Re: [RFA 08/15] Remove readin and compunit_symtab fields from psymtab

[Simon Marchi <simark at simark dot ca>]

On 2018-10-05 9:24 p.m., Tom Tromey wrote:
>>>>>> "Tom" == Tom Tromey <tom@tromey.com> writes:
> 
>>>>>> "Simon" == Simon Marchi <simark@simark.ca> writes:
> Simon> As Pedro pointed out a few times, unordered_map is an hash map with open
> Simon> hashing.  So we change what was previously a single pointer dereference
> Simon> to a lookup in a hash table, followed with a linear search in a linked
> Simon> list.  If this map is looked up very frequently, maybe we should consider
> Simon> using an htab_t instead?  Some profiling data would help, but it seems
> Simon> like psymtab_read_in_p is called quite a bit when looking up symbols...
> 
> [...]
> 
> Tom> I still haven't tried any performance measurements.
> Tom> I'm not quite sure what to try.
> 
> I ran the gdb.perf tests and on some tests, it seems that the results
> are within the noise; but on backtrace (2048) and skip-function-3000 (4000)
> gdb is about 4% slower.  I don't know if this is enough to worry about.
> 
> Tom
> 

That sounds like a reasonable trade-off in exchange for the feature of decoupling
psymtabs from objfiles.  Perhaps we can put it on the list of "candidates for htab_t"
(could be a comment next to the field), for when we finally bring the C++ wrappers
from gcc :).

Simon