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<p>This is incorrect. <br>
</p>
<p>IRCE's current sole known user happens to be a compiler for a
GCed language, but there is no (intentional) dependence on that
fact. It should work on arbitrary IR. <br>
</p>
<p>Loop predication (the form in IndVars) triggers for arbitrary
IR. The separate pass depends on semantics of guards which is
related to deopt semantics, but *not* GC. <br>
</p>
<p>Philip<br>
</p>
<div class="moz-cite-prefix">On 5/11/21 7:17 AM, Jie He wrote:<br>
</div>
<blockquote type="cite"
cite="mid:CABSkmVHR_-=xbxfoGOBxQbToWRXDB0FbAhfdpi5Amx7ZCQfPAw@mail.gmail.com">
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<div dir="ltr">as I know, current IRCE implementation relies on
some preconditions. it's intended to language runtime with
garbage collection, not for loop vectorization.
<div>the same is true for loop predication, which is also
helpful for eliminating condition check within a loop.</div>
<div><br>
</div>
<div>Jie He</div>
<div>B.R</div>
</div>
<br>
<div class="gmail_quote">
<div dir="ltr" class="gmail_attr">On Tue, 11 May 2021 at 20:50,
Jingu Kang via llvm-dev <<a
href="mailto:llvm-dev@lists.llvm.org" moz-do-not-send="true">llvm-dev@lists.llvm.org</a>>
wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px
0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div style="overflow-wrap: break-word;" lang="EN-US">
<div class="gmail-m_6473269197180563542WordSection1">
<p class="MsoNormal">Hi Philip,</p>
<p class="MsoNormal"> </p>
<p class="MsoNormal">I have extended your suggestion
slightly more as below.</p>
<p class="MsoNormal"> </p>
<p class="MsoNormal">
newbound1 = min(n, c)</p>
<p class="MsoNormal">
newbound2 = max(n, c)</p>
<p class="MsoNormal"> while (iv < n) {
while(iv < newbound1) {</p>
<p class="MsoNormal"> A A</p>
<p class="MsoNormal"> if (iv < c)
B</p>
<p class="MsoNormal"> B C</p>
<p class="MsoNormal"> C }</p>
<p class="MsoNormal"> } iv =
newbound1</p>
<p class="MsoNormal">
while (iv < newbound2) {</p>
<p class="MsoNormal"> A</p>
<p class="MsoNormal"> C</p>
<p class="MsoNormal"> }</p>
<p class="MsoNormal"> </p>
<p class="MsoNormal">I have implemented a simple pass to
split bound of loop, which has conditional branch with
IV, as above example.
<a href="https://reviews.llvm.org/D102234"
target="_blank" moz-do-not-send="true">https://reviews.llvm.org/D102234</a>
It is initial version. If possible, please review it.</p>
<p class="MsoNormal"> </p>
<p class="MsoNormal">Thanks</p>
<p class="MsoNormal">JinGu Kang</p>
<p class="MsoNormal"> </p>
<div
style="border-top:none;border-right:none;border-bottom:none;border-left:1.5pt
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<div
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<p class="MsoNormal"><b>From:</b> Jingu Kang <<a
href="mailto:Jingu.Kang@arm.com" target="_blank"
moz-do-not-send="true">Jingu.Kang@arm.com</a>>
<br>
<b>Sent:</b> 04 May 2021 12:45<br>
<b>To:</b> Philip Reames <<a
href="mailto:listmail@philipreames.com"
target="_blank" moz-do-not-send="true">listmail@philipreames.com</a>>;
Jingu Kang <<a href="mailto:Jingu.Kang@arm.com"
target="_blank" moz-do-not-send="true">Jingu.Kang@arm.com</a>><br>
<b>Cc:</b> <a
href="mailto:llvm-dev@lists.llvm.org"
target="_blank" moz-do-not-send="true">llvm-dev@lists.llvm.org</a><br>
<b>Subject:</b> RE: [llvm-dev] Enabling IRCE pass
or Adding something similar in the pipeline of new
pass manager</p>
</div>
</div>
<p class="MsoNormal"> </p>
<p class="MsoNormal">Philip, I appreciate your kind
comments.</p>
<p><span style="font-size:10pt;font-family:"Courier
New"">>In this example, forming the full
pre/main/post loop structure of IRCE is overkill.
Instead, we could simply restrict the loop bounds in
the following manner:</span></p>
<pre>><a href="http://loop.ph" target="_blank" moz-do-not-send="true">loop.ph</a>:</pre>
<pre>> ;; Warning: psuedo code, might have edge conditions wrong</pre>
<pre>> %c = icmp sgt %iv, %n</pre>
<pre>> %min = umax(%n, %a)</pre>
<pre>> br i1 %c, label %exit, label %<a href="http://loop.ph" target="_blank" moz-do-not-send="true">loop.ph</a></pre>
<pre>> </pre>
<pre>>loop.ph.split:</pre>
<pre>> br label %loop</pre>
<pre>> </pre>
<pre>>loop:</pre>
<pre>> %iv = phi i64 [ %inc, %loop ], [ 1, %<a href="http://loop.ph" target="_blank" moz-do-not-send="true">loop.ph</a> ]</pre>
<pre>> %src.arrayidx = getelementptr inbounds i64, i64* %src, i64 %iv </pre>
<pre>> %val = load i64, i64* %src.arrayidx</pre>
<pre>> %dst.arrayidx = getelementptr inbounds i64, i64* %dst, i64 %iv </pre>
<pre>> store i64 %val, i64* %dst.arrayidx</pre>
<pre>> %inc = add nuw nsw i64 %iv, 1</pre>
<pre>> %cond = icmp eq i64 %inc, %min</pre>
<pre>> br i1 %cond, label %exit, label %loop</pre>
<pre>> </pre>
<pre>>exit:</pre>
<pre>> ret void</pre>
<pre>>}</pre>
<pre>> </pre>
<pre>>I'm not quite sure what to call this transform, but it's not IRCE. If this example is actually general enough to cover your use cases, it's going to be a lot easier to judge profitability on than the general form of iteration set splitting</pre>
<p class="MsoNormal"> </p>
<p class="MsoNormal">I agree with you. If the llvm
community is ok to accept above approach as a pass or
a part of a certain pass, I would be happy to
implement it because I am aiming to handle this case
with llvm upstream.</p>
<p class="MsoNormal"> </p>
<p><span style="font-size:10pt;font-family:"Courier
New"">>Another way to frame this special
case might be to recognize the conditional block can
be inverted into an early exit. (Reasoning: %iv is
strictly increasing, condition is monotonic, path if
not taken has no observable effect) Consider:</span></p>
<pre>><a href="http://loop.ph" target="_blank" moz-do-not-send="true">loop.ph</a>:</pre>
<pre>> br label %loop</pre>
<pre>> </pre>
<pre>>loop:</pre>
<pre>> %iv = phi i64 [ %inc, %for.inc ], [ 1, %<a href="http://loop.ph" target="_blank" moz-do-not-send="true">loop.ph</a> ]</pre>
<pre>> %cmp = icmp sge i64 %iv, %a</pre>
<pre>> br i1 %cmp, label %exit, label %for.inc</pre>
<pre>> </pre>
<pre>>for.inc:</pre>
<pre>> %src.arrayidx = getelementptr inbounds i64, i64* %src, i64 %iv </pre>
<pre>> %val = load i64, i64* %src.arrayidx</pre>
<pre>> %dst.arrayidx = getelementptr inbounds i64, i64* %dst, i64 %iv </pre>
<pre>> store i64 %val, i64* %dst.arrayidx</pre>
<pre>> %inc = add nuw nsw i64 %iv, 1</pre>
<pre>> %cond = icmp eq i64 %inc, %n</pre>
<pre>> br i1 %cond, label %exit, label %loop</pre>
<pre>> </pre>
<pre>>exit:</pre>
<pre>> ret void</pre>
<pre>>}</pre>
<p><span style="font-size:10pt;font-family:"Courier
New"">>Once that's done, the multiple exit
vectorization work should vectorize this loop.
Thinking about it, I really like this variant.
</span></p>
<p class="MsoNormal"> I have not looked at the multiple
exit vectorization work yet but it looks we could
consider the inverted condition as early exit’s
condition.</p>
<p><span style="font-size:10pt;font-family:"Courier
New"">>The costing here seems quite off. I
have not looked at how the vectorize costs
predicated loads on hardware without predication,
but needing to scalarize a conditional VF-times and
form a vector again does not have a cost of 3
million. This could definitely be improved.</span></p>
<p class="MsoNormal">I agree with you.</p>
<p class="MsoNormal"> </p>
<p class="MsoNormal">Additionally, if possible, I would
like to suggest to enable or add transformations in
order to help vectorization. For example, as removing
conditional branch inside loop, we could split a loop
with dependency, which blocks vectorization, into
vectorizable loop and non-vectorizable one using
transformations like loop distribution. I am not sure
why these features have not been enabled as default on
pass manager but it would make more loops
vectorizable.</p>
<p class="MsoNormal"> </p>
<p class="MsoNormal">Thanks</p>
<p class="MsoNormal">JinGu Kang</p>
</div>
</div>
</div>
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-- <br>
<div dir="ltr" class="gmail_signature">Best Regards<br>
He Jie 何杰</div>
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