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<base href="https://bugs.llvm.org/">
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<body><table border="1" cellspacing="0" cellpadding="8">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [IndVarSimplify] ReplaceExitValue=cheap cost modelling is too pessimistic"
href="https://bugs.llvm.org/show_bug.cgi?id=44668">44668</a>
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</tr>
<tr>
<th>Summary</th>
<td>[IndVarSimplify] ReplaceExitValue=cheap cost modelling is too pessimistic
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>Linux
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>Scalar Optimizations
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>lebedev.ri@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
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<div>
<pre>Apologies in advance for the crudeness of the "reduced" test case.
I'm providing the full sample for now, to be sure that i don't overreduce it.
<a href="https://godbolt.org/z/ZstrDC">https://godbolt.org/z/ZstrDC</a>
If we look at the original IR (see '-lcssa' output), we see:
<...>
%10 = load i32, i32* %width, align 8, !tbaa !15
%16 = zext i32 %10 to i64
; ^ loop-invariant, available before loop entry etc etc
<...>
for.body: ; preds = %for.body,
%omp.inner.for.body
%indvars.iv158 = phi i64 [ %indvars.iv.next159, %for.body ], [ 1,
%omp.inner.for.body ]
%indvars.iv = phi i64 [ %indvars.iv.next, %for.body ], [ 2,
%omp.inner.for.body ]
<...>
%indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
%cmp11 = icmp ult i64 %indvars.iv.next, %16
%indvars.iv.next159 = add nuw nsw i64 %indvars.iv158, 1
br i1 %cmp11, label %for.body, label %for.cond.invoke.cont15_crit_edge
for.cond.invoke.cont15_crit_edge: ; preds = %for.body
%indvars.iv.lcssa = phi i64 [ %indvars.iv, %for.body ]
%.lcssa = phi i16 [ %35, %for.body ]
%conv.i.i.i74.le = sext i16 %.lcssa to i32
%36 = trunc i64 %indvars.iv.lcssa to i32
br label %invoke.cont15
invoke.cont15: ; preds =
%for.cond.invoke.cont15_crit_edge, %omp.inner.for.body
%col.0.lcssa = phi i32 [ %36, %for.cond.invoke.cont15_crit_edge ], [ 1,
%omp.inner.for.body ]
%idxprom.i.i.i.i72.lcssa = phi i64 [ %indvars.iv.lcssa,
%for.cond.invoke.cont15_crit_edge ], [ 1, %omp.inner.for.body ]
<...>
So %indvars.iv* is used outside of the loop.
I suspect this is one of the reasons preventing this loop from being
vectorized.
Indeed, IndVarSimplify *can* deal with this, but it doesn't due to costmodel,
with '-lcssa -indvars -replexitval=always', we get the 'expected':
%10 = load i32, i32* %width, align 8, !tbaa !15
%16 = zext i32 %10 to i64
%17 = icmp ugt i64 %16, 3
%umax = select i1 %17, i64 %16, i64 3
%18 = add i64 %umax, -1
<...>
for.body: ; preds =
%for.body.preheader, %for.body
%indvars.iv158 = phi i64 [ %indvars.iv.next159, %for.body ], [ 1,
%for.body.preheader ]
%indvars.iv = phi i64 [ %indvars.iv.next, %for.body ], [ 2,
%for.body.preheader ]
%29 = add nsw i64 %indvars.iv158, -1
<...>
%indvars.iv.next159 = add nuw nsw i64 %indvars.iv158, 1
br i1 %cmp11, label %for.body, label %for.cond.invoke.cont15_crit_edge
for.cond.invoke.cont15_crit_edge: ; preds = %for.body
%.lcssa = phi i16 [ %37, %for.body ]
%conv.i.i.i74.le = sext i16 %.lcssa to i32
%38 = trunc i64 %18 to i32
br label %invoke.cont15
invoke.cont15: ; preds =
%for.cond.invoke.cont15_crit_edge, %omp.inner.for.body
%col.0.lcssa = phi i32 [ %38, %for.cond.invoke.cont15_crit_edge ], [ 1,
%omp.inner.for.body ]
%idxprom.i.i.i.i72.lcssa = phi i64 [ %18, %for.cond.invoke.cont15_crit_edge
], [ 1, %omp.inner.for.body ]
So in old case, in %invoke.cont15 bb, the incoming value for
%col.0.lcssa was essentially %indvars.iv, while now it is %18,
with cost=3 extra instructions.
Is this something fixable in IndVarSimplify?</pre>
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