[llvm-dev] LiveInterval error with 2 dead defs

[Quentin Colombet via llvm-dev]

Hi Matt,

The expectation is that each LiveInterval should only have values that are connected somehow, otherwise these should be split into different liveinterval.
I don’t think anything bad would happen when this assumption is broken (hence why it looks fine in your example), but it certainly puts needless constraints on the live-ranges.

I’m guessing Jakob added that check to make sure the splitting mechanism would create one live-range per connected component.

If you’re hitting that problem that means you miss somewhere a call to ConnectedVNInfoEqClasses::Distribute.

Cheers,
-Quentin
> On Sep 9, 2019, at 3:36 PM, Arsenault, Matthew <Matthew.Arsenault at amd.com> wrote:
> 
> Hi,
>  
> I’m hitting a machine verifier error in a trivial testcase which I don’t understand. There are 2 dead defs of the same register:
> ---
> name:            multiple_connected_compnents_dead
> tracksRegLiveness: true
> body:             |
>   bb.0:
>     dead %0:vgpr_32 = V_MOV_B32_e32 0, implicit $exec
>     dead %0:vgpr_32 = V_MOV_B32_e32 1, implicit $exec
>  
> ...
>  
> The live intervals look OK to me with 1 valno per instruction, for the life of the instruction like I would expect. The verifier does not like it however:
>  
> $ llc -mtriple=amdgcn-amd-amdhsa -mcpu=gfx906 -verify-machineinstrs -run-pass=machine-scheduler -o - -verify-misched foo.mir
>  
> # Before machine scheduling.
> ********** INTERVALS **********
> %0 [16r,16d:1)[32r,32d:0)  0 at 32r 1 at 16r weight:0.000000e+00
> RegMasks:
> ********** MACHINEINSTRS **********
> # Machine code for function multiple_connected_components_dead: NoPHIs, TracksLiveness
>  
> 0B           bb.0:
> 16B          dead %0:vgpr_32 = V_MOV_B32_e32 0, implicit $exec
> 32B          dead %0:vgpr_32 = V_MOV_B32_e32 1, implicit $exec
>  
> # End machine code for function multiple_connected_components_dead.
>  
> *** Bad machine code: Multiple connected components in live interval ***
> - function:    multiple_connected_components_dead
> - interval:    %0 [16r,16d:1)[32r,32d:0)  0 at 32r 1 at 16r weight:0.000000e+00
> 0: valnos 0
> 1: valnos 1
> LLVM ERROR: Found 1 machine code errors.
>  
> What is the problem here?
>  
> -Matt

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