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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - GVNHoist leads to miscompile of llvm test-suite SingleSource/Benchmarks/Misc/fbench.c"
href="https://llvm.org/bugs/show_bug.cgi?id=28880">28880</a>
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<th>Summary</th>
<td>GVNHoist leads to miscompile of llvm test-suite SingleSource/Benchmarks/Misc/fbench.c
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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
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<th>OS</th>
<td>All
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>Scalar Optimizations
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>matze@braunis.de
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
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<th>Classification</th>
<td>Unclassified
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<p>
<div>
<pre>GVN-hoist appears to miscompile SingleSource/Benchmarks/Misc/fbench.c as this
builder:
<a href="http://lab.llvm.org:8080/green/job/Performance_O3flto/">http://lab.llvm.org:8080/green/job/Performance_O3flto/</a>
is broken after r277786 "GVN-hoist: enable by default".
Citing Sebastians analysis: "
think there is a problem with the MemorySSA when compiling fbench with -Ofast
The problem appears with -mllvm -gvn-max-hoisted=63, (at 62 the
program is not miscompiled.)
before.ll is the MSSA dump at 62 and after.ll is the MSSA for 63.
We end up hoisting two loads past a store, and the loads do not link
to the store: they are both liveOnEntry:
; MemoryUse(liveOnEntry)
%19 = load double, double* @axis_slope_angle, align 8, !tbaa !5
and
; MemoryUse(liveOnEntry)
%22 = load double, double* @axis_slope_angle, align 8, !tbaa !5
past a store to the global variable:
; 7 = MemoryDef(liveOnEntry)
store double 0.000000e+00, double* @axis_slope_angle, align 8, !tbaa !5
"</pre>
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