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2018-Nov-01 22:27 UTC

[llvm-dev] RFC: Adding debug information to LLVM to support Fortran

Regarding flags, I was just thinking that maybe we should invent a new
DISubprogramFlags type.  DISubprogram already has a few bitfields for
subprogram-specific things, Fortran will want 3 more, and there's no reason
to fill up the generic DIFlags with more bits that are used in only one class.

I agree that the array stuff needs to be designed with an eye to handling how
other languages do arrays, and leverage the common aspects.  Several languages
have runtime-sized arrays and it would be nice to handle them all the same way.
However the CHARACTER type probably does want to be DW_TAG_string_type rather
than an array. COBOL also has strings as a fundamental type.

I guess we'll have to learn what all the Fortran array stuff actually means
now…
--paulr

From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of Adrian
Prantl via llvm-dev
Sent: Thursday, November 01, 2018 6:12 PM
To: Eric Schweitz
Cc: llvm-dev at lists.llvm.org
Subject: Re: [llvm-dev] RFC: Adding debug information to LLVM to support Fortran

Thanks for sharing your plans, I made a few comments inline I noticed on my
first quick read-through.

-- adrian


On Nov 1, 2018, at 1:29 PM, Eric Schweitz via llvm-dev <llvm-dev at
lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:


From: flang-dev <flang-dev-bounces at
lists.flang-compiler.org<mailto:flang-dev-bounces at
lists.flang-compiler.org>> On Behalf Of Eric Schweitz (PGI)
Sent: Thursday, November 01, 2018 1:02 PM
To: flang-dev at lists.flang-compiler.org<mailto:flang-dev at
lists.flang-compiler.org>
Subject: [Flang-dev] RFC: Adding debug information to LLVM to support Fortran

In order to support debugging in the Flang project, work has been done to extend
LLVM debug information for the Fortran language. The changes are currently
available at https://github.com/flang-compiler/llvm.

In order to upstream these changes into LLVM itself, three smaller changesets,
described below, will be uploaded to
https://reviews.llvm.org<https://reviews.llvm.org/> for code review.

1. Elemental, Pure, and Recursive Procedures
DWARF 4 defines attributes for these Fortran procedure specifiers:
DW_AT_elemental, DW_AT_pure, DW_AT_recursive, resp. LLVM has a way of informing
the DWARF generator of simple boolean attributes in the metadata via the flags
parameter. We have added these new values to the existing collection of flags.
!60 = !DISubprogram(…, flags: DIFlagElemental)
!61 = !DISubprogram(…, flags: DIFlagPure)
!62 = !DISubprogram(…, flags: DIFlagRecursive)

That change should be fairly straightforward, although we are starting to run
out on bits in DIFLags, so we may need to come up with a clever encoding (such
as reusing bits that aren't used in a DISubprogram context).




2. Fortran Type Support
2.1 CHARACTER Intrinsic Type
There is no analog in C for the Fortran CHARACTER type. The Fortran CHARACTER
type maps to the DWARF tag, DW_TAG_string_type. We have added a new named DI to
LLVM to generate this DWARF information.
!21 = !DIStringType(name: “character(5)”, size: 40)

This produces the following DWARF information.
DW_TAG_string_type:
                DW_AT_name: “character(5)”
                DW_AT_byte_size: 5

CHARACTER types can also have deferred length. This is supported in the new
metadata as follows.
!22 = !DIStringType(name: “character(*)!1”, size: 32, stringLength: !23,
stringLengthExpression: !DIExpression())
!23 = !DILocalVariable(scope: !3, arg: 4, file: !4, type: !5, flags:
DIFlagArtificial)
Can you take a look at how variable-length arrays in C99 are implemented in
Clang at the moment? It would be nice to use a similar scheme here.


This will generate the following DWARF information.
DW_TAG_string_type:
                DW_AT_name: character(*)!1
DW_AT_string_length: 0x9b (location list)
DW_AT_byte_size: 4

2.2 Fortran Array Types and Bounds
In this section we refer to the DWARF tag, DW_TAG_array_type, which is used to
describe Fortran arrays.
However in Fortran, arrays are not types but are rather runtime data objects, a
multidimensional rectangular set of scalar data of homogeneous type.  An array
object has dimensions (rank and corank) and extents in those dimensions. The
rank and ranges of the extents of an array may not be known until runtime.
Arrays may be reshaped, acted upon in whole or in part, or otherwise be
referenced (perhaps even in reverse order) non-contiguously. Furthermore arrays
may be allocated and deallocated at runtime and aliased through other POINTER
objects. In short, Fortran array objects are not readily mappable to the C
family of languages model of arrays, and more expressive DWARF information is
required.
2.2.1 Explicit array dimensions
An array may be given a constant size as in the following example. The example
shows a two-dimensional array, named array, that has indices from 1 to 10 for
the rows and 2 to 11 for the columns.
TYPE(t) :: array(10,2:11)

For this declaration, the compiler generates the following LLVM metadata.
!100 = !DIFortranArrayType(baseType: !7, elements: !101)

Since the DI* hierarchy really just is the DWARF type hierarchy, I don't
think we will need to introduce any fortran-specific names for arrays.


!101 = !{ !102, !103 }
!102 = !DIFortranSubrange(constLowerBound: 1, constUpperBound: 10)
!103 = !DIFortranSubrange(constLowerBound: 2, constUpperBound: 11)

The DWARF generated for this is as follows. (DWARF asserts in the standard that
arrays are interpreted as column-major.)
DW_TAG_array_type:
                DW_AT_name: array
                DW_AT_type: 4d08 ;TYPE(t)
DW_TAG_subrange_type:
DW_AT_type: int
DW_AT_lower_bound: 1
DW_AT_upper_bound: 10
DW_TAG_subrange_type:
DW_AT_type: int
DW_AT_lower_bound: 2
DW_AT_upper_bound: 11

2.2.2 Adjustable arrays
By adjustable arrays, we mean that an array may have its size passed explicitly
as another argument.
SUBROUTINE subr2(array2,N)
  INTEGER :: N
 TYPE(t) :: array2(N)

In this case, the compiler expresses the !DISubrange as an expression that
references the dummy argument, N.
call void @llvm.dbg.declare(metadata i64* %N, metadata !113, metadata
!DIExpression())
…
!110 = !DIFortranArrayType(baseType: !7, elements: !111)
!111 = !{ !112 }
!112 = !DIFortranSubrange(lowerBound: 1, upperBound: !113, upperBoundExpression:
!DIExpression(DW_OP_deref))
It would be better (and much more robust in presence of optimizations) if the
DIExpression were part of a @llvm.dbg.declare / value intrinsic tying the
DILocalVariable to an LLVM SSA value.
!113 = !DILocalVariable(scope: !2, name: “zb1”, file: !3, type: !4, flags:
DIFlagArtificial)

It turned out that gdb didn’t properly interpret location lists or variable
references in the DW_AT_lower_bound and DW_AT_upper_bound attribute forms, so
the compiler must generate either a constant or a block with the DW_OP
operations for each of them.
DW_TAG_array_type:
                DW_AT_name: array2
                DW_AT_type: 4d08 ;TYPE(t)
DW_TAG_subrange_type:
DW_AT_type: int
DW_AT_lower_bound: 1
DW_AT_upper_bound: 2 byte block: 91 70

2.2.3 Assumed size arrays
An assumed size array leaves the last dimension of the array unspecified.
SUBROUTINE subr3(array3)
  TYPE(t) :: array3(*)

The compiler generates DWARF information without an upper bound, such as in this
snippet.
DW_TAG_array_type
DW_AT_name: array3
DW_TAG_subrange_type
DW_AT_type = int
DW_AT_lower_bound = 1

This DWARF is produced by omission of the upper bound information.
!122 = !DIFortranSubrange(lowerBound: 1)

2.2.4 Assumed shape arrays
Fortran also has assumed shape arrays, which allow extra state to be passed into
the procedure to describe the shape of the array dummy argument. This extra
information is the array descriptor, generated by the compiler, and passed as a
hidden argument.
SUBROUTINE subr4(array4)
  TYPE(t) :: array4(:,:)

In this case, the compiler generates DWARF expressions to access the results of
the procedure’s usage of the array descriptor argument when it computes the
lower bound (DW_AT_lower_bound) and upper bound (DW_AT_upper_bound).
…
call void @llvm.dbg.declare(metadata i64* %4, metadata !134, metadata
!DIExpression())
call void @llvm.dbg.declare(metadata i64* %8, metadata !136, metadata
!DIExpression())
call void @llvm.dbg.declare(metadata i64* %9, metadata !137, metadata
!DIExpression())
call void @llvm.dbg.declare(metadata i64* %13, metadata !139, metadata
!DIExpression())
…
!130 = !DIFortranArrayType(baseType: !80, elements: !131)
!131 = !{ !132, !133 }
!132 = !DISubrange(lowerBound: !134, lowerBoundExpression:
!DIExpression(DW_OP_deref), upperBound: !136, upperBoundExpression:
!DIExpression(DW_OP_deref))
!133 = !DISubrange(lowerBound: !137, lowerBoundExpression:
!DIExpression(DW_OP_deref), upperBound: !139, upperBoundExpression:
!DIExpression(DW_OP_deref))
same here.

!134 = !DILocalVariable(scope: !2, file: !3, type: !9, flags: DIArtificial)
!136 = !DILocalVariable(scope: !2, file: !3, type: !9, flags: DIArtificial)
!137 = !DILocalVariable(scope: !2, file: !3, type: !9, flags: DIArtificial)
!139 = !DILocalVariable(scope: !2, file: !3, type: !9, flags: DIArtificial)

The DWARF generated for this is as follows.
DW_TAG_array_type:
DW_AT_name: array4
DW_AT_type: 4d08 ;TYPE(t)
DW_TAG_subrange_type:
DW_AT_type: int
DW_AT_lower_bound: 2 byte block: 91 78
DW_AT_upper_bound: 2 byte block: 91 70
DW_TAG_subrange_type:
DW_AT_type: int
DW_AT_lower_bound: 2 byte block: 91 68
DW_AT_upper_bound: 2 byte block: 91 60

2.2.5 Assumed rank arrays and coarrays
This changeset does not address DWARF 5 extensions to support assumed rank
arrays or coarrays.

3. Fortran COMMON Block
COMMON blocks are a feature of Fortran that has no direct analog in C languages,
but they are similar to data sections in assembly language programming. A COMMON
block is a named area of memory that holds a collection of variables. Fortran
subprograms may map the COMMON block memory area to their own, possibly
distinct, non-empty list of variables.  A Fortran COMMON block might look like
the following example.

COMMON /ALPHA/ I, J

For this construct, the compiler generates a new scope-like DI construct
(!DICommonBlock) into which variables (see I, J above) can be placed. As the
common block implies a range of storage with global lifetime, the !DICommonBlock
refers to a !DIGlobalVariable.  The Fortran variable that comprise the COMMON
block are also linked via metadata to offsets within the global variable that
stands for the entire common block.

@alpha_ = common global %alphabytes_  zeroinitializer, align 64, !dbg !27, !dbg
!30, !dbg !33
!14 = distinct !DISubprogram(…)
!20 = distinct !DICommonBlock(scope: !14, declaration: !25, name:
"alpha")
!25 = distinct !DIGlobalVariable(scope: !20, name: "common alpha",
type: !24)
!27 = !DIGlobalVariableExpression(var: !25, expr: !DIExpression())
!29 = distinct !DIGlobalVariable(scope: !20, name: "i", file: !3,
type: !28)
!30 = !DIGlobalVariableExpression(var: !29, expr: !DIExpression())
!31 = distinct !DIGlobalVariable(scope: !20, name: "j", file: !3,
type: !28)
!32 = !DIExpression(DW_OP_plus_uconst, 4)
!33 = !DIGlobalVariableExpression(var: !31, expr: !32)

The DWARF generated for this is as follows.

DW_TAG_common_block:
DW_AT_name: alpha
DW_AT_location: @alpha_+0
DW_TAG_variable:
DW_AT_name: common alpha
DW_AT_type: array of 8 bytes
DW_AT_location: @alpha_+0
DW_TAG_variable:
DW_AT_name: i
DW_AT_type: integer*4
DW_AT_location: @alpha+0
DW_TAG_variable:
DW_AT_name: j
DW_AT_type: integer*4
DW_AT_location: @alpha+4

--
Eric


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Sohail Somani (Fizz Buzz Inc.) via llvm-dev

2018-Nov-01 22:35 UTC

head link

[llvm-dev] RFC: Adding debug information to LLVM to support Fortran

On Thu, Nov 1, 2018, at 6:27 PM, via llvm-dev wrote:

> Regarding flags, I was just thinking that maybe we should invent a new
> DISubprogramFlags type.  DISubprogram already has a few bitfields for
> subprogram-specific things, Fortran will want 3 more, and there's no
> reason to fill up the generic DIFlags with more bits that are used in
> only one class.>  
> I agree that the array stuff needs to be designed with an eye to
> handling how other languages do arrays, and leverage the common
> aspects.  Several languages have runtime-sized arrays and it would be
> nice to handle them all the same way.> However the CHARACTER type
probably does want to be DW_TAG_string_type
> rather than an array. COBOL also has strings as a fundamental type.>  
> I guess we'll have to learn what all the Fortran array stuff actually
> means now…> --paulr

May I suggest changing DIFlags from an enum to a simple int wrapper that
behaves like an enum and can implement encoding? It is true that a lot
of the bits are left unused in everyday contexts, so there is a
possibility to encode more data.For example:

DIFlags flags = FortranRelatedContext; flags |= SomeFortranFlag;So when
assigning SomeFortranFlag the appropriate DIFlags operator would
assert that the specific context is active.I’ve had to extend DIFlags and in the
end, just increased the size of
the data type, but a clever encoding with a non-clever interface would
halve memory usage currently used for DIFlags in my circumstance.— Sohail Somani
Fizz Buzz Inc. Booking schedule:
https://sohailsomani.youcanbook.me

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Eric Schweitz (PGI) via llvm-dev

2018-Nov-02 19:03 UTC

head link

[llvm-dev] RFC: Adding debug information to LLVM to support Fortran

Hi Sohail,

Thanks for your thoughts.  You can find the patch here, and I believe it is
rather close to what you suggest.

https://reviews.llvm.org/D54043

--
Eric


From: Sohail Somani (Fizz Buzz Inc.) via llvm-dev <llvm-dev at
lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>
Date: Thu, Nov 1, 2018 at 3:35 PM
Subject: Re: [llvm-dev] RFC: Adding debug information to LLVM to support Fortran
To: <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>


On Thu, Nov 1, 2018, at 6:27 PM, via llvm-dev wrote:

Regarding flags, I was just thinking that maybe we should invent a new
DISubprogramFlags type.  DISubprogram already has a few bitfields for
subprogram-specific things, Fortran will want 3 more, and there's no reason
to fill up the generic DIFlags with more bits that are used in only one class.



I agree that the array stuff needs to be designed with an eye to handling how
other languages do arrays, and leverage the common aspects.  Several languages
have runtime-sized arrays and it would be nice to handle them all the same way.

However the CHARACTER type probably does want to be DW_TAG_string_type rather
than an array. COBOL also has strings as a fundamental type.



I guess we'll have to learn what all the Fortran array stuff actually means
now…

--paulr

May I suggest changing DIFlags from an enum to a simple int wrapper that behaves
like an enum and can implement encoding? It is true that a lot of the bits are
left unused in everyday contexts, so there is a possibility to encode more data.

For example:

DIFlags flags = FortranRelatedContext;

flags |= SomeFortranFlag;

So when assigning SomeFortranFlag the appropriate DIFlags operator would assert
that the specific context is active.

I’ve had to extend DIFlags and in the end, just increased the size of the data
type, but a clever encoding with a non-clever interface would halve memory usage
currently used for DIFlags in my circumstance.

—
Sohail Somani
Fizz Buzz Inc.
Booking schedule: https://sohailsomani.youcanbook.me

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llvm dev - Nov 2018 - RFC: Adding debug information to LLVM to support Fortran

[llvm-dev] RFC: Adding debug information to LLVM to support Fortran

[llvm-dev] RFC: Adding debug information to LLVM to support Fortran

[llvm-dev] RFC: Adding debug information to LLVM to support Fortran