[lldb-dev] [RFC] Fast Conditional Breakpoints (FCB)

[Ismail Bennani via lldb-dev]

I built Clang (and LLVM) in Release Mode with Debug Info (-O2),
and got these results:

|   Dwarf Occurences   |    Occurences   |
|----------------------|-----------------|
| DW\_OP\_deref        |    1,570        |
| DW\_OP\_const        |    3,791        |
| DW\_OP\_addr         |    9,528        |
| DW\_OP\_lit          |    62,826       |
| DW\_OP\_fbreg        |    205,382      |
| DW\_OP\_piece        |    242,888      |
| DW\_OP\_stack\_value |    992,261      |
| DW\_OP\_breg         |    1,006,070    |
| DW\_OP\_reg          |    5,175,831    |
| **Total**            |  **7,700,147**  |


I could technically implement the logic to support DW_OP_reg, DW_OP_breg
and DW_OP_stack_value fairly easily (which still represents 90% of all ops).

However, DW_OP_piece is a more complex operation since it combines
several other operations, and would require more work.

This would also imply that there will 2 DWARF Expression Interpreter in
LLDB, hence twice as much code to maintain … I’ll try to see if I can
use the existing interpreter for this feature.

Ismail

> On Aug 14, 2019, at 3:42 PM, Finkel, Hal J. <hfinkel at anl.gov> wrote:
> 
> 
> On 8/14/19 3:52 PM, Ismail Bennani via lldb-dev wrote:
>> Hi everyone,
>> 
>> I’m Ismail, a compiler engineer intern at Apple. As a part of my internship,
>> I'm adding Fast Conditional Breakpoints to LLDB, using code patching.
>> 
>> ...
>> 
>> Since all the registers are already mapped to a structure, I should
>> be able to support more __DWARF Operations__ in the future.
>> 
>> After collecting some metrics on the __Clang__ binary, built at __-O0__,
>> the debug info shows that __99%__ of the most used DWARF Operations are :
>> 
>> |DWARF Operation|         Occurrences       |
>> |---------------|---------------------------|
>> |DW\_OP_fbreg   |         2 114 612         |
>> |DW\_OP_reg     |           820 548         |
>> |DW\_OP_constu  |           267 450         |
>> |DW\_OP_addr    |            17 370         |
>> 
>> |   __Top 4__   | __3 219 980 Occurrences__ |
>> |---------------|---------------------------|
>> |   __Total__   | __3 236 859 Occurrences__ |
>> 
>> Those 4 operations are the one that I'll support for now.
>> To support more complex expressions, we would need to JIT-compile
>> a DWARF expression interpreter.
> 
> 
> First, this all sounds really useful.
> 
> Out of curiosity, how do these statistics change if you compile Clang 
> with -O1? Many of my users need to debug slightly-optimized code.
> 
>  -Hal
> 
> 
> -- 
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory
>