r236059 - Debug Info: Represent local anonymous unions as anonymous unions

[Eric Christopher]

On Wed, Apr 29, 2015 at 8:20 AM Adrian Prantl <aprantl at apple.com> wrote:

>
> > On Apr 28, 2015, at 6:26 PM, David Blaikie <dblaikie at gmail.com> wrote:
> >
> >
> >
> > On Tue, Apr 28, 2015 at 4:01 PM, Adrian Prantl <aprantl at apple.com>
> wrote:
> > Author: adrian
> > Date: Tue Apr 28 18:01:24 2015
> > New Revision: 236059
> >
> > URL: http://llvm.org/viewvc/llvm-project?rev=236059&view=rev
> > Log:
> > Debug Info: Represent local anonymous unions as anonymous unions
> > in the debug info. This patch deletes a hack that emits the members
> > of local anonymous unions as local variables.
> >
> > Besides being morally wrong, the existing representation using local
> > variables breaks internal assumptions about the local variables' storage
> > size.
> >
> > Compiling
> >
> > ```
> >    void fn1() {
> >      union {
> >        int i;
> >        char c;
> >      };
> >      i = c;
> >    }
> >
> > ```
> >
> > with -g -O3 -verify will cause the verifier to fail after SROA splits
> > the 32-bit storage for the "local variable" c into two pieces because the
> > second piece is clearly outside the 8-bit range that is expected for a
> > variable of type char. Given the choice I'd rather fix the debug
> > representation than weaken the verifier.
> >
> > Debuggers generally already know how to deal with anonymous unions when
> > they are members of C++ record types, but they may have problems finding
> > the local anonymous struct members in the expression evaluator.
> >
> > Seems we're not so lucky:
> http://lab.llvm.org:8011/builders/clang-x86_64-ubuntu-gdb-75/builds/21650/steps/gdb-75-check/logs/gdb.cp__anon-union.exp
> >
> > Could you revert & I'll look into getting you a reduced test
> case/demonstration of the issue? (can you run GDB 7.5? Perhaps a simple
> test case would demonstrate the issue if you're lucky, otherwise I can
> reduce one from the failing test case)
>
> I reverted the commit in r236110. I probably won’t need a reduction — my
> guess from the log is that gdb expects a local variable to be present.
>
> My suggestion is to emit local artificial shadow variables and then weaken
> the Verifier to not verify artificial variables. In a next step, we could
> use the new debugger tuning target feature to make the artificial local
> variables and the weakened verifier a gdb-specific behavior, file a bug
> against gdb, and eventually remove it altogether.
>
>
FWIW I don't want to use the "tuning" parameters to also affect correctness.

-eric


> -- adrian
>
> >
> >
> > rdar://problem/20730771
> >
> > Modified:
> >     cfe/trunk/lib/CodeGen/CGDebugInfo.cpp
> >     cfe/trunk/test/CodeGenCXX/debug-info-anon-union-vars.cpp
> >
> > Modified: cfe/trunk/lib/CodeGen/CGDebugInfo.cpp
> > URL:
> http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/CodeGen/CGDebugInfo.cpp?rev=236059&r1=236058&r2=236059&view=diff
> >
> ==============================================================================
> > --- cfe/trunk/lib/CodeGen/CGDebugInfo.cpp (original)
> > +++ cfe/trunk/lib/CodeGen/CGDebugInfo.cpp Tue Apr 28 18:01:24 2015
> > @@ -2835,31 +2835,6 @@ void CGDebugInfo::EmitDeclare(const VarD
> >        return;
> >      } else if (isa<VariableArrayType>(VD->getType()))
> >        Expr.push_back(llvm::dwarf::DW_OP_deref);
> > -  } else if (const RecordType *RT =
> dyn_cast<RecordType>(VD->getType())) {
> > -    // If VD is an anonymous union then Storage represents value for
> > -    // all union fields.
> > -    const RecordDecl *RD = cast<RecordDecl>(RT->getDecl());
> > -    if (RD->isUnion() && RD->isAnonymousStructOrUnion()) {
> > -      for (const auto *Field : RD->fields()) {
> > -        llvm::MDType *FieldTy = getOrCreateType(Field->getType(), Unit);
> > -        StringRef FieldName = Field->getName();
> > -
> > -        // Ignore unnamed fields. Do not ignore unnamed records.
> > -        if (FieldName.empty() && !isa<RecordType>(Field->getType()))
> > -          continue;
> > -
> > -        // Use VarDecl's Tag, Scope and Line number.
> > -        auto *D = DBuilder.createLocalVariable(
> > -            Tag, Scope, FieldName, Unit, Line, FieldTy,
> > -            CGM.getLangOpts().Optimize, Flags, ArgNo);
> > -
> > -        // Insert an llvm.dbg.declare into the current block.
> > -        DBuilder.insertDeclare(Storage, D,
> DBuilder.createExpression(Expr),
> > -                               llvm::DebugLoc::get(Line, Column, Scope),
> > -                               Builder.GetInsertBlock());
> > -      }
> > -      return;
> > -    }
> >    }
> >
> >    // Create the descriptor for the variable.
> >
> > Modified: cfe/trunk/test/CodeGenCXX/debug-info-anon-union-vars.cpp
> > URL:
> http://llvm.org/viewvc/llvm-project/cfe/trunk/test/CodeGenCXX/debug-info-anon-union-vars.cpp?rev=236059&r1=236058&r2=236059&view=diff
> >
> ==============================================================================
> > --- cfe/trunk/test/CodeGenCXX/debug-info-anon-union-vars.cpp (original)
> > +++ cfe/trunk/test/CodeGenCXX/debug-info-anon-union-vars.cpp Tue Apr 28
> 18:01:24 2015
> > @@ -21,8 +21,26 @@ int test_it() {
> >    return (c == 1);
> >  }
> >
> > +// This is not necessary (and actually harmful because it breaks IR
> assumptions)
> > +// for local variables.
> > +void foo() {
> > +  union {
> > +    int i;
> > +    char c;
> > +  };
> > +  i = 8;
> > +}
> > +
> >  // CHECK: [[FILE:.*]] = !MDFile(filename:
> "{{.*}}debug-info-anon-union-vars.cpp",
> >  // CHECK: !MDGlobalVariable(name: "c",{{.*}} file: [[FILE]], line:
> 6,{{.*}} isLocal: true, isDefinition: true
> >  // CHECK: !MDGlobalVariable(name: "d",{{.*}} file: [[FILE]], line:
> 6,{{.*}} isLocal: true, isDefinition: true
> >  // CHECK: !MDGlobalVariable(name: "a",{{.*}} file: [[FILE]], line:
> 6,{{.*}} isLocal: true, isDefinition: true
> >  // CHECK: !MDGlobalVariable(name: "b",{{.*}} file: [[FILE]], line:
> 6,{{.*}} isLocal: true, isDefinition: true
> > +// CHECK: !MDLocalVariable(
> > +// CHECK-NOT: name:
> > +// CHECK: type: ![[UNION:[0-9]+]]
> > +// CHECK: ![[UNION]] = !MDCompositeType(tag: DW_TAG_union_type,
> > +// CHECK-NOT: name:
> > +// CHECK: elements
> > +// CHECK: !MDDerivedType(tag: DW_TAG_member, name: "i", scope:
> ![[UNION]],
> > +// CHECK: !MDDerivedType(tag: DW_TAG_member, name: "c", scope:
> ![[UNION]],
> >
> >
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> >
>
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