llvm dev - Apr 2017 - GSoC project on finding and fixing bugs with debug info

Hi,

I would like to learn more about the GSoC project to find and fix bugs that
changes the generated code when it is compiled with the debug info option (
http://llvm.org/OpenProjects.html#gsoc17). I have built llvm with clang and
I compiled the hello.c program found in http://llvm.org/docs/Gettin
gStarted.html with and without the -g flag. I disassembled both using
llvm-dis and took the diff of them. The diff file does show differences
(see attached) and as I understand this shouldn't be the case.

I would like to know the following about this project.

   - What is the scope of this project? Are the bugs that cause these
   differences and how many of them there are, known? If not what should be
   the deliverable of the project?
   - Which are the test programs that can be used to discover these bugs?


Thanks in advance!
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9c9
< define i32 @main() local_unnamed_addr #0 {
---
> define i32 @main() local_unnamed_addr #0 !dbg !6 {
11,12c11,12
<   %puts = tail call i32 @puts(i8* getelementptr inbounds ([12 x i8], [12 x
i8]* @str, i64 0, i64 0))
<   ret i32 0
---
>   %puts = tail call i32 @puts(i8* getelementptr inbounds ([12 x i8], [12 x
i8]* @str, i64 0, i64 0)), !dbg !10
>   ret i32 0, !dbg !11
21c21,23
< !llvm.ident = !{!0}
---
> !llvm.dbg.cu = !{!0}
> !llvm.module.flags = !{!3, !4}
> !llvm.ident = !{!5}
23c25,36
< !0 = !{!"clang version 5.0.0 (trunk 299830)"}
---
> !0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer:
"clang version 5.0.0 (trunk 299830)", isOptimized: true,
runtimeVersion: 0, emissionKind: FullDebug, enums: !2)
> !1 = !DIFile(filename: "helloworld.c", directory:
"/home/himeshi/Projects/gsoc/programs")
> !2 = !{}
> !3 = !{i32 2, !"Dwarf Version", i32 4}
> !4 = !{i32 2, !"Debug Info Version", i32 3}
> !5 = !{!"clang version 5.0.0 (trunk 299830)"}
> !6 = distinct !DISubprogram(name: "main", scope: !1, file: !1,
line: 3, type: !7, isLocal: false, isDefinition: true, scopeLine: 3,
isOptimized: true, unit: !0, variables: !2)
> !7 = !DISubroutineType(types: !8)
> !8 = !{!9}
> !9 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
> !10 = !DILocation(line: 4, column: 3, scope: !6)
> !11 = !DILocation(line: 5, column: 3, scope: !6)

The llvm-dis tool will disassemble a file into LLVM IR.  Clang normally compiles
to target machine instructions.  You want to disassemble the compiled object
into machine instructions for your target.  llvm-objdump can do this, or the GNU
tools objdump and (if your target uses ELF) readelf.  We are specifically
looking for differences in the machine instructions, in the '.text'
sections.

There may be one or two existing bug reports in this area, for example bug
22344.  But the main point of the project is to find new examples, and either
reduce the test cases to reasonable size and report them as new bugs, or fix
them.

I believe the primary value of this project is that you will be forced to learn
to understand the behavior of many different parts of the compiler.  Differences
that are triggered by debug info might start anywhere.  It could begin in an IR
optimization pass that does not account for debug information properly; it could
be in some machine-function pass, which might be target-independent or
target-specific.  The nature of how LLVM encodes some debug information, by
introducing IR instructions and machine-IR instructions, means that these
differences could arise nearly anywhere.

I think the project description notes that there is a reasonably large body of
source code in the LLVM 'test-suite' project.  These programs could be
used as a starting point in the search for differences introduced by –g.  But,
probably any reasonable sized project that you can build using clang would
provide you with some examples.

Please write back if you have additional questions.  I am very interested in
having someone take on this project, although for personal reasons I do not
think I can mentor someone this summer.
--paulr

From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of Himeshi
De Silva via llvm-dev
Sent: Tuesday, April 11, 2017 7:22 AM
To: llvm-dev at lists.llvm.org
Subject: [llvm-dev] GSoC project on finding and fixing bugs with debug info

Hi,

I would like to learn more about the GSoC project to find and fix bugs that
changes the generated code when it is compiled with the debug info option
(http://llvm.org/OpenProjects.html#gsoc17). I have built llvm with clang and I
compiled the hello.c program found in http://llvm.org/docs/GettingStarted.html
with and without the -g flag. I disassembled both using llvm-dis and took the
diff of them. The diff file does show differences (see attached) and as I
understand this shouldn't be the case.

I would like to know the following about this project.

  *   What is the scope of this project? Are the bugs that cause these
differences and how many of them there are, known? If not what should be the
deliverable of the project?
  *   Which are the test programs that can be used to discover these bugs?

Thanks in advance!

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