[PR58315] reset inlined debug vars at return-to point

[Alexandre Oliva]

On Feb 25, 2015, Jakub Jelinek <jakub@redhat.com> wrote:

> On Wed, Feb 25, 2015 at 06:17:33PM -0300, Alexandre Oliva wrote:
>> My measurements, for a not particularly unusual testcase, showed an
>> overall reduction of 63% in compile time, as indicated yesterday.  Now,
>> who should bear the burden of collecting evidence to back up the claims
>> against the change?  Are those concerns enough to hold it up?

> Can you e.g. run dwlocstat on some larger C++ binaries built without and
> with your patch?  I believe dwlocstat is supposed to count only the
> instructions where the variables or parameters are in scope, so should be
> exactly what we care about here.

Erhm...  I don't think that would cover the case you were worried about,
namely inspecting variables of an inlined function while at a statement
moved out of the function ranges.

Anyway, I've run dwlocstat and inspected the results.  There is indeed a
significant reduction in the coverage, so I looked into that.

What I found out is that the reduction is an improvement on yet another
long-standing var-tracking issue: if a function is called within a loop
and we inline it, bindings from the call in one iteration of the loop
will carry over onto the subsequent iteration until a new binding
occurs.  This accounts for all of the coverage reductions I've
investigated.

This, in turn, suggests that introducing reset stmts when variables go
out of scope even in local blocks will further reduce debug info,
although in some cases it might have the opposite effect.  E.g., if the
actual live range of a variable is scattered across multiple
non-contiguous blocks, stopping the binding from being used in
intervening blocks where the variable is not live will cause additional
entries in the location list.  I've observed this effect with the
proposed patch, too, but it removes a lot more nonsensical entries than
it adds entries to account for not covering intervening ranges by
accident.

-- 
Alexandre Oliva, freedom fighter    http://FSFLA.org/~lxoliva/
You must be the change you wish to see in the world. -- Gandhi
Be Free! -- http://FSFLA.org/   FSF Latin America board member
Free Software Evangelist|Red Hat Brasil GNU Toolchain Engineer