[llvm-dev] RFC: Supporting macros in LLVM debug info

Aboud, Amjad via llvm-dev
2015-11-03T18:19:19Z

> Not necessarily, if we kept the macros in order in the list of macros
attached to the CU, which I imagine we would.
OK, now I understand what you are aiming for. I really do not favor one on the
other.
But, can you explain what is the advantage of the parent approach over the
children approach?
If any, the children approach seems to be the one reduces the LLVM IR size, is
not it?

Regards,
Amjad

From: David Blaikie [mailto:dblaikie at gmail.com]
Sent: Tuesday, November 03, 2015 18:46
To: Aboud, Amjad
Cc: llvm-dev at lists.llvm.org
Subject: Re: [llvm-dev] RFC: Supporting macros in LLVM debug info



On Tue, Nov 3, 2015 at 12:16 AM, Aboud, Amjad <amjad.aboud at
intel.com<mailto:amjad.aboud at intel.com>>
wrote:
> Do we really need to touch the AST? Or would it be reasonable to wire up
the CGDebugInfo directly to the PPCallbacks, if it isn't already? (perhaps
it is already wired up for other reasons?)
This sound as a good idea, I will check that approach.
PPCallbacks is only an interface, has nothing connected to it, but we will
create a new class, which implement PPCallbacks, for macros.

Right - I was wondering if CGDebugInfo already implemented PPCallbacks or was
otherwise being notified of PPCallback related things, possibly through a layer
or two of indirection.

So we can connect whatever we want to that class.
The only drawback with this approach, is that we can test the frontend using the
generated  LLVM IR, i.e. the whole path, instead of having two tests, AST for
testing the parser, and LLVM IR for testing the Sema.

We don't usually do direct AST tests in Clang for debug info (or for many
things, really) - we just do source -> llvm IR anyway, so that's nothing
out of the ordinary.


> I wonder if it'd be better to use a parent chain style approach
(DIMacro has a DIMacroFile it refers to, each DIMacroFile has another one that
it refers to, up to null)?
> (does it ever make sense/need to have a DIMacroFile without any macros in
it? I assume not?)
First, it seems that GCC does emit MacroFile that has no macros inside (I
understand that it might not be useful, but I am not sure if we should ignore
that or not).

Yeah, that's weird - I'd sort of be inclined to skip it until we know
what it's useful for.

Second, I assume that you are suggesting the parent chain style instead to the
current children style, right?

Correct

In this case, won’t it make the debug emitter code much complicated to figure
out the DFS tree,

I don't quite imagine it would be more complicated - we would just be
building the file parent chain as we go, and keeping the current macro file
around to be used as the parent to any macros we create.

which should be emitted for the macros, not mentioning the macro order which
will be lost?

Not necessarily, if we kept the macros in order in the list of macros attached
to the CU, which I imagine we would.

Also, remember that the command line macros have no DIMacroFile parent.

Fair - they could have the null parent, potentially.

However, if you meant to use the parent chain in addition to the children list,
then what extra information it will give us?


>Might be good to start with dwarfdump support - seems useful regardless of
anything else?
I agree, and in fact, I already have this code implemented, will upload it for
review soon.

Cool


Thanks,
Amjad

From: David Blaikie [mailto:dblaikie at gmail.com<mailto:dblaikie at
gmail.com>]
Sent: Tuesday, November 03, 2015 00:32
To: Aboud, Amjad
Cc: llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] RFC: Supporting macros in LLVM debug info



On Wed, Oct 28, 2015 at 7:56 AM, Aboud, Amjad via llvm-dev <llvm-dev at
lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
Hi,
I would like to implement macro debug info support in LLVM.
Below you will find 4 parts:
1.      Background  on what does it mean to debug macros.
2.      A brief explanation on how to represent macro debug info in DWARF 4.0.
3.      The suggested design.
4.      A full example: Source -> AST -> LLVM IR -> DWARF.

Feel free to skip first two parts if you think you know the background.
Please, let me know if you have any comment or feedback on this approach.

Thanks,
Amjad

[Background]
There are two kind of macro definition:
1. Simple macro definition, e.g.  #define M1 Value1
2. Function macro definition, e.g. #define M2(x, y)  (x) + (y)
Macro scope starts with the "#define" directive and ends with
"#undef" directive.

GDB supports debugging macros. This means, it can evaluate the macro expression
for all macros, which have a scope that interleaves with the current breakpoint.
For example:
GDB command: print M2(3, 5)
GDB Result: 8

GDB can evaluate the macro expression based on the ".debug_macroinfo"
section (DWARF 4.0).

[DWARF 4.0 ".debug_macroinfo" section]
In this section there are 4 kinds of entries
1.      DW_MACROINFO_define
2.      DW_MACROINFO_undef
3.      DW_MACROINFO_start_file
4.      DW_MACROINFO_end_file

Note: There is a 5th kind of entry for vendor specific macro information, that
we do not need to support.

The first two entries contain information about the line number where the macro
is defined/undefined, and a null terminated string, which contain the macro name
(followed by the replacement value in case of a definition, or a list of
parameters then the replacement value in case of function macro definition).
The third entry contains information about the line where the file was included
followed by the file id (an offset into the files table in the debug line
section).
The fourth entry contains nothing, and it just close the previous entry of third
kind (start_file) .

Macro definition and file including entries must appear at the same order as
they appear in the source file. Where all macro entries between
"start_file" and "end_file" entries represent macros appears
directly/indirectly in the included file.

Special cases:
1.      The main source file should be the first "start_file" entry in
the sequence, and should have line number "0".
2.      Command line/Compiler definitions must also have line number
"0" but must appear before the first "start_file" entry.
3.      Command line include files, must also have line number "0" but
will appear straight after the "start_file" of the main source.

[Design]
To support macros the following components need to be modified: Clang, LLVM IR,
Dwarf Debug emitter.

In clang, we need to handle these source directives:
1.      #define
2.      #undef
3.      #include
The idea is to make a use of "PPCallbacks" class, which allows
preprocessor to notify the parser each time one of the above directives occurs.
These are the callbacks that should be implemented:
"MacroDefined", "MacroUndefined", "FileChanged",
and "InclusionDirective".

AST will be extended to support two new DECL types: "MacroDecl" and
"FileIncludeDecl".

Do we really need to touch the AST? Or would it be reasonable to wire up the
CGDebugInfo directly to the PPCallbacks, if it isn't already? (perhaps it is
already wired up for other reasons?)

Where "FileIncludeDecl" AST might contain other
"FileIncludeDecl"/"MacroDecl" ASTs.
These two new AST DECLs are not part of TranslationUnitDecl and are handled
separately (see AST example below).

In the LLVM IR, metadata debug info will be extended to support new DIs as well:
"DIMacro", "DIFileInclude", and "MacroNode".
The last, is needed as we cannot use DINode as a base class of
"DIMacro" and DIFileInclude" nodes.

DIMacro will contain:
•        type (definition/undefinition).
•        line number (interger).
•        name (null terminated string).
•        replacement value  (null terminated string - optional).

DIFileMacro will contain:
•        line number (interger).
•        file (DIFile).
•        macro list (MacroNodeArray) - optional.

I wonder if it'd be better to use a parent chain style approach (DIMacro has
a DIMacroFile it refers to, each DIMacroFile has another one that it refers to,
up to null)?
(does it ever make sense/need to have a DIMacroFile without any macros in it? I
assume not?)


Might be good to start with dwarfdump support - seems useful regardless of
anything else?


In addition, the DICompileUnit will contain a new optional field of macro list
of type (MacroNodeArray).

Finally, I assume that macro support should be disabled by default, and there
should be a flag to enable this feature. I would say that we should introduce a
new specific flag, e.g. "-gmacro", that could be used with
"-g".

[Example]
Here is an example that demonstrate the macro support from
Source->AST->LLVM IR->DWARF.

Source
========================================================mainfile.c:
--------------------------------------------------------------------------------------
1. #define M1 Value1
2. #include "myfile.h"
3. #define M2( x , y)   ( (x)    + (y)  * Value2)
--------------------------------------------------------------------------------------

myfile.h:
--------------------------------------------------------------------------------------
1.
2.
3.
4. #undef M1
5. #define M1 NewValue1
--------------------------------------------------------------------------------------

myfile2.h:
--------------------------------------------------------------------------------------
1. #define M4 Value4
--------------------------------------------------------------------------------------
========================================================
Command line:
clang -c -g -gmacro -O0 -DM3=Value3 -include myfile2.h mainfile.c


AST
========================================================MacroDecl 0xd6c5c0
<<invalid sloc>> <invalid sloc> __llvm__ defined
MacroDecl 0xd6c618 <<invalid sloc>> <invalid sloc> __clang__
defined

… <More compiler macros> …

MacroDecl 0x11c01b0 <<invalid sloc>> <invalid sloc> M3 defined
FileIncludeDecl 0x11c0208 <mainfile.c:1:1> col:1
|-FileIncludeDecl 0x11c0238 <myfile2.h:1:1> col:1
| `-MacroDecl 0x11c0268 <<invalid sloc>> <invalid sloc> M4
defined
|-MacroDecl 0x11c02c0 <mainfile.c:1:9> col:9 M1 defined
|-FileIncludeDecl 0x11c0318 <myfile.h:1:1> col:1
| |-MacroDecl 0x11c0348 <line:4:8> col:8 M1 undefined
| `-MacroDecl 0x11c03a0 <line:5:9> col:9 M1 defined
`-MacroDecl 0x11c03f8 <mainfile.c:3:9> col:9 M2 defined
TranslationUnitDecl 0xd6c078 <<invalid sloc>> <invalid sloc>
|-TypedefDecl 0xd6c330 <<invalid sloc>> <invalid sloc>
implicit __int128_t '__int128'
|-TypedefDecl 0xd6c370 <<invalid sloc>> <invalid sloc>
implicit __uint128_t 'unsigned __int128'
|-TypedefDecl 0xd6c3c8 <<invalid sloc>> <invalid sloc>
implicit __builtin_ms_va_list 'char *'
`-TypedefDecl 0xd6c590 <<invalid sloc>> <invalid sloc>
implicit __builtin_va_list 'struct __va_list_tag [1]'
========================================================

LLVM IR
========================================================target datalayout =
"e-m:e-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-pc-linux"

!llvm.dbg.cu<http://llvm.dbg.cu> = !{!0}
!llvm.module.flags = !{!327}
!llvm.ident = !{!328}

!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer:
"clang version 3.8.0 (trunk 251321)", isOptimized: false,
runtimeVersion: 0, emissionKind: 1, enums: !2, macros: !3)
!1 = !DIFile(filename: "mainfile.c", directory: "/")
!2 = !{}
!3 = !{!4, !5, … <More compiler macros> … , !312, !313}
!4 = !DIMacro(macro type: DW_MACINFO_define, name: "__llvm__", value:
!"1")
!5 = !DIMacro(macro type: DW_MACINFO_define, name: "__clang__", value:
!"1")

… <More compiler macros> …

!312 = !DIMacro(macro type: DW_MACINFO_define, name: "M3", value:
!"Value3")
!313 = !DIFileInclude(file: !314, nodes: !315)
!314 = !DIFile(filename: "mainfile.c", directory: "/")
!315 = !{!316, !320, !321, !326}
!316 = !DIFileInclude(file: !317, nodes: !318)
!317 = !DIFile(filename: "myfile2.h", directory: "/")
!318 = !{!319}
!319 = !DIMacro(macro type: DW_MACINFO_define, name: "M4", value:
!"Value4")
!320 = !DIMacro(macro type: DW_MACINFO_define, name: "M1", line: 1,
value: !"Value1")
!321 = !DIFileInclude(line: 2, file: !322, nodes: !323)
!322 = !DIFile(filename: "myfile.h", directory: "/")
!323 = !{!324, !325}
!324 = !DIMacro(macro type: DW_MACINFO_undef, name: "M1", line: 4)
!325 = !DIMacro(macro type: DW_MACINFO_define, name: "M1", line: 5,
value: !"NewValue1")
!326 = !DIMacro(macro type: DW_MACINFO_define, name: "M2(x,y)", line:
3, value: !"( (x) + (y) * Value2)")
!327 = !{i32 2, !"Debug Info Version", i32 3}
!328 = !{!"clang version 3.8.0 (trunk 251321)"}
========================================================

DWARF
========================================================Command line:
llvm-dwarfdump.exe -debug-dump=macro mainfile.o
--------------------------------------------------------------------------------------
mainfile3.o:  file format ELF64-x86-64

.debug_macinfo contents:
DW_MACINFO_define - lineno: 0 macro: __llvm__ 1
DW_MACINFO_define - lineno: 0 macro: __clang__ 1

… <More compiler macros> …

DW_MACINFO_define - lineno: 0 macro: M3 Value3
DW_MACINFO_start_file - lineno: 0 filenum: 1
  DW_MACINFO_start_file - lineno: 0 filenum: 2
    DW_MACINFO_define - lineno: 0 macro: M4 Value4
  DW_MACINFO_end_file
  DW_MACINFO_define - lineno: 1 macro: M1 Value1
  DW_MACINFO_start_file - lineno: 2 filenum: 3
    DW_MACINFO_undef - lineno: 4 macro: M1
    DW_MACINFO_define - lineno: 5 macro: M1 NewValue1
  DW_MACINFO_end_file
  DW_MACINFO_define - lineno: 3 macro: M2(x,y) ( (x) + (y) * Value2)
DW_MACINFO_end_file

--------------------------------------------------------------------------------------
Command line: llvm-dwarfdump.exe -debug-dump=line mainfile.o
--------------------------------------------------------------------------------------
.debug_line contents:

… <Other line table Info> …

                Dir  Mod Time   File Len   File Name
                ---- ---------- ---------- ---------------------------
file_names[  1]    1 0x00000000 0x00000000 mainfile.c
file_names[  2]    1 0x00000000 0x00000000 myfile2.h
file_names[  3]    1 0x00000000 0x00000000 myfile.h
========================================================

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