https://llvm.org/bugs/show_bug.cgi?id=31122
Bug ID: 31122
Summary: [ARM] Assert in register allocation during last chance
recoloring
Product: tools
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: llc
Assignee: unassignedb...@nondot.org
Reporter: ssija...@codeaurora.org
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
Created attachment 17634
--> https://llvm.org/bugs/attachment.cgi?id=17634&action=edit
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.
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