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

[Finkel, Hal J. via lldb-dev]

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