[PATCH] Avoid empty .debug_loc entries and lists

[Duncan P. N. Exon Smith]

> On 2015 Jun 4, at 16:09, David Blaikie <dblaikie at gmail.com> wrote:
> 
> Judging by the code, does this cause us not to create a variable if it has no locations? That's probably not quite right - even if we have no locations, it's probably a good idea/correct to describe the variable so shadowing, etc, works correctly (if someone tries to print the variable the compiler still finds the right variable but just reports that it doesn't know the location)

So the code here already has an early return if there are no ranges:

    for (const auto &I : DbgValues) {
      InlinedVariable IV = I.first;
      if (Processed.count(IV))
        continue;

      // Instruction ranges, specifying where IV is accessible.
      const auto &Ranges = I.second;
      if (Ranges.empty())
        continue;

My patch effectively hits this `continue` more often.  The difference
is cases where we *thought* we knew how to emit ranges, but in fact
didn't know how.

I suppose I assumed the intended behaviour was "don't emit the
variable if we don't have any ranges for it" since that's what was
going on, but are you saying this `continue` is a bug?  (Or am I
totally missing your point?)

> On 2015 Jun 4, at 16:13, Adrian Prantl <aprantl at apple.com> wrote:
> 
> Even worse, when it is a function argument, omitting the variable will effectively change the function signature. (Yes, there is also the subroutine type...).

No, there's a separate loop that guarantees all the function arguments
get created.

    // Collect info for variables that were optimized out.
    for (const DILocalVariable *DV : SP->getVariables()) {
      if (!Processed.insert(InlinedVariable(DV, nullptr)).second)
        continue;
      if (LexicalScope *Scope = LScopes.findLexicalScope(DV->getScope())) {
        ensureAbstractVariableIsCreatedIfScoped(InlinedVariable(DV, nullptr),
                                                Scope->getScopeNode());
        ConcreteVariables.push_back(make_unique<DbgVariable>(
            DV, /* IA */ nullptr, /* Expr */ nullptr, this));
        InfoHolder.addScopeVariable(Scope, ConcreteVariables.back().get());
      }
    }

I haven't touched this.