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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Perf regressions with enable-new-pm due to SpeculateAroundPHIs splitting backedges"
href="https://bugs.llvm.org/show_bug.cgi?id=48821">48821</a>
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<th>Summary</th>
<td>Perf regressions with enable-new-pm due to SpeculateAroundPHIs splitting backedges
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
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<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
</td>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>Scalar Optimizations
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>bjorn.a.pettersson@ericsson.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
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<p>
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<pre>Downstream we have seen performance regressions when using -enable-new-pm.
Basically the effect is that our backend passes fail to detect that hardware
loops can be use.
We've tracked it down to the SpecualteAroundPHIs pass modifying loops by doing
splitting on the back edge of the loop. And I wonder if that is expected (and
if there are any examples where it would be profitable to do that).
Consider a test case like this:
define void @hello(i16 %n) {
entry:
%cmp1 = icmp slt i16 %n, 8
br i1 %cmp1, label %for.body, label %for.end
for.body:
%i.02 = phi i16 [ %add, %for.body ], [ 0, %entry ]
%add = add nuw nsw i16 %i.02, 1
%cmp = icmp slt i16 %add, %n
br i1 %cmp, label %for.body, label %for.end
for.end:
ret void
}
when compiled with "opt -passes=spec-phis -S" we get
define void @hello(i16 %n) {
entry:
%cmp1 = icmp slt i16 %n, 8
br i1 %cmp1, label %entry.for.body_crit_edge, label %for.end
entry.for.body_crit_edge:
%add.1 = add nuw nsw i16 0, 1
br label %for.body
for.body:
%add.phi = phi i16 [ %add.0, %for.body.for.body_crit_edge ], [ %add.1,
%entry.for.body_crit_edge ]
%cmp = icmp slt i16 %add.phi, %n
br i1 %cmp, label %for.body.for.body_crit_edge, label %for.end
for.body.for.body_crit_edge:
%add.0 = add nuw nsw i16 %add.phi, 1
br label %for.body
for.end:
ret void
}
Is this a good transformation, or should SpecualateAroundPHIs avoid splitting
backedges?
Note that something like this might do the trick (not sure if it is an ideal
solution though):
diff --git a/llvm/lib/Transforms/Scalar/SpeculateAroundPHIs.cpp
b/llvm/lib/Transforms/Scalar/SpeculateAroundPHIs.cpp
index 9b18c945d950..41377f0572d5 100644
--- a/llvm/lib/Transforms/Scalar/SpeculateAroundPHIs.cpp
+++ b/llvm/lib/Transforms/Scalar/SpeculateAroundPHIs.cpp
@@ -792,6 +792,13 @@ static bool tryToSpeculatePHIs(SmallVectorImpl<PHINode *>
&PNs,
return false;
}
+ for (auto &Pred : PredSet) {
+ if (DT.dominates(PNs[0]->getParent(), Pred)) {
+ LLVM_DEBUG(dbgs() << " Involves a back-edge\n");
+ return false;
+ }
+ }
+
I don't know if any in-tree targets suffers the same way from
SpeculateAroundPHIs. Maybe people could do some benchmarks with the above patch
for their targets to see if there is any impacts.</pre>
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