Re: [PATCH 3/6] gdbsupport: use dynamic partitioning in gdb::parallel_for_each

[Simon Marchi <simon.marchi@polymtl.ca>]

On 6/13/25 3:56 PM, Simon Marchi wrote:
> On 6/13/25 3:22 PM, Simon Marchi wrote:
>> Another idea I had (but didn't try) was to make lambdas receive a
>> "magic" range object, like athis:
>>
>>   [&] (gdb::dynamic_range<dwarf2_per_cu *> range)
>>     {
>>       for (dwarf2_per_cu *cu : range)
>>         process_unit (cu);
>>     }
>>
>> "range" would get work items from the work queue in batches, but yield
>> one at a time.  It would reach its end when the work queue is empty.
>> It will be a bit of work to implement, but it would have the advantage
>> that any per-worker state could be right there in the lambda, like we
>> have today.
> 
> Ah, I remembered why I didn't do this.  It wouldn't play well with how
> minimal_symbol_reader::install() currently uses gdb::parallel_for_each.
> Currently, it receives one contiguous range [start,end).  After having
> computed the demangled names and hashes for the whole range, each worker
> locks a mutex and installs the names in a shared hash table.
> 
> In my patch, it's similar, but each worker installs the names in the
> share hash table at the end of each small range it receives.
> 
> With my "magic" iterator idea, it wouldn't be clear when and how to
> install the names in the shared hash table.

One way to make this work would be to make the iterator yield batches,
so you would have two levels of iteration:

  [&] (gdb::dynamic_range<minimal_symbol *, 1000> range)
    {
      for (gdb::batch<minimal_symbol *> batch : range)
        {
	  for (minimal_symbol *msym : batch)
	    {
	      // compute demangled name and hash
	    }

	  // lock mutex

	  for (minimal_symbol *msym : batch)
	    {
	      // install msym in the shared hash table
	    }
	}
    }

Let me know if you see an easier solution to this.

Simon