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

[David Blaikie]

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)


>
> 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]],
>
>
> _______________________________________________
> cfe-commits mailing list
> cfe-commits at cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/cfe-commits/attachments/20150428/4b856167/attachment.html>