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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - [SCEV] New Trip Count Computation cause Overflow assertion in MachineBlockFrequency"
href="http://llvm.org/bugs/show_bug.cgi?id=21622">21622</a>
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<th>Summary</th>
<td>[SCEV] New Trip Count Computation cause Overflow assertion in MachineBlockFrequency
</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>
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<th>OS</th>
<td>Linux
</td>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>normal
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Global Analyses
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>zhaoshiz@codeaurora.org
</td>
</tr>
<tr>
<th>CC</th>
<td>apazos@codeaurora.org, chandlerc@gmail.com, llvmbugs@cs.uiuc.edu, mcrosier@codeaurora.org
</td>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
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<div>
<pre>Created <span class=""><a href="attachment.cgi?id=13380" name="attach_13380" title="C Test case">attachment 13380</a> <a href="attachment.cgi?id=13380&action=edit" title="C Test case">[details]</a></span>
C Test case
<span class="quote">> clang --target=aarch64-linux-gnu -mcpu=cortex-a57 -Ofast test.c -mllvm -unroll-allow-partial -mllvm -unroll-threshold=1000 -c -o test.o
> clang-3.6: /prj/llvm-arm/home/nightly/src/community-mainline/llvm/lib/Analysis/BlockFrequencyInfoImpl.cpp:125: void combineWeight({anonymous}::Weight&, const Weight&): Assertion `W.Amount < W.Amount + OtherW.Amount && "Unexpected overflow"' failed.</span >
The commit below changes how SCEV computes loop trip count:
<span class="quote">> - dyn_cast<SCEVConstant>(getBackedgeTakenCount(L));
> + dyn_cast<SCEVConstant>(getExitCount(L, ExitingBlock));</span >
This results in a loop previously considered runtime (trip count is 0) becomes
a compile time loop (trip count is 255).
With '-unroll-allow-partial' and '-unroll-threshold=1000', loop unroll pass
chooses to unroll this loop by a factor of 85.
As a result, a substantial number of branches are targeting the same landing
block, eventually overflow BlockFrequency calculation.
commit ed05e3703e07dfeae33acdd3cc5ad07b5f5b86c6
Author: Mark Heffernan <<a href="mailto:meheff@google.com">meheff@google.com</a>>
Date: Fri Oct 10 10:39:11 2014
This patch de-pessimizes the calculation of loop trip counts in
ScalarEvolution in the presence of multiple exits. Previously all
loops exits had to have identical counts for a loop trip count to be
considered computable. This pessimization was implemented by calling
getBackedgeTakenCount(L) rather than getExitCount(L, ExitingBlock)
inside of ScalarEvolution::getSmallConstantTripCount() (see the FIXME
in the comments of that function). The pessimization was added to fix
a corner case involving undefined behavior (pr/16130). This patch more
precisely handles the undefined behavior case allowing the pessimization
to be removed.
ControlsExit replaces IsSubExpr to more precisely track the case where
undefined behavior is expected to occur. Because undefined behavior is
tracked more precisely we can remove MustExit from ExitLimit. MustExit
was used to track the case where the limit was computed potentially
assuming undefined behavior even if undefined behavior didn't necessarily
occur.
git-svn-id: <a href="https://llvm.org/svn/llvm-project/llvm/trunk@219517">https://llvm.org/svn/llvm-project/llvm/trunk@219517</a>
91177308-0d34-0410-b5e6-96231b3b80d8</pre>
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