[LLVMdev] Multiple connected components in live interval

Andrew Trick
Thu, 16 Apr 2015 22:50:37 -0000

> On Apr 16, 2015, at 6:58 AM, Jonas Paulsson <jonas.paulsson at
ericsson.com> wrote:
>
> 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?

That’s right. It looks like a copy was inserted,

> %vreg3<def> = COPY %vreg46


breaking the live interval, and a new LI was not created. Maybe the splitter did
it? You would need to look at debug-only=regalloc.

Andy

>
> 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
>
>
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