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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 - LoopVectorizer bails out when ShortTripCount=8, MaxVF=16"
href="https://bugs.llvm.org/show_bug.cgi?id=34438">34438</a>
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<th>Summary</th>
<td>LoopVectorizer bails out when ShortTripCount=8, MaxVF=16
</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>
<tr>
<th>OS</th>
<td>Windows NT
</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>Loop Optimizer
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>zvi.rackover@intel.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>For this case:
target datalayout =
"e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
target triple = "x86_64-apple-macosx10.8.0"
; This loop will be vectorized as the trip count is below the threshold but no
; scalar iterations are needed. This should work also when MaxVL > TripCount
;
define void @small_tc(float* noalias nocapture %A, float* noalias nocapture
readonly %B) {
entry:
br label %for.body
for.body:
%indvars.iv = phi i64 [ 0, %entry ], [ %indvars.iv.next, %for.body ]
%arrayidx = getelementptr inbounds float, float* %B, i64 %indvars.iv
%0 = load float, float* %arrayidx, align 4, !llvm.mem.parallel_loop_access
!3
%arrayidx2 = getelementptr inbounds float, float* %A, i64 %indvars.iv
%1 = load float, float* %arrayidx2, align 4, !llvm.mem.parallel_loop_access
!3
%add = fadd fast float %0, %1
store float %add, float* %arrayidx2, align 4, !llvm.mem.parallel_loop_access
!3
%indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
%exitcond = icmp eq i64 %indvars.iv.next, 8
br i1 %exitcond, label %for.end, label %for.body, !llvm.loop !4
for.end:
ret void
}
!3 = !{!3}
!4 = !{!4}
The loop is vectorized with:
opt -loop-vectorize -mtriple=x86_64-apple-macosx10.8.0 -mcpu=core-i7
(ShortTripCount=8, VF=8)
The loop is not vectorized with:
(ShortTripCount=8, VF=16)
opt -loop-vectorize -mtriple=x86_64-apple-macosx10.8.0 -mcpu=skx
It appears that we are bailing out here:
*********************************************
if (TC % MaxVF != 0) {
// If the trip count that we found modulo the vectorization factor is not
// zero then we require a tail.
// FIXME: look for a smaller MaxVF that does divide TC rather than give
up.
// FIXME: return None if loop requiresScalarEpilog(<MaxVF>), or look for a
// smaller MaxVF that does not require a scalar epilog.
...
return None;
}
***********************************************
Looks like the first FIXME is the solution.</pre>
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