[PATCH] Debug Info: Represent local anonymous unions as anonymous unions.

[Adrian Prantl]

Hi echristo, dblaikie,

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.

Why? 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.

Eric, do you know whether the local variable trick is necessary for GDB compatibility?

REPOSITORY
  rL LLVM

http://reviews.llvm.org/D9329

Files:
  lib/CodeGen/CGDebugInfo.cpp
  test/CodeGenCXX/debug-info-anon-union-vars.cpp

Index: lib/CodeGen/CGDebugInfo.cpp
===================================================================
--- lib/CodeGen/CGDebugInfo.cpp
+++ lib/CodeGen/CGDebugInfo.cpp
@@ -2835,31 +2835,6 @@
       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.
Index: test/CodeGenCXX/debug-info-anon-union-vars.cpp
===================================================================
--- test/CodeGenCXX/debug-info-anon-union-vars.cpp
+++ test/CodeGenCXX/debug-info-anon-union-vars.cpp
@@ -21,8 +21,26 @@
   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]],

EMAIL PREFERENCES
  http://reviews.llvm.org/settings/panel/emailpreferences/
-------------- next part --------------
A non-text attachment was scrubbed...
Name: D9329.24581.patch
Type: text/x-patch
Size: 2825 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/cfe-commits/attachments/20150428/5bf2433d/attachment.bin>