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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - [ARM] Assert in register allocation during last chance recoloring"
href="https://llvm.org/bugs/show_bug.cgi?id=31122">31122</a>
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<th>Summary</th>
<td>[ARM] Assert in register allocation during last chance recoloring
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<th>Product</th>
<td>tools
</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>Linux
</td>
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<th>Status</th>
<td>NEW
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<th>Severity</th>
<td>normal
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>llc
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>ssijaric@codeaurora.org
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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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<div>
<pre>Created <span class=""><a href="attachment.cgi?id=17634" name="attach_17634" title="Test case for register allocation">attachment 17634</a> <a href="attachment.cgi?id=17634&action=edit" title="Test case for register allocation">[details]</a></span>
Test case for register allocation
There are two parts to this problem. Compiled using "llc -debug-only=regalloc
test.ll", the attached test case fails with
VirtRegMap.h:119: void llvm::VirtRegMap::clearVirt(unsigned int): Assertion
`Virt2PhysMap[virtReg] != NO_PHYS_REG && "attempt to clear a not assigned
virtual register"' failed.
The code looks like:
%vreg647<def> = COPY %vreg645; QQQQPR_with_dsub_7_then_ssub_0:%vreg647
QQQQPR:%vreg645
%vreg657<def> = COPY %vreg655; QQQQPR_with_dsub_7_then_ssub_0:%vreg657
QQQQPR:%vreg655
%vreg679<def> = COPY %vreg678; QPR_VFP2:%vreg679 QPR:%vreg678
%vreg657:dsub_7_then_ssub_0<def> = VDIVS %vreg679:ssub_2,
%vreg647:dsub_7_then_ssub_0, pred:14, pred:%noreg;
QQQQPR_with_dsub_7_then_ssub_0:%vreg657,%vreg647 QPR_VFP2:%vreg679
%vreg659<def> = COPY %vreg657; QQQQPR:%vreg659
QQQQPR_with_dsub_7_then_ssub_0:%vreg657
This problem occurs during last chance recoloring, where %vreg679 doesn’t get
allocated (and is marked as not spillable), so last chance recoloring kicks in.
It tries to allocate Q0 to %vreg679, where Q0 is assigned to %vreg647, which
interferes with %vreg679. Upon trying to allocate a register to %vreg647,
%vreg679 gets evicted from Q0. But %vreg679 is a fixed register, so it
shouldn’t get evicted. Recoloring is thus deemed successful, and upon
returning from tryRecoloringCandidates, we assert when trying to unassign
%vreg679.
I can get around this by moving the virtual register under consideration in
tryLastChanceRecoloring (%vreg679) to RS_Done stage right before the call to
tryRecolringCandidates, and then restoring the original stage afterward. This
prevents the eviction of %vreg679 during recoloring. Alternatively, I can
check FixedRegisters in canEvictInterference and return false.
After this assert is gone, I run into “LLVM ERROR: ran out of registers during
register allocation”.
To get past this problem, we may need to spill an actual physical register to
free it up for %vreg679 if last chance recoloring fails.</pre>
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