[llvm-dev] Assert in register allocation during last chance recoloring

[Sanjin Sijaric via llvm-dev]

Hi,

 

I opened the bug #31122 for this issue.

 

There are two parts to this problem. Compiled using "llc
-debug-only=regalloc test.ll", the test case attached to the bug 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_Q1_Q2_Q3 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 the assert is gone, I run into "LLVM ERROR: ran out of registers
during register allocation".  Increasing lcr-max-depth doesn't help.

 

To get past this error, I am thinking about spilling an actual physical
register if last chance recoloring fails.  For example,

 

P = ..

 

virtX = .. Last chance recoloring fails, 

 

. = P

 

.. = virtX

 

will result in something along the lines of

 

P = ..

 

spill P to fi#0 (original P)

P = .. (virtX is now in P)

..

Spill P to fi#1 (this is virtX)

Reload P form from fi#0 (original P)

. = P

..

Reload P from fi#1 (this is virtX)

= P

 

Does anyone have any other suggestions about dealing with this problem?

 

Thank you,

Sanjin

 

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