Abstract LLDB is a modern, high-performance debugger in the LLVM family of projects, and is built as a modular and reusable set of components on top of the Clang/LLVM foundation. It was originally developed for Mac OS X, but now supports FreeBSD and Linux as well, with ongoing work for Windows support. This presentation will provide an overview of the design of LLDB, compare it with the existing GNU debugger in the FreeBSD base system, and present the path to importing LLDB as FreeBSD's debugger. Speaker bio Ed Maste manages project development for the FreeBSD Foundation and works in an engineering support role with Robert Watson's research group at the University of Cambridge Computer Laboratory. He has been a FreeBSD committer since 2005.

1. The LLDB Debugger
in FreeBSD
Ed Maste
EuroBSDCon 2014

2. Why a New Debugger?

3. Why a New Debugger?
Upstream
FreeBSD

4. Why a New Debugger?

5. Why a New Debugger?

6. Why a New Debugger?
. . .

7. Why a New Debugger?

8. A New Debugger
• GDB
• BSDdbg
• ngdb (written in D)
• pathdb

9. A New Debugger
• GDB
• BSDdbg
• ngdb (written in D)
• pathdb
• LLDB

10. LLDB History
0
200
400
600
800
2010-052010-072010-092010-112011-012011-032011-052011-072011-092011-112012-012012-032012-052012-072012-092012-112013-012013-032013-052013-072013-092013-112014-012014-032014-052014-07
LinesofCode(000s)
Lines of code data from Black Duck Open Hub (formerly Ohloh)

11. LLDB History
0
200
400
600
800
2010-052010-072010-092010-112011-012011-032011-052011-072011-092011-112012-012012-032012-052012-072012-092012-112013-012013-032013-052013-072013-092013-112014-012014-032014-052014-07
LinesofCode(000s)
Lines of code data from Black Duck Open Hub (formerly Ohloh)

12. 0
200
400
600
800
2010-052010-072010-092010-112011-012011-032011-052011-072011-092011-112012-012012-032012-052012-072012-092012-112013-012013-032013-052013-072013-092013-112014-012014-032014-052014-07
LLDB History
LinesofCode(000s)
Lines of code data from Black Duck Open Hub (formerly Ohloh)

13. 0
200
400
600
800
2010-052010-072010-092010-112011-012011-032011-052011-072011-092011-112012-012012-032012-052012-072012-092012-112013-012013-032013-052013-072013-092013-112014-012014-032014-052014-07
LLDB History
LinesofCode(000s)
Lines of code data from Black Duck Open Hub (formerly Ohloh)

14. 0
200
400
600
800
2010-052010-072010-092010-112011-012011-032011-052011-072011-092011-112012-012012-032012-052012-072012-092012-112013-012013-032013-052013-072013-092013-112014-012014-032014-052014-07
LLDB History
LinesofCode(000s)
Lines of code data from Black Duck Open Hub (formerly Ohloh)

15. LLDB Design

19. LLDB Use

20. Syntax
GDB!
% gdb a.out
(gdb) break main
Breakpoint 1 at 0x100000f33: file main.c, line 4
(gdb) run
LLDB!
% lldb a.out
(lldb) breakpoint set --name main
Breakpoint created: 1: name = 'main', locations = 1
(lldb) process launch

21. Syntax
GDB!
(gdb) run
(gdb) r
(gdb) step
(gdb) s
(gdb) info break
(gdb) info args
&
(gdb) info args
LLDB!
(lldb) process launch
(lldb) run
(lldb) r
(lldb) thread step-in
(lldb) step
(lldb) s
(lldb) breakpoint list
(lldb) br l
(lldb) frame variable
(lldb) fr v

22. Demo

23. FreeBSD LLDB Status
Testsuite
• 311 tests run
• Failures have PRs in LLVM's bugzilla

24. FreeBSD LLDB Status
Targets
• amd64
• i386 (code in tree)
• MIPS (userland core files)
• ARM (in LLDB core, not FreeBSD/Linux)
• AArch64
• PowerPC / PowerPC64
• Sparc64
• ia64

25. FreeBSD LLDB Status
Userland core file debugging
• Works on 9.2+ and HEAD
• Testing always welcome
Userland live debugging (ptrace)
• Process launch
• Process attach by pid
• Process attach by name
• Breakpoints
• Watchpoints
• Threads

26. FreeBSD LLDB Status
Kernel core file debugging
• GSoC proof of concept
• Consider switch to ELF dumps
!
Kernel live debugging
• GSoC proof of concept
• /dev/mem
• gdb remote (serial)
• remote debugging over Ethernet (TBD)
!

27. FreeBSD LLDB Status
Remote Debugging - GDB Protocol
• Testing required
!
Debugserver
• lldb-gdbserver (llgs) in progress on Linux
!
Cross debugging
• Cross-arch and Cross-OS
• Minor issues
• Needs testing

28. FreeBSD LLDB Roadmap
• Source in contrib/llvm/tools/lldb
• FreeBSD build infrastructure in the tree
• Optional build, in /etc/src.conf
WITH_LLDB=YES
!
• Clang 3.5 import
• LLDB update
• Enable by default

29. FreeBSD LLDB Roadmap
Python
• not in base system
• currently a compile-time option
• move to a run-time option with a special bridge port

31. Front Ends
• Built-in text user interface
• Codelite
• XCode
• GDB/MI-compatible interface
• Eclipse
• Many others

34. Expression Evaluation
(lldb) expr some_variable

35. Expression Evaluation
void
$__lldb_expr(void *$__lldb_arg)
{
some_variable;
}

36. Expression Evaluation
#undef NULL
#define NULL (__null)
typedef signed __INT8_TYPE__ int8_t;
typedef unsigned __INT8_TYPE__ uint8_t;
…
void
$__lldb_expr(void *$__lldb_arg)
{
some_variable;
}

37.
void
$__lldb_expr(void *$__lldb_arg)
{
some_variable;
}
Expression Evaluation
ClangExpressionDeclMap::FindExternalVisibleDecls[2] for
'some_variable' in a 'TranslationUnit'
CEDM::FEVD[2] Searching the root namespace
CEDM::FEVD[2] Found variable some_variable,
returned static int &some_variable (original int)

38.
void
$__lldb_expr(void *$__lldb_arg)
{
some_variable;
}
Expression Evaluation
Last statement is an lvalue with type: int
T *$__lldb_expr_result_ptr = …

39.
void
$__lldb_expr(void *$__lldb_arg)
{
some_variable;
}
Expression Evaluation
Last statement is an lvalue with type: int
*(arg + 0) = *(arg + 8)

40. Expression Evaluation
; Function Attrs: nounwind
define void @"_Z12$__lldb_exprPv"(i8* %"$__lldb_arg") #0 {
%1 = alloca i8*, align 8, !clang.decl.ptr !8
store i8* %"$__lldb_arg", i8** $1, align 8
%2 = load i8* @"_ZGVZ12$__lldb_exprPvE23$__lldb_expr_result_ptr", align 1
%3 = icmp eq i8 %2, 0
br i1 %3, label %4, label %6
; <label>:4 ; preds = %0
%5 = load i32** @_ZL13some_variable, align 8
store i32* %5, i32** @"_ZZ12$__lldb_exprPvE23$__lldb_expr_result_ptr", align 8
store i8 1, i8* @"_ZGVZ12$__lldb_exprPvE23$__lldb_expr_result_ptr"
br label %6
; <label>:6 ; press = %4, %0
ret void
}

41. Expression Evaluation
; Function Attrs: nounwind
define void @"_Z12$__lldb_exprPv"(i8* %"$__lldb_arg") #0 {
%1 = getelementptr i8* %"$__lldb_arg", i32 8
%2 = bitcast i8* %1 to i32**
%3 = getelementptr i8* %"$__lldb_arg", i32 0
%4 = bitcast i8* %3 to i32**
%5 = alloca i8*, align 8, !clang.decl.ptr !8
store i8* %"$__lldb_arg", i8** %5, align 8
%6 = icmp eq i8 0, 0
br i1 %6, label %7, label %9
!
; <label>:7 ; preds = %0
%8 = load i32** %2, align 8
store i32* %8, i32** %4, align 8
br label %9
!
; <label>:9 ; preds = %7, %0
ret void
}