[llvm-bugs] [Bug 39553] New: Fix "LLVM ERROR: ran out of registers during register allocation" for LDDWRdPtrQ with the Z register

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=39553

            Bug ID: 39553
           Summary: Fix "LLVM ERROR: ran out of registers during register
                    allocation" for LDDWRdPtrQ with the Z register
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: AVR
          Assignee: unassignedbugs at nondot.org
          Reporter: me at dylanmckay.io
                CC: llvm-bugs at lists.llvm.org

We have an AVR-specific workaround for a limitation of the register
allocator that only exposes itself on targets with high register
contention like AVR, which only has three pointer registers.

The three pointer registers are X, Y, and Z.
In most nontrivial functions, Y is reserved for the frame pointer,
as per the calling convention. This leaves X and Z. Some instructions,
such as LPM ("load program memory"), are only defined for the Z
register. Sometimes this just leaves X.

When the backend generates a LDDWRdPtrQ instruction with Z as the 
destination pointer, it usually trips up the register allocator
with this error message:

  LLVM ERROR: ran out of registers during register allocation

Here is a description from the author Peter Nimmervoll

  As far as I understand the problem occurs when LDDWRdPtrQ uses
  the ptrdispregs register class as target register. This should work, but 
  the allocator can't deal with this for some reason. So from my testing,
  it seams like (and I might be totally wrong on this) the allocator reserves
  the Z register for the ICALL instruction and then the register class
  ptrdispregs only has 1 register left and we can't use Y for source and 
  destination. Removing the Z register from DREGS fixes the problem but 
  removing Y register does not.

More information about the original bug and its investigation can be found on
the avr-rust issue
tracker at https://github.com/avr-rust/rust/issues/37.

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