBankSelect. Would it be a big effort to make it work
bottom-up instead? I'm guessing one of the biggest areas
would be the check whether a cross-bank copy is needed
as well as calculating the overall cost for alternative
mappings as now all usages of the current instruction
would have to be checked instead of the much more
limited operands. How big of an impact would this have?<br
class="">
</div>
</div>
</blockquote>
<div><br class="">
</div>
<div>It’s been a while since I looked at the implementation
but I would expect this to be significant.</div>
<div><br class="">
</div>
<div>I think we should step back and check what we want before
investing any time in some rewrite. For instance, I don’t
see what bottom-up fundamentally gives us. It seems like a
workaround to me.</div>
<div>That said, it would work either way!</div>
<div><br class="">
</div>
<div>Cheers,</div>
<div>-Quentin</div>
<br class="">
<blockquote type="cite" class="">
<div class="">
<div class=""><br class="">
Cheers,<br class="">
<br class="">
Dominik<br class="">
<br class="">
_______________________________________________<br
class="">
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class="">
</div>
</div>
</blockquote>
</div>
<br class="">
</div>
</blockquote>
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