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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Spills of implicit-defs seem useless"
href="https://bugs.llvm.org/show_bug.cgi?id=33311">33311</a>
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<th>Summary</th>
<td>Spills of implicit-defs seem useless
</td>
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<th>Product</th>
<td>libraries
</td>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Windows NT
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>Register Allocator
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>mcrosier@codeaurora.org
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</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
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<p>
<div>
<pre>While analyzing the hottest function in SPEC2000/mesa I noticed the following
code sequence:
str w8, sp,#260
str w8, sp,#244
str w8, sp,#256
str w8, sp,#248
str w8, sp,#252
str s1, sp,#284
str s1, sp,#280
str s1, sp,#288
str s1, sp,#276
str s1, sp,#272
At first it wasn't clear to me why these instructions weren't being paired by
the load store optimizer. These stores are spills of implicit-defs being
inserted by the register allocator. AFAICT these implicit-defs are coming from
variables that are undefined on some path in the CFG. I think the fix would be
to not spill these implicit-defs and update the register allocator's internal
data structures accordingly, but this is well outside my area of expertise.
To better understand how frequently this issue occurs I inserted a statistic in
the InlineSpiller to count the number of implicit-def spills. Across SPEC200X
there were ~200 static spills of which 61 came from SPEC2000/mesa.</pre>
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