[LLVMdev] Multiple connected components in live interval

Jonas Paulsson
Thu, 16 Apr 2015 13:58:14 -0000

Hi,

I have come across a csmith generated test case that made the
MachineVerifier spit out:

*** Bad machine code: Multiple connected components in live interval ***

Having looked at what this might mean, it seems that
ConnectedVNInfoEqClasses::Classify() was called on the LI in question by
the verifier, and that it returned two equivalence classes, instead of
just one, which is demanded by the verifier. Does this mean that there
should never be
any ValNos in a LiveInterval that are not connected? In other words
should such an LI never exist, but rather two different LIs?

I have tried to run this on in-tree targets, but unfortunately they did
not reproduce the condition.
I will therefore try to explain:

The options to llc are -optimize-regalloc -O0. The function is
meaningless - with -O3 it just returns zero.
It contains two nested loops, with a call inside the inner loop, which
is a CFG-diamond.

The PHI-nodes look like this in the inner loop:

BB#5:  // Inner loop header
    Predecessors according to CFG: BB#1 BB#4
      vreg7<def> = PHI %vreg29, <BB#1>, %vreg4, <BB#4>
      ...
    Successors according to CFG: BB#2 BB#6

BB#2:
    Predecessors according to CFG: BB#5
    ...
    Successors according to CFG: BB#3 BB#4

BB#3:
    Predecessors according to CFG: BB#2
      call()
       %vreg46<def> = COPY %return_reg
       %vreg3<def> = COPY %vreg46;
       use of %vreg 46

    Successors according to CFG: BB#4

BB#4:
    Predecessors according to CFG: BB#2 BB#3
      %vreg4<def> = PHI %vreg7, <BB#2>, %vreg3, <BB#3>
    Successors according to CFG: BB#5

The observation I made here is that %vreg7 and %vreg4 are sort of nested
PHI nodes, while there are no other users of the registers than the PHI
nodes themselves. There is however a use of %vreg46, which later gets
coalesced with %vreg64, which will include as well the two PHI nodes.

This is the code with the two equivalence classes, when verifier aborts:

2272B   BB#1: derived from LLVM BB %bb3
             Predecessors according to CFG: BB#8
2304B           %vreg64<def> = mov 0
2448B           jmp <BB#5>
             Successors according to CFG: BB#5

2592B   BB#3: derived from LLVM BB %bb6
             Predecessors according to CFG: BB#2
2704B           callr <ga:@safe_div_func_uint64_t_u_u>
2736B           %vreg64<def> = COPY %return_reg
2768B           use of %vreg64
2784B           use of %vreg64
2816B           jmp <BB#4>
             Successors according to CFG: BB#4

*** Bad machine code: Multiple connected components in live interval ***
- function:    func_61
- interval:    %vreg64 [2304r,2336r:0)[2736r,2784r:3) 0 at 2304r 1 at x 2 at x
3 at 2736r
0: valnos 0
1: valnos 1 2 3
LLVM ERROR: Found 1 machine code errors.

Two small live ranges of %vreg64 (originated from %vreg7 and %vreg4),
which look ok to me, but the verifier does not like it.

Can anyone give me any background or any hint on what might be the
problem here?

thanks,

Jonas Paulsson