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<base href="https://bugs.llvm.org/">
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<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [SLP] slp-vectorizer-hor miscompiles unrolled &"
href="https://bugs.llvm.org/show_bug.cgi?id=40310">40310</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>[SLP] slp-vectorizer-hor miscompiles unrolled &
</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>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>Loop Optimizer
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>fedor.v.sergeev@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=21322" name="attach_21322" title="IR that demonstrates slp-vectorizer miscompile">attachment 21322</a> <a href="attachment.cgi?id=21322&action=edit" title="IR that demonstrates slp-vectorizer miscompile">[details]</a></span>
IR that demonstrates slp-vectorizer miscompile
Attached/cited IR is a manually reduced result of optimizing a Java code:
---
void mainTest(int param, int values[], int len) {
int mask = 31;
for (int i = 0; i < len; i++) {
values[i] = param--;
for (int ix = 0; ix < 16; ix++)
param &= mask++;
}
}
---
] cat before-slp.ll
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128-ni:1"
target triple = "x86_64-unknown-linux-gnu"
define void @mainTest(i32 %param, i8* align 8 %vals, i32 %len) {
bci_15.preheader:
br label %bci_15
bci_15:
%local_0_ = phi i32 [ %v43, %bci_15 ], [ %param, %bci_15.preheader ]
%local_4_ = phi i32 [ %v44, %bci_15 ], [ 31, %bci_15.preheader ]
%v12 = add i32 %local_0_, -1
%v13 = add i32 %local_4_, 1
%v14 = and i32 %local_4_, %v12
%v15 = add i32 %local_4_, 2
%v16 = and i32 %v13, %v14
%v17 = add i32 %local_4_, 3
%v18 = and i32 %v15, %v16
%v19 = add i32 %local_4_, 4
%v20 = and i32 %v17, %v18
%v21 = add i32 %local_4_, 5
%v22 = and i32 %v19, %v20
%v23 = add i32 %local_4_, 6
%v24 = and i32 %v21, %v22
%v25 = add i32 %local_4_, 7
%v26 = and i32 %v23, %v24
%v27 = add i32 %local_4_, 8
%v28 = and i32 %v25, %v26
%v29 = add i32 %local_4_, 9
%v30 = and i32 %v27, %v28
%v31 = add i32 %local_4_, 10
%v32 = and i32 %v29, %v30
%v33 = add i32 %local_4_, 11
%v34 = and i32 %v31, %v32
%v35 = add i32 %local_4_, 12
%v36 = and i32 %v33, %v34
%v37 = add i32 %local_4_, 13
%v38 = and i32 %v35, %v36
%v39 = add i32 %local_4_, 14
%v40 = and i32 %v37, %v38
%v41 = add i32 %local_4_, 15
%v42 = and i32 %v39, %v40
%v43 = and i32 %v41, %v42
%v44 = add i32 %local_4_, 16
br i1 true, label %bci_15, label %loopexit
loopexit:
ret void
}
]
These add's and and's are a result of unrolled inner loop that does param =
&mask. Unfortunately, when horizontal reduction is applied it starts spitting
out undef-s:
] bin/opt -slp-vectorizer before-slp.ll -S | grep -c undef
23
]
And these undefs essentially lead to 'param' calculation to be undef'ed (and
become 0 in my case). This is a miscompile, and is rather subtle and hard to
track :(
If horizontal reduction is disabled there are no undefs and the code is
semantically correct. (also my original Java test starts passing, so I'm pretty
sure that my miscompile is caused by this).
] bin/opt -slp-vectorizer before-slp.ll -slp-vectorize-hor=false -S | grep -c
undef
0
]</pre>
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