[LLVMdev] Machine code sinking pass

[Jonas Paulsson]

Hi,

I ran into MachineVerifier "Virtual register killed in block, but needed live out."

It was MachineSinking:PerformTrivialForwardCoalescing() that coalesced a COPY inside a single-block loop, but left the
kill-flag and then MachineVerifier complains that a register in vregsRequired is killed in MBB.

In the example, %vreg520 is replaced by %vreg368 by PerformTrivialForwardCoalescing(), without clearing the kill flag:

BB#13: derived from LLVM BB %CF250
    Predecessors according to CFG: BB#12 BB#13 BB#14
       ...
        %vreg520<def> = COPY %vreg368
        %vreg568<def,tied1> = cmp %vreg341<tied0>, %vreg520<kill>
        brr_cond <BB#13>
        brr_uncond <BB#14>
    Successors according to CFG: BB#13, BB#14

Into

BB#13: derived from LLVM BB %CF250
    Predecessors according to CFG: BB#12 BB#13 BB#14
        ...
        %vreg568<def,tied1> = cmp %vreg341<tied0>, %vreg368<kill>
        brr_cond <BB#13>
        brr_uncond <BB#14>

=>

*** Bad machine code: Virtual register killed in block, but needed live out. ***
- function:    autogen_SD15028
- basic block: BB#13 CF250 (0x1c75890)
Virtual register %vreg368 is used after the block.

There is only one use of %vreg368 in the function.

One thing that strikes me is that one might want to clear the kill flag for a use inside a loop of a register defined prior to
the loop?

Best regards,

Jonas Paulsson

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