llvm dev - Jan 2018 - StripDeadDebugInfo for static inline functions.

Hi all,

I would like to understand the strip-dead-debug-info transformation.
In my test case there is a static inline function with two local variables.
It appears that the function is already inlined before
strip-dead-debug-info starts its work. As a result the DICompileUnit is
cleaned and its subprograms list has no reference to the DISubprogram for
the inlined function, but as there is reference from the DILocalVariables
to the DISubprogram it remains (IMHO).
This caused to a segmentation fault while running llc on the optimized IR
file at function "constructAbstractSubprogramScopeDIE" in file
DwarfDebug.cpp because the scope of the DISubprogram is null (I assume it
should be the DICompileUnit).

So I would like to know what would be the correct solution for this issue:
Update optimization to not remove reference from the subprograms list, or
to skip finding scope in llc.

Any help is highly appreciated.

Thanks,
Arsen


-- 

If it's not fun you're not doing it right -- Fran Tarkenton
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I'm not as familiar with all the ins and outs of metadata as maybe I should
be, but ultimately the inlined function should have a DWARF description
contained within the description of the caller (which is why you're seeing
the call to constructAbstractSubprogramScopeDIE).  That suggests that the
DISubprogram for the inlined function ought to remain, and its scope should be
the DICompileUnit.
--paulr

From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of Arsen
Hakobyan via llvm-dev
Sent: Friday, January 12, 2018 1:57 AM
To: llvm-dev at lists.llvm.org
Subject: [llvm-dev] StripDeadDebugInfo for static inline functions.

Hi all,

I would like to understand the strip-dead-debug-info transformation.
In my test case there is a static inline function with two local variables.
It appears that the function is already inlined before strip-dead-debug-info
starts its work. As a result the DICompileUnit is cleaned and its subprograms
list has no reference to the DISubprogram for the inlined function, but as there
is reference from the DILocalVariables to the DISubprogram it remains (IMHO).
This caused to a segmentation fault while running llc on the optimized IR file
at function "constructAbstractSubprogramScopeDIE" in file
DwarfDebug.cpp because the scope of the DISubprogram is null (I assume it should
be the DICompileUnit).

So I would like to know what would be the correct solution for this issue:
Update optimization to not remove reference from the subprograms list, or to
skip finding scope in llc.

Any help is highly appreciated.

Thanks,
Arsen


--

If it's not fun you're not doing it right -- Fran Tarkenton
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Hi Paul,

Thanks for your response.
Let me actually post more details visualizing my case. Assuming that can
help.

so the IR before the opt tool is running is:

; Function Attrs: nounwind

define i16 @main() #0 !dbg !13 {

entry:

  %retval = alloca i16, align 1

    ...

}


; Function Attrs: inlinehint nounwind

define internal void @delay(i16 %d) #4 !dbg !69 {

entry:

  %d.addr = alloca i16, align 1

  %i = alloca i16, align 1

  ...

}


!llvm.dbg.cu = !{!0}

!llvm.module.flags = !{!84, !85}

!llvm.ident = !{!86}


!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer:
"clang version 3.8.0 Revision 72230", isOptimized: true,
runtimeVersion: 0,
emissionKind: 1, enums: !2, retainedTypes: !3, subprograms: !12, globals:
!73)

...

!12 = !{!13, !54, !57, !60, !61, !62, !65, !66, !67, !68, !69}

!13 = distinct !DISubprogram(name: "main", scope: !14, file: !14,
line: 15,
type: !15, isLocal: false, isDefinition: true, scopeLine: 15, isOptimized:
true, variables: !17)

...

!54 = distinct !DISubprogram(name: ...

!57 = distinct !DISubprogram(name: ...

!60 = distinct !DISubprogram(name: ...

!61 = distinct !DISubprogram(name: ...

!62 = distinct !DISubprogram(name: ...

!65 = distinct !DISubprogram(name: ...

!66 = distinct !DISubprogram(name: ...

!67 = distinct !DISubprogram(name: ...

!68 = distinct !DISubprogram(name: ...

!69 = distinct !DISubprogram(name: "delay", scope: !14, file: !14,
line: 9,
type: !55, isLocal: true, isDefinition: true, scopeLine: 9, flags:
DIFlagPrototyped, isOptimized: true, variables: !70)

!70 = !{!71, !72}

!71 = !DILocalVariable(name: "d", arg: 1, scope: !69, file: !14, line:
9,
type: !11)

!72 = !DILocalVariable(name: "i", scope: !69, file: !14, line: 9,
type: !11)

...




The IR after opt is:

; Function Attrs: nounwind

define i16 @main() #0 !dbg !13 {

entry:

  ...

}


!llvm.dbg.cu = !{!0}

!llvm.module.flags = !{!64, !65}

!llvm.ident = !{!66}


!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer:
"clang version 3.8.0 Revision 72230", isOptimized: true,
runtimeVersion: 0,
emissionKind: 1, enums: !2, retainedTypes: !3, subprograms: !12, globals:
!54)

...

!12 = !{!13}

!13 = distinct !DISubprogram(name: "main", scope: !14, file: !14,
line: 15,
type: !15, isLocal: false, isDefinition: true, scopeLine: 15, isOptimized:
true, variables: !17)

!14 = !DIFile(filename: "main.c", directory:
"path_to_directory")

...

!68 = !DILocalVariable(name: "d", arg: 1, scope: !69, file: !14, line:
9,
type: !11)

!69 = distinct !DISubprogram(name: "delay", scope: !14, file: !14,
line: 9,
type: !70, isLocal: true, isDefinition: true, scopeLine: 9, flags:
DIFlagPrototyped, isOptimized: true, variables: !72)

...

!73 = !DILocalVariable(name: "i", scope: !69, file: !14, line: 9,
type: !11)

...


I checked the StripSymbol.cpp file and there is no difference between
version I am using (from clang 3.8) and current (6.0.0) version.

As you can see the !12 node does not contain reference to !69 after
optimizations which I think is incorrect. I assume that !69 has not been
removed because there are two nodes (!68 and !73) referenced to it as the
scope. But that should force the transformations to not remove the link
from !12. Is it right?

Thank you very much.
Arsen


On Fri, Jan 12, 2018 at 10:21 PM, Robinson, Paul <paul.robinson at
sony.com>
wrote:
> I'm not as familiar with all the ins and outs of metadata as maybe I
> should be, but ultimately the inlined function should have a DWARF
> description contained within the description of the caller (which is why
> you're seeing the call to constructAbstractSubprogramScopeDIE).  That
> suggests that the DISubprogram for the inlined function ought to remain,
> and its scope should be the DICompileUnit.
>
> --paulr
>
>
>
> *From:* llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] *On Behalf Of
*Arsen
> Hakobyan via llvm-dev
> *Sent:* Friday, January 12, 2018 1:57 AM
> *To:* llvm-dev at lists.llvm.org
> *Subject:* [llvm-dev] StripDeadDebugInfo for static inline functions.
>
>
>
> Hi all,
>
>
>
> I would like to understand the strip-dead-debug-info transformation.
>
> In my test case there is a static inline function with two local variables.
>
> It appears that the function is already inlined before
> strip-dead-debug-info starts its work. As a result the DICompileUnit is
> cleaned and its subprograms list has no reference to the DISubprogram for
> the inlined function, but as there is reference from the DILocalVariables
> to the DISubprogram it remains (IMHO).
>
> This caused to a segmentation fault while running llc on the optimized IR
> file at function "constructAbstractSubprogramScopeDIE" in file
> DwarfDebug.cpp because the scope of the DISubprogram is null (I assume it
> should be the DICompileUnit).
>
>
>
> So I would like to know what would be the correct solution for this issue:
> Update optimization to not remove reference from the subprograms list, or
> to skip finding scope in llc.
>
>
>
> Any help is highly appreciated.
>
>
>
> Thanks,
>
> Arsen
>
>
>
>
>
> --
>
>
>
> If it's not fun you're not doing it right -- Fran Tarkenton
>


-- 

If it's not fun you're not doing it right -- Fran Tarkenton
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