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<base href="http://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Failure to compute loop count (and, thus, vectorize)"
href="http://llvm.org/bugs/show_bug.cgi?id=19394">19394</a>
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<th>Summary</th>
<td>Failure to compute loop count (and, thus, vectorize)
</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>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Loop Optimizer
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>hfinkel@anl.gov
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=12363" name="attach_12363" title="test case">attachment 12363</a> <a href="attachment.cgi?id=12363&action=edit" title="test case">[details]</a></span>
test case
We currently are unable to vectorize (or do much of anything else) with the
following loop:
omp.lb.le.global_ub..lr.ph.split.us: ; preds = %entry
%12 = sext i32 %7 to i64
br label %omp.lb.le.global_ub..us
omp.lb.le.global_ub..us: ; preds =
%omp.lb_ub.check_pass.us, %omp.lb.le.global_ub..lr.ph.split.us
%indvars.iv14 = phi i64 [ %indvars.iv.next15, %omp.lb_ub.check_pass.us ], [
%12, %omp.lb.le.global_ub..lr.ph.split.us ]
%13 = trunc i64 %indvars.iv14 to i32
%omp.idx.le.ub.us = icmp sgt i32 %13, %10
br i1 %omp.idx.le.ub.us, label %omp.loop.end.loopexit, label
%omp.lb_ub.check_pass.us
omp.lb_ub.check_pass.us: ; preds =
%omp.lb.le.global_ub..us
%14 = load double* %ref3, align 8, !tbaa !4
%arrayidx.us = getelementptr inbounds [10000000 x double]* @c, i64 0, i64
%indvars.iv14
%15 = load double* %arrayidx.us, align 8, !tbaa !4
%mul5.us = fmul double %14, %15
%arrayidx6.us = getelementptr inbounds [10000000 x double]* @b, i64 0, i64
%indvars.iv14
store double %mul5.us, double* %arrayidx6.us, align 8, !tbaa !4
%indvars.iv.next15 = add nsw i64 %indvars.iv14, 1
br label %omp.lb.le.global_ub..us
omp.loop.end.loopexit: ; preds =
%omp.lb.le.global_ub..us
br label %omp.loop.end
this is important because this is what is generated by Intel's OpenMP branch
(after optimization) from this basic loop (the outlined portion):
ssize_t j;
#pragma omp parallel for
for (j=0; j<STREAM_ARRAY_SIZE; j++)
c[j] = a[j];
running opt -analyze -scalar-evolution -S < /tmp/stream_omt35.ll demonstrates
the problem. SE seems to understand the loop induction variable well enough:
%indvars.iv.next15 = add nsw i64 %indvars.iv14, 1
--> {(1 + (sext i32 %7 to i64)),+,1}<nsw><%omp.lb.le.global_ub..us>
Exits: <<Unknown>>
but it cannot compute an expression for the backedge count:
Determining loop execution counts for: @.omp_microtask.35
Loop %omp.lb.le.global_ub..us: Unpredictable backedge-taken count.
Loop %omp.lb.le.global_ub..us: Unpredictable max backedge-taken count.
We should either enhance SE, or maybe there is a different want to formulate
the loop in the frontend that will be friendlier to the existing SE
implementation.</pre>
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