via llvm-dev
2021-Feb-10 13:53 UTC
[llvm-dev] [DebugInfo]: Representing constants in debug-info
I don't see any use of DW_TAG_constant in the LLVM tree, except in the
DWARFLinker, which isn't what you need.
Most things not needed by C-family languages aren't supported, because to
date nobody has needed them (or if they did, they implemented the support
downstream). The closest similar thing would be enumerator constants; you could
probably imitate what's done for those fairly easily.
--paulr
From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of Tomar,
Sourabh Singh via llvm-dev
Sent: Wednesday, February 10, 2021 12:54 AM
To: llvm-dev <llvm-dev at lists.llvm.org>
Cc: George, Jini Susan <JiniSusan.George at amd.com>; Chen, Chih-Ping
<chih-ping.chen at intel.com>; Sharma, Alok Kumar <AlokKumar.Sharma at
amd.com>
Subject: [llvm-dev] [DebugInfo]: Representing constants in debug-info
[AMD Public Use]
Hi Everyone,
Is there a way of representing **constants** in LLVM debug-info ? Languages such
as FORTRAN has constants
i.e, consider the following Fortran snippet
[...]
Module foo
Integer, parameter :: bar = 200 ! Constant
End module foo
[...]
A front-end may choose to emit as Global Constant in LLVM IR as:
[...]
@bar.. = internal constant i32 200
A naïve attempt to represent it as GlobalVariable(or constant) as
[...]
!7 = !DIGlobalVariableExpression(var: !8, expr: !DIExpression(DW_OP_consts,
200))
!8 = distinct !DIGlobalVariable(name: "bar", scope: !2, file: !3,
line: 3, type: !9, isLocal: false, isDefinition: true)
This will materialize in DWARF as:
0x0000004a: DW_TAG_variable
DW_AT_name ("bar")
...
DW_AT_location (DW_OP_addr 0x2007d4, DW_OP_consts +200)
// This is incorrect, pointing to data section
[...]
Gfortran is representing this as: **DW_TAG_constant**
0x00000055: **DW_TAG_constant**
DW_AT_name ("bar")
...
DW_AT_type (0x0000006a "const integer(kind=4)")
DW_AT_external (true)
DW_AT_const_value (0xc8)
Do we have Metadata analog of (DW_TAG_constant) ? I think the primary question
here is to how represent this ?
Any inputs appreciated!
Thanks.. much!
Sourabh.
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Tomar, Sourabh Singh via llvm-dev
2021-Feb-12 17:30 UTC
[llvm-dev] [DebugInfo]: Representing constants in debug-info
[AMD Public Use]
Hi Paul,
Thanks for the hints for a possible implementation by mimicking something close
to Enumerations. I noticed 2 problems doing so:
1. Enumerators are integers, while a constant can be of any type(at least
basic types provided by the base language).
2. Restricting Scope of a Constant, Scoping concerns with Enumerator as
depicted in https://bugs.llvm.org/show_bug.cgi?id=49153
We did some digging in LLVM history, seems like there were some attempts for
bringing DW_TAG_constant back in 2010;
However things are not clear with respect to overall support, as a result it was
dropped.
Second last commit cites some GDB related concerns(back in 2010) for removal of
DW_TAG_constant from LLVM.
[...]
commit b407bb2789e0c1daea1b7d7e896e6a600e408cf0
Author: Duncan P. N. Exon Smith <dexonsmith at apple.com<mailto:dexonsmith
at apple.com>>
Date: Mon Feb 9 22:48:04 2015 +0000
DebugInfo: Remove DW_TAG_constant
Remove handling for DW_TAG_constant. We started producing it in
r110656, but reverted that in r110876 without dropping the support.
Finish the job.
llvm-svn: 228623
commit 4d597e82684d462bc98f2ff934e37d3c5e38a0c9
Author: Devang Patel <dpatel at apple.com<mailto:dpatel at
apple.com>>
Date: Wed Aug 11 23:17:54 2010 +0000
Even if a variable has constant value all the time, it is still a variable
in gdb's eyes.
Tested by scope.exp in gdb testsuite.
llvm-svn: 110876
commit b219746c80be195e914d6f620427217818476ba8
Author: Devang Patel <dpatel at apple.com<mailto:dpatel at
apple.com>>
Date: Tue Aug 10 07:11:13 2010 +0000
Handle TAG_constant for integers.
llvm-svn: 110656
[...]
Now, that GDB has support for DW_TAG_constant is pretty decent. So I wanted to
take this opportunity to define a new DIConstant metadata which will eventually
materialize as DW_TAG_constant in DWARF.
Supporting arguments with respect to this:
1. Languages such as Fortran and the new Frontend Flang(now part of LLVM) can
utilize this for producing debug-info.
2. Other languages that have named constant and there potential
front-ends(living down-stream or not part of LLVM) should be able utilize this
too.
Thanks ..much!
Sourabh.
From: paul.robinson at sony.com <paul.robinson at sony.com>
Sent: Wednesday, February 10, 2021 7:23 PM
To: Tomar, Sourabh Singh <SourabhSingh.Tomar at amd.com>; llvm-dev at
lists.llvm.org
Cc: George, Jini Susan <JiniSusan.George at amd.com>; chih-ping.chen at
intel.com; Sharma, Alok Kumar <AlokKumar.Sharma at amd.com>
Subject: RE: [DebugInfo]: Representing constants in debug-info
[CAUTION: External Email]
I don't see any use of DW_TAG_constant in the LLVM tree, except in the
DWARFLinker, which isn't what you need.
Most things not needed by C-family languages aren't supported, because to
date nobody has needed them (or if they did, they implemented the support
downstream). The closest similar thing would be enumerator constants; you could
probably imitate what's done for those fairly easily.
--paulr
From: llvm-dev <llvm-dev-bounces at lists.llvm.org<mailto:llvm-dev-bounces
at lists.llvm.org>> On Behalf Of Tomar, Sourabh Singh via llvm-dev
Sent: Wednesday, February 10, 2021 12:54 AM
To: llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at
lists.llvm.org>>
Cc: George, Jini Susan <JiniSusan.George at
amd.com<mailto:JiniSusan.George at amd.com>>; Chen, Chih-Ping
<chih-ping.chen at intel.com<mailto:chih-ping.chen at intel.com>>;
Sharma, Alok Kumar <AlokKumar.Sharma at amd.com<mailto:AlokKumar.Sharma at
amd.com>>
Subject: [llvm-dev] [DebugInfo]: Representing constants in debug-info
[AMD Public Use]
Hi Everyone,
Is there a way of representing **constants** in LLVM debug-info ? Languages such
as FORTRAN has constants
i.e, consider the following Fortran snippet
[...]
Module foo
Integer, parameter :: bar = 200 ! Constant
End module foo
[...]
A front-end may choose to emit as Global Constant in LLVM IR as:
[...]
@bar.. = internal constant i32 200
A naïve attempt to represent it as GlobalVariable(or constant) as
[...]
!7 = !DIGlobalVariableExpression(var: !8, expr: !DIExpression(DW_OP_consts,
200))
!8 = distinct !DIGlobalVariable(name: "bar", scope: !2, file: !3,
line: 3, type: !9, isLocal: false, isDefinition: true)
This will materialize in DWARF as:
0x0000004a: DW_TAG_variable
DW_AT_name ("bar")
...
DW_AT_location (DW_OP_addr 0x2007d4, DW_OP_consts +200)
// This is incorrect, pointing to data section
[...]
Gfortran is representing this as: **DW_TAG_constant**
0x00000055: **DW_TAG_constant**
DW_AT_name ("bar")
...
DW_AT_type (0x0000006a "const integer(kind=4)")
DW_AT_external (true)
DW_AT_const_value (0xc8)
Do we have Metadata analog of (DW_TAG_constant) ? I think the primary question
here is to how represent this ?
Any inputs appreciated!
Thanks.. much!
Sourabh.
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