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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 - Infinite loop in DAG combining"
href="https://bugs.llvm.org/show_bug.cgi?id=40521">40521</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Infinite loop in DAG combining
</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>Common Code Generator Code
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>mikael.holmen@ericsson.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=21403" name="attach_21403" title="reproducer">attachment 21403</a> <a href="attachment.cgi?id=21403&action=edit" title="reproducer">[details]</a></span>
reproducer
Reproduce with
llc bbi-23735.ll -o -
With -debug we see some repeated printouts:
Combining: t28: ch = store<(store 5 into %ir.res2, align 8), trunc to i36>
t126:1, t129, FrameIndex:i64<1>, undef:i64
Combining: t126: i64,ch = load<(dereferenceable load 4 from %ir.f, align 8),
zext from i32> t0, FrameIndex:i64<0>, undef:i64
Combining: t129: i64 = and t127, Constant:i64<4095>
Creating new node: t130: i32 = truncate t126
Creating constant: t131: i32 = Constant<4095>
Creating new node: t132: i32 = srl t130, Constant:i8<15>
Creating new node: t133: i32 = and t132, Constant:i32<4095>
Creating new node: t134: i64 = zero_extend t133
... into: t134: i64 = zero_extend t133
Combining: t126: i64,ch = load<(dereferenceable load 4 from %ir.f, align 8),
zext from i32> t0, FrameIndex:i64<0>, undef:i64
Combining: t28: ch = store<(store 5 into %ir.res2, align 8), trunc to i36>
t126:1, t134, FrameIndex:i64<1>, undef:i64
Combining: t134: i64 = zero_extend t133
Combining: t133: i32 = and t132, Constant:i32<4095>
Combining: t131: i32 = Constant<4095>
Combining: t132: i32 = srl t130, Constant:i8<15>
Combining: t130: i32 = truncate t126
Creating new node: t135: i32,ch = load<(dereferenceable load 4 from %ir.f,
align 8)> t0, FrameIndex:i64<0>, undef:i64
... into: t135: i32,ch = load<(dereferenceable load 4 from %ir.f, align 8)>
t0, FrameIndex:i64<0>, undef:i64
Combining: t28: ch = store<(store 5 into %ir.res2, align 8), trunc to i36>
t135:1, t134, FrameIndex:i64<1>, undef:i64
Combining: t134: i64 = zero_extend t133
Creating new node: t136: i64,ch = load<(dereferenceable load 4 from %ir.f,
align 8), zext from i32> t0, FrameIndex:i64<0>, undef:i64
Creating new node: t137: i64 = srl t136, Constant:i8<15>
Creating constant: t138: i64 = Constant<4095>
Creating new node: t139: i64 = and t137, Constant:i64<4095>
Replacing.1 t134: i64 = zero_extend t133
With: t139: i64 = and t137, Constant:i64<4095>
and 0 other values
Combining: t28: ch = store<(store 5 into %ir.res2, align 8), trunc to i36>
t136:1, t139, FrameIndex:i64<1>, undef:i64
Combining: t136: i64,ch = load<(dereferenceable load 4 from %ir.f, align 8),
zext from i32> t0, FrameIndex:i64<0>, undef:i64
Combining: t139: i64 = and t137, Constant:i64<4095>
Creating new node: t140: i32 = truncate t136
Creating constant: t141: i32 = Constant<4095>
Creating new node: t142: i32 = srl t140, Constant:i8<15>
Creating new node: t143: i32 = and t142, Constant:i32<4095>
Creating new node: t144: i64 = zero_extend t143
... into: t144: i64 = zero_extend t143
Combining: t136: i64,ch = load<(dereferenceable load 4 from %ir.f, align 8),
zext from i32> t0, FrameIndex:i64<0>, undef:i64
Combining: t28: ch = store<(store 5 into %ir.res2, align 8), trunc to i36>
t136:1, t144, FrameIndex:i64<1>, undef:i64
This starts happening with r329516 but I've no idea if that commit is really to
blame or if it just exposes some old problem.
r329516:
[DAGCombiner] Fold (zext (and/or/xor (shl/shr (load x), cst), cst))
In our real world application, we found the following optimization is
missed in DAGCombiner
(zext (and/or/xor (shl/shr (load x), cst), cst)) -> (and/or/xor (shl/shr
(zextload x), (zext cst)), (zext cst))
If the user of original zext is an add, it may enable further lea
optimization on x86.
This patch add a new function CombineZExtLogicopShiftLoad to do this
optimization.
Differential Revision: <a href="https://reviews.llvm.org/D44402">https://reviews.llvm.org/D44402</a></pre>
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