llvm dev - Oct 2019 - LiveInterval error with 2 dead defs

The associated patch caused a compilation problems on Hexagon:
https://bugs.llvm.org/show_bug.cgi?id=43302

The splitting of a live interval should not be done automatically upon creation.
Calling LIS->getInterval(Reg) should not go around changing the code behind
the scenes.

There is already a function “splitSeparateComponents” that does that.  It should
be added where it’s missing.

--
Krzysztof Parzyszek  kparzysz at quicinc.com<mailto:kparzysz at
quicinc.com>   AI tools development

From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of Quentin
Colombet via llvm-dev
Sent: Monday, September 9, 2019 7:17 PM
To: Arsenault, Matthew <Matthew.Arsenault at amd.com>
Cc: llvm-dev <llvm-dev at lists.llvm.org>
Subject: [EXT] Re: [llvm-dev] LiveInterval error with 2 dead defs

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<mailto: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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> On Oct 7, 2019, at 14:04, Krzysztof Parzyszek via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> The associated patch caused a compilation problems on Hexagon:
https://bugs.llvm.org/show_bug.cgi?id=43302
<https://bugs.llvm.org/show_bug.cgi?id=43302>
>  
> The splitting of a live interval should not be done automatically upon
creation.  Calling LIS->getInterval(Reg) should not go around changing the
code behind the scenes.
>  
> There is already a function “splitSeparateComponents” that does that.  It
should be added where it’s missing.
>  
The patch is using splitSeparateComponents. Where else would it go? The problem
is with the initial live interval creation.

-Matt
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I have an alternative at https://reviews.llvm.org/D68666.

--
Krzysztof Parzyszek  kparzysz at quicinc.com<mailto:kparzysz at
quicinc.com>   AI tools development

From: Matt Arsenault <whatmannerofburgeristhis at gmail.com> On Behalf Of
Matt Arsenault
Sent: Monday, October 7, 2019 4:50 PM
To: Krzysztof Parzyszek <kparzysz at quicinc.com>
Cc: Quentin Colombet <qcolombet at apple.com>; Matthew P Arsenault
<Matthew.Arsenault at amd.com>; llvm-dev at lists.llvm.org; Pranav
Bhandarkar <pranavb at quicinc.com>
Subject: [EXT] Re: [llvm-dev] LiveInterval error with 2 dead defs




On Oct 7, 2019, at 14:04, Krzysztof Parzyszek via llvm-dev <llvm-dev at
lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:

The associated patch caused a compilation problems on Hexagon:
https://bugs.llvm.org/show_bug.cgi?id=43302

The splitting of a live interval should not be done automatically upon creation.
Calling LIS->getInterval(Reg) should not go around changing the code behind
the scenes.

There is already a function “splitSeparateComponents” that does that.  It should
be added where it’s missing.


The patch is using splitSeparateComponents. Where else would it go? The problem
is with the initial live interval creation.

-Matt
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