[PATCH] Refactor debug info lexical block generation

[Aboud, Amjad]

This would work just fine for function static variable.
In fact, this was my first thought when I tried to fix this specific problem.

But it will not solve the other two cases, the typedef and the record.
Do you have any though on how to use same design to solve the type cases?

For example:

int foo(bool b, int  x) {
  if (b) {
    typedef short A;
    A a = x;
   return a;
  } else {
    typedef float A;
    A a = x;
   return a;
 }
}

Regards,
Amjad

From: David Blaikie [mailto:dblaikie at gmail.com]
Sent: Wednesday, May 27, 2015 21:42
To: Aboud, Amjad
Cc: reviews+D9960+public+41c324f590718ccc at reviews.llvm.org; Eric Christopher; Benjamin Kramer; llvm-commits at cs.uiuc.edu; Robinson, Paul
Subject: Re: [PATCH] Refactor debug info lexical block generation



On Wed, May 27, 2015 at 11:32 AM, Aboud, Amjad <amjad.aboud at intel.com<mailto:amjad.aboud at intel.com>> wrote:
I am not sure what is imported entry is, and if it could be located inside a lexical block.

When you create an imported entry you call the “getOrCreateContextDIE”, this function will check for, Compile-Unit, File, Namespace, Type, Subprogram context.
If none match, it calls “getDIE” function:

DIE *DwarfUnit::getDIE(const DINode *D) const {
  if (isShareableAcrossCUs(D))
    return DU->getDIE(D);
  return MDNodeToDieMap.lookup(D);
}

Maybe imported entries fit with the first condition check of “isShareableAcrossCUs”, but the function static variables will have a lexical block context and when we will look for this context in the “MDNodeToDieMap” map it will not be there, because we do not map lexical block MDNode to lexical block DIE.

Take a look in DwarfCompileUnit::cosntructScopeDIE, around half way through, you'll find this code:
    unsigned ChildScopeCount;

    // We create children here when we know the scope DIE is not going to be
    // null and the children will be added to the scope DIE.
    createScopeChildrenDIE(Scope, Children, &ChildScopeCount);

    // Skip imported directives in gmlt-like data.
    if (!includeMinimalInlineScopes()) {
      // There is no need to emit empty lexical block DIE.
      for (const auto &E : DD->findImportedEntitiesForScope(DS))
        Children.push_back(
            constructImportedEntityDIE(cast<DIImportedEntity>(E.second)));
    }

<<It seems to me like the code there that the logic for static variables should go here too - the same way imported entities are dealt with. Then, if we have any static variables, imported entities, we create this scope - otherwise we put it in the parent>>
    // If there are only other scopes as children, put them directly in the
    // parent instead, as this scope would serve no purpose.
    if (Children.size() == ChildScopeCount) {
      FinalChildren.insert(FinalChildren.end(),
                           std::make_move_iterator(Children.begin()),
                           std::make_move_iterator(Children.end()));
      return;
    }
    ScopeDIE = constructLexicalScopeDIE(Scope);
    assert(ScopeDIE && "Scope DIE should not be null.");



Also, right now, we decide if to create the lexical block DIE or not depends on the local variable children only, as the function static variables DIE and the type DIEs were not attached yet to the lexical block DIE.
This, is why I also suggest to create all lexical blocks at first and once all its children are created, we can revisit this lexical block FIE and decide if it worth collapsing with parent.

Regards,
Amjad


From: David Blaikie [mailto:dblaikie at gmail.com<mailto:dblaikie at gmail.com>]
Sent: Wednesday, May 27, 2015 21:14
To: reviews+D9960+public+41c324f590718ccc at reviews.llvm.org<mailto:reviews%2BD9960%2Bpublic%2B41c324f590718ccc at reviews.llvm.org>
Cc: Aboud, Amjad; Eric Christopher; Benjamin Kramer; llvm-commits at cs.uiuc.edu<mailto:llvm-commits at cs.uiuc.edu>; Robinson, Paul
Subject: Re: [PATCH] Refactor debug info lexical block generation

Perhaps it'd be good to have a general design discussion here, rather than in the form of line-by-line code review. A few broad issues have been brought up by both Paul and Frederic that sound reasonable to me, though I've not looked at the code in detail yet.


Is there any reason this wouldn't be implemented the same way as the imported entity work? Where we check for imported entities at scope construction time to decide whether to create that scope or not.

On Sun, May 24, 2015 at 9:00 AM, Amjad Aboud <amjad.aboud at intel.com<mailto:amjad.aboud at intel.com>> wrote:
Hi echristo, bkramer, dblaikie,

This patch is needed for resolving Bug 19238.
It is the first part which will be followed by D9758.

The idea is to create all debug info lexical block DIEs and add them to the DIScope->DIE map.
Latter, just before emitting the DIEs, we will search the DIE tree and collapse all redundant (useless) DIE lexical blocks.

REPOSITORY
  rL LLVM

http://reviews.llvm.org/D9960

Files:
  include/llvm/CodeGen/DIE.h
  lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp
  lib/CodeGen/AsmPrinter/DwarfDebug.cpp
  lib/CodeGen/AsmPrinter/DwarfDebug.h
  test/DebugInfo/useless_lexical_scope.ll

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