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

[Finkel, Hal J. via lldb-dev]

On 8/16/19 10:27 AM, Adrian Prantl wrote:
>
>> On Aug 15, 2019, at 2:03 PM, Ismail Bennani via lldb-dev <lldb-dev at lists.llvm.org> wrote:
>>
>> 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.
> I strongly agree that unless the code can be shared, the JIT-ed DWARF expression interpreter should be kept as simple as possible and aim to support the lion's share of DWARF expressions encountered in a typical program, but making it support 100% is a lot of effort and maintenance burden with very diminishing returns.


+1

(and, thanks for the data! I think it would be useful to support the 
things that we can easily support, but the more complicated things 
should be weighed carefully against the maintenance costs)

  -Hal


>
> -- adrian
>
>> 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
>>>
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-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory