Adding debug info to support Fortran (part 3)

[schweitz]

Summary

3. Fortran COMMON Block

COMMON blocks are a feature of Fortran that has no direct analog in C languages, but they are similar to data sections in assembly language programming. A COMMON block is a named area of memory that holds a collection of variables. Fortran subprograms may map the COMMON block memory area to their own, possibly distinct, non-empty list of variables. A Fortran COMMON block might look like the following example.

COMMON /ALPHA/ I, J

For this construct, the compiler generates a new scope-like DI construct (!DICommonBlock) into which variables (see I, J above) can be placed. As the common block implies a range of storage with global lifetime, the !DICommonBlock refers to a !DIGlobalVariable. The Fortran variable that comprise the COMMON block are also linked via metadata to offsets within the global variable that stands for the entire common block.

@alpha_ = common global %alphabytes_ zeroinitializer, align 64, !dbg !27, !dbg !30, !dbg !33
!14 = distinct !DISubprogram(…)
!20 = distinct !DICommonBlock(scope: !14, declaration: !25, name: "alpha")
!25 = distinct !DIGlobalVariable(scope: !20, name: "common alpha", type: !24)
!27 = !DIGlobalVariableExpression(var: !25, expr: !DIExpression())
!29 = distinct !DIGlobalVariable(scope: !20, name: "i", file: !3, type: !28)
!30 = !DIGlobalVariableExpression(var: !29, expr: !DIExpression())
!31 = distinct !DIGlobalVariable(scope: !20, name: "j", file: !3, type: !28)
!32 = !DIExpression(DW_OP_plus_uconst, 4)
!33 = !DIGlobalVariableExpression(var: !31, expr: !32)

The DWARF generated for this is as follows.

DW_TAG_common_block:
DW_AT_name: alpha
DW_AT_location: @alpha_+0
DW_TAG_variable:
DW_AT_name: common alpha
DW_AT_type: array of 8 bytes
DW_AT_location: @alpha_+0
DW_TAG_variable:
DW_AT_name: i
DW_AT_type: integer*4
DW_AT_location: @Alpha+0
DW_TAG_variable:
DW_AT_name: j
DW_AT_type: integer*4
DW_AT_location: @Alpha+4

[JohnReagan]

I don't understand the need for the "common alpha" global variable. For VMS Fortran on Itanium (not LLVM), we generate the following DWARF for

COMMON /ALPHA/ I,J

0x0000008f:     DW_TAG_common_block [5] *
                  DW_AT_name [DW_FORM_string]	("ALPHA")
                  DW_AT_location [DW_FORM_block1]	(<0x09> 03 00 00 00 00 00 00 00 00 )

0x000000a0:       DW_TAG_base_type [6]  
                    DW_AT_name [DW_FORM_string]	("integer*4")
                    DW_AT_encoding [DW_FORM_data1]	(0x05)
                    DW_AT_byte_size [DW_FORM_udata]	(4)

0x000000ad:       DW_TAG_variable [4]  
                    DW_AT_name [DW_FORM_string]	("I")
                    DW_AT_type [DW_FORM_ref4]	(cu + 0x00a0 => {0x000000a0})
                    DW_AT_location [DW_FORM_block1]	(<0x09> 03 00 00 00 00 00 00 00 00 )

0x000000be:       DW_TAG_variable [4]  
                    DW_AT_name [DW_FORM_string]	("J")
                    DW_AT_type [DW_FORM_ref4]	(cu + 0x00a0 => {0x000000a0})
                    DW_AT_location [DW_FORM_block1]	(<0x09> 03 00 00 00 00 00 00 00 00 )

0x000000cf:       NULL

0x000000d0:     DW_TAG_common_inclusion [7]  
                  DW_AT_common_reference [DW_FORM_ref4]	(cu + 0x008f => {0x0000008f})

with 3 DIRLSB64s relocations for the DW_AT_locations

[I don't have an Itanium obdump so forgive our platform-specific format]

                   Relocation Entry 3.
00000AE8 00000048    Sec. 21. offset affected:   0000000000000098                                       rela$pq_r_offset
                     Relocation Entry Info:      0000000700000027                                       rela$q_r_info
00000AF0 00000050      Relocation Type:                  00000027          DIR64LSB                     rela$l_r_type
00000AF4 00000054      Sec 12. Sym Index:                00000007       7.                              rela$l_r_sym
                       Symbol Bound to Section:          00000008       8. "ALPHA"
00000AF8 00000058    Relocation addend:          0000000000000000                                       rela$q_r_addend
                   Relocation Entry 4.
00000B00 00000060    Sec. 21. offset affected:   00000000000000B6                                       rela$pq_r_offset
                     Relocation Entry Info:      0000000700000027                                       rela$q_r_info
00000B08 00000068      Relocation Type:                  00000027          DIR64LSB                     rela$l_r_type
00000B0C 0000006C      Sec 12. Sym Index:                00000007       7.                              rela$l_r_sym
                       Symbol Bound to Section:          00000008       8. "ALPHA"
00000B10 00000070    Relocation addend:          0000000000000000                                       rela$q_r_addend
                   Relocation Entry 5.
00000B18 00000078    Sec. 21. offset affected:   00000000000000C7                                       rela$pq_r_offset
                     Relocation Entry Info:      0000000700000027                                       rela$q_r_info
00000B20 00000080      Relocation Type:                  00000027          DIR64LSB                     rela$l_r_type
00000B24 00000084      Sec 12. Sym Index:                00000007       7.                              rela$l_r_sym
                       Symbol Bound to Section:          00000008       8. "ALPHA"
00000B28 00000088    Relocation addend:          0000000000000004                                       rela$q_r_addend

[aprantl]

This will also need assembler roundtrip tests in tests/Assembler.

lib/IR/LLVMContextImpl.h

805

Why does a common block have an alignment?

[schweitz]

In D54327#1293281, @JohnReagan wrote:

I don't understand the need for the "common alpha" global variable.

It's not strictly required to have it explicitly named. Our implementation allocates a COMMON block as an "untyped" block of memory and each subprogram can, of course, remap the storage units to its own list of variable names. The DW_TAG_common_block does require a DW_AT_location, and the !DIGlobalVariable fulfills that requirement.

[schweitz]

lib/IR/LLVMContextImpl.h

805

Good point. It seems like the alignment of the global storage would suffice.

[dblaikie]

include/llvm/IR/DebugInfoMetadata.h

2619

Probably skip this as it's the implicit default?

(I see this seems to have been defaulted all over the Metadata hierarchy - so far as I can tell, still unnecessary in all those places & no need to propagate this around further?)

[JohnReagan]

In D54327#1293577, @schweitz wrote:

In D54327#1293281, @JohnReagan wrote:

I don't understand the need for the "common alpha" global variable.

It's not strictly required to have it explicitly named. Our implementation allocates a COMMON block as an "untyped" block of memory and each subprogram can, of course, remap the storage units to its own list of variable names. The DW_TAG_common_block does require a DW_AT_location, and the !DIGlobalVariable fulfills that requirement.

Ah, yes I see. I forgot that for us, we have a platform-specific ELF section flag for "overlay" sections which are common blocks. We don't need the extra hidden weak variable to be the intermediate allocator.

[schweitz]

Remove alignment (not needed for DW_TAG_common_block).
Remove default dtor. (not needed)
Add both an llc test and a llvm-as | llvm-dis loop test.

[schweitz]

I believe all the comments have been addressed. Is there anything else required on our end with this one?

[probinson]

I am curious how debug info for common blocks plays with LTO. I think we would not want different common-block descriptions to be de-duplicated just because the global name was the same.

[schweitz]

In D54327#1310565, @probinson wrote:

I am curious how debug info for common blocks plays with LTO. I think we would not want different common-block descriptions to be de-duplicated just because the global name was the same.

Our existing compilers do not make use of LLVM's LTO. Is there a suggestion on how to correctly deal with LTO's de-duplication?

[aprantl]

In D54327#1422996, @schweitz wrote:

In D54327#1310565, @probinson wrote:

I am curious how debug info for common blocks plays with LTO. I think we would not want different common-block descriptions to be de-duplicated just because the global name was the same.

Our existing compilers do not make use of LLVM's LTO. Is there a suggestion on how to correctly deal with LTO's de-duplication?

You can simulate the relevant effect of LTO by producing two .ll files and running llvm-link on them. Alternatively you can also run llvm-lto/llvm-lto2 on them for a more complete simulation.

[schweitz]

In D54327#1425054, @aprantl wrote:

You can simulate the relevant effect of LTO by producing two .ll files and running llvm-link on them. Alternatively you can also run llvm-lto/llvm-lto2 on them for a more complete simulation.

We admit that we're not entirely sure what we're looking for exactly, and we are not familiar with llvm-lto. However, we conducted an experiment using llvm-lto as follows.

1. We wrote a Fortran program using separate source files. 2 of these files contained subroutines that have COMMON /foo/ specifications. These COMMON blocks define a series of variables under different names. (For example, variable a1 and a2 map to the same memory location in the same /foo/ COMMON block but each is from a different SUBROUTINE scopes.)

2. We ran llvm-lto on the resulting .bc files with -O3 and -O0.

3. Debugging the resulting program, we see that the variables in one subroutine are present and can be queried. The variables in the second subroutine are present but have no location information. We see the same behavior regardless of the optimization level.

This is a different result than we obtain when we do not manually intervene with LTO. When compiling with our Fortran compiler and no LTO, we can set breakpoints in both of the aforementioned routines and all the expected COMMON variables are present, can be queried as to value, and have location information in the DWARF.

We are not clear if this is a problem versus working as intended, and if it is the former what, if anything, we ought to do about it.

[schweitz]

Updated the diff ahead in time.

[schweitz]

Just wondering if there are any more to-dos with this one. Thanks in advance.

[aprantl]

I think @probinson had some questions about common blocks and LTO?

[aprantl]

Mechanically, this patch LGTM.

[probinson]

Regarding your LTO-like experiments, that result is more or less what I thought would happen. In a normal compilation, each CU gets its own description of the types, and the fact that they are different in different CUs doesn't matter. Within each CU's context, the common-block description is complete and will be used correctly by the debugger.
In LTO, however, the IR linker wants to de-duplicate debug information, to avoid multiple copies of the same type in the final object file. I suspect that de-duplication (which IIRC is founded on the C++ "one definition rule") probably should not apply to Fortran, or at least not to common blocks.
This is not something you need to solve today, but I think after this lands, you should file a bug to the effect that LTO with Fortran can produce incorrect debug information, specifically for common blocks. That will at least keep track of this factoid somewhere that we can find it again.

So: One test comment, you'll need to file that bug, and I am embarrassed to say I haven't looked at the code part of the patch. But if @dblaikie or @aprantl are happy that's good enough for me.

test/DebugInfo/Generic/DICommonBlock.ll

13 ↗

(On Diff #192481)

Remove the 'target triple' line so this test will still work on bots that don't build the X86 target.

[schweitz]

Thanks, Paul. I can certainly file a bug after this lands for tracking purposes.

Process question: should this review be updated with the bug information/link?

I'll remove the line in the test ...

test/DebugInfo/Generic/DICommonBlock.ll

13 ↗

(On Diff #192481)

Thanks for catching that, Paul. I'll remove that line.

[schweitz]

Removes the target triple from the test.

[probinson]

In D54327#1453361, @schweitz wrote:

Thanks, Paul. I can certainly file a bug after this lands for tracking purposes.

Process question: should this review be updated with the bug information/link?

Yes, please post the url for the bug after you file it. You don't need to mention it in the commit log for the patch though.

[aprantl]

test/DebugInfo/Generic/DICommonBlock.ll

2 ↗

(On Diff #193530)

; RUN: %llc_dwarf -O0 -filetype=obj %s -o - | \
; RUN: llvm-dwarfdump -v -debug-info

is the verbose dump (-v) needed?

[schweitz]

Thanks for accepting the changes, Adrian.

I just tried the test without the -v option, and it passed. It appears to not be required.

As a reminder, I don't have permissions to land changes myself.

[aprantl]

The change to the LLParser broke two tests:

Failing Tests (2):
    LLVM :: Assembler/invalid-diglobalvariable-empty-name.ll
    LLVM :: Assembler/invalid-diglobalvariable-missing-name.ll

  Expected Passes    : 22549
  Expected Failures  : 66
  Unsupported Tests  : 7854
  Unexpected Failures: 2

Can you please take a look?

[schweitz]

Right, those failures make sense to me.

It seems okay for our purposes here to tweak this patch slightly to not depend on support for anonymous global variables.

I'll do that and post a new patch.

[schweitz]

A couple small changes to correct the issues.

• Testing: 1 tests, single process --

PASS: LLVM :: Assembler/invalid-diglobalvariable-empty-name.ll (1 of 1)
Testing Time: 0.18s

Expected Passes    : 1

• Testing: 1 tests, single process --

PASS: LLVM :: Assembler/invalid-diglobalvariable-missing-name.ll (1 of 1)
Testing Time: 0.05s

Expected Passes    : 1

[schweitz]

In D54327#1455171, @schweitz wrote:

I just tried the test without the -v option, and it passed. It appears to not be required.

I also took the -v out of this latest patch set.

[probinson]

In D54327#1453928, @probinson wrote:

In D54327#1453361, @schweitz wrote:

Thanks, Paul. I can certainly file a bug after this lands for tracking purposes.

Process question: should this review be updated with the bug information/link?

Yes, please post the url for the bug after you file it. You don't need to mention it in the commit log for the patch though.

@schweitz Please remember to file this bug. If you have, please post the pointer here.

[schweitz]

@probinson Thank you for the reminder.

https://bugs.llvm.org/show_bug.cgi?id=41472

include/llvm-c/DebugInfo.h

enum {
   LLVMDITemplateTypeParameterMetadataKind,
   LLVMDITemplateValueParameterMetadataKind,
   LLVMDIGlobalVariableMetadataKind,
   LLVMDILocalVariableMetadataKind,
   LLVMDILabelMetadataKind,
   LLVMDIObjCPropertyMetadataKind,
   LLVMDIImportedEntityMetadataKind,
   LLVMDIMacroMetadataKind,
-  LLVMDIMacroFileMetadataKind
+  LLVMDIMacroFileMetadataKind,
+  LLVMDICommonBlockMetadataKind
 };
 typedef unsigned LLVMMetadataKind;
 
 /**
  * An LLVM DWARF type encoding.
  */
 typedef unsigned LLVMDWARFTypeEncoding;
 

include/llvm/Bitcode/LLVMBitCodes.h

enum MetadataCodes {
   METADATA_MACRO = 33,           // [distinct, macinfo, line, name, value]
   METADATA_MACRO_FILE = 34,      // [distinct, macinfo, line, file, ...]
   METADATA_STRINGS = 35,         // [count, offset] blob([lengths][chars])
   METADATA_GLOBAL_DECL_ATTACHMENT = 36, // [valueid, n x [id, mdnode]]
   METADATA_GLOBAL_VAR_EXPR = 37,        // [distinct, var, expr]
   METADATA_INDEX_OFFSET = 38,           // [offset]
   METADATA_INDEX = 39,                  // [bitpos]
   METADATA_LABEL = 40,                  // [distinct, scope, name, file, line]
+  METADATA_COMMON_BLOCK = 44,     // [distinct, scope, name, variable,...]
 };
 
 // The constants block (CONSTANTS_BLOCK_ID) describes emission for each
 // constant and maintains an implicit current type value.
 enum ConstantsCodes {
   CST_CODE_SETTYPE = 1,          // SETTYPE:       [typeid]
   CST_CODE_NULL = 2,             // NULL
   CST_CODE_UNDEF = 3,            // UNDEF

include/llvm/IR/DIBuilder.h

DISubprogram *createMethod(
         DIScope *Scope, StringRef Name, StringRef LinkageName, DIFile *File,
         unsigned LineNo, DISubroutineType *Ty, bool isLocalToUnit,
         bool isDefinition, unsigned Virtuality = 0, unsigned VTableIndex = 0,
         int ThisAdjustment = 0, DIType *VTableHolder = nullptr,
         DINode::DIFlags Flags = DINode::FlagZero, bool isOptimized = false,
         DITemplateParameterArray TParams = nullptr,
         DITypeArray ThrownTypes = nullptr);
 
+    /// Create common block entry for a Fortran common block
+    /// \param Scope       Scope of this common block
+    /// \param Name        The name of this common block
+    /// \param File        The file this common block is defined
+    /// \param LineNo      Line number
+    /// \param VarList     List of variables that a located in common block
+    DICommonBlock *createCommonBlock(DIScope *Scope, DIGlobalVariable *decl,
+                                     StringRef Name, DIFile *File,
+                                     unsigned LineNo);
+
     /// This creates new descriptor for a namespace with the specified
     /// parent scope.
     /// \param Scope       Namespace scope
     /// \param Name        Name of this namespace
     /// \param ExportSymbols True for C++ inline namespaces.
     DINamespace *createNameSpace(DIScope *Scope, StringRef Name,
                                  bool ExportSymbols);
 

include/llvm/IR/DebugInfoMetadata.h

static bool classof(const Metadata *MD) {
     case DICompositeTypeKind:
     case DISubroutineTypeKind:
     case DIFileKind:
     case DICompileUnitKind:
     case DISubprogramKind:
     case DILexicalBlockKind:
     case DILexicalBlockFileKind:
     case DINamespaceKind:
+    case DICommonBlockKind:
     case DITemplateTypeParameterKind:
     case DITemplateValueParameterKind:
     case DIGlobalVariableKind:
     case DILocalVariableKind:
     case DILabelKind:
     case DIObjCPropertyKind:
     case DIImportedEntityKind:
     case DIModuleKind:
static bool classof(const Metadata *MD) {
     case DICompositeTypeKind:
     case DISubroutineTypeKind:
     case DIFileKind:
     case DICompileUnitKind:
     case DISubprogramKind:
     case DILexicalBlockKind:
     case DILexicalBlockFileKind:
     case DINamespaceKind:
+    case DICommonBlockKind:
     case DIModuleKind:
       return true;
     }
   }
 };
 
 /// File.
 ///
public:
   Metadata *getRawTemplateParams() const { return getOperand(7); }
   MDTuple *getTemplateParams() const { return getOperandAs<MDTuple>(7); }
 
   static bool classof(const Metadata *MD) {
     return MD->getMetadataID() == DIGlobalVariableKind;
   }
 };
 
+class DICommonBlock : public DIScope {
+  unsigned LineNo;
+
+  friend class LLVMContextImpl;
+  friend class MDNode;
+
+  DICommonBlock(LLVMContext &Context, StorageType Storage, unsigned LineNo,
+                ArrayRef<Metadata *> Ops)
+      : DIScope(Context, DICommonBlockKind, Storage, dwarf::DW_TAG_common_block,
+                Ops), LineNo(LineNo) {}
+
+  static DICommonBlock *getImpl(LLVMContext &Context, DIScope *Scope,
+                                DIGlobalVariable *Decl, StringRef Name,
+                                DIFile *File, unsigned LineNo,
+                                StorageType Storage,
+                                bool ShouldCreate = true) {
+    return getImpl(Context, Scope, Decl, getCanonicalMDString(Context, Name),
+                   File, LineNo, Storage, ShouldCreate);
+  }
+  static DICommonBlock *getImpl(LLVMContext &Context, Metadata *Scope,
+                                Metadata *Decl, MDString *Name, Metadata *File,
+                                unsigned LineNo, 
+                                StorageType Storage, bool ShouldCreate = true);
+
+  TempDICommonBlock cloneImpl() const {
+    return getTemporary(getContext(), getScope(), getDecl(), getName(),
+                        getFile(), getLineNo());
+  }
+
+public:
+  DEFINE_MDNODE_GET(DICommonBlock,
+                    (DIScope *Scope, DIGlobalVariable *Decl, StringRef Name,
+                     DIFile *File, unsigned LineNo),
+                    (Scope, Decl, Name, File, LineNo))
+  DEFINE_MDNODE_GET(DICommonBlock,
+                    (Metadata *Scope, Metadata *Decl, MDString *Name,
+                     Metadata *File, unsigned LineNo),
+                    (Scope, Decl, Name, File, LineNo))
+
+  TempDICommonBlock clone() const { return cloneImpl(); }
+
+  DIScope *getScope() const { return cast_or_null<DIScope>(getRawScope()); }
+  DIGlobalVariable *getDecl() const {
+    return cast_or_null<DIGlobalVariable>(getRawDecl());
+  }
+  StringRef getName() const { return getStringOperand(2); }
+  DIFile *getFile() const { return cast_or_null<DIFile>(getRawFile()); }
+  unsigned getLineNo() const { return LineNo; }
+
+  Metadata *getRawScope() const { return getOperand(0); }
+  Metadata *getRawDecl() const { return getOperand(1); }
+  MDString *getRawName() const { return getOperandAs<MDString>(2); }
+  Metadata *getRawFile() const { return getOperand(3); }
+
+  static bool classof(const Metadata *MD) {
+    return MD->getMetadataID() == DICommonBlockKind;
+  }
+};
+
 /// Local variable.
 ///
 /// TODO: Split up flags.
 class DILocalVariable : public DIVariable {
   friend class LLVMContextImpl;
   friend class MDNode;
 
   unsigned Arg : 16;

include/llvm/IR/Metadata.def

 HANDLE_SPECIALIZED_MDNODE_LEAF_UNIQUABLE(DIGlobalVariable)
 HANDLE_SPECIALIZED_MDNODE_LEAF_UNIQUABLE(DILocalVariable)
 HANDLE_SPECIALIZED_MDNODE_LEAF_UNIQUABLE(DILabel)
 HANDLE_SPECIALIZED_MDNODE_LEAF_UNIQUABLE(DIObjCProperty)
 HANDLE_SPECIALIZED_MDNODE_LEAF_UNIQUABLE(DIImportedEntity)
 HANDLE_SPECIALIZED_MDNODE_BRANCH(DIMacroNode)
 HANDLE_SPECIALIZED_MDNODE_LEAF_UNIQUABLE(DIMacro)
 HANDLE_SPECIALIZED_MDNODE_LEAF_UNIQUABLE(DIMacroFile)
+HANDLE_SPECIALIZED_MDNODE_LEAF_UNIQUABLE(DICommonBlock)
 
 #undef HANDLE_METADATA
 #undef HANDLE_METADATA_LEAF
 #undef HANDLE_METADATA_BRANCH
 #undef HANDLE_MDNODE_LEAF
 #undef HANDLE_MDNODE_LEAF_UNIQUABLE
 #undef HANDLE_MDNODE_BRANCH
 #undef HANDLE_SPECIALIZED_MDNODE_LEAF
 #undef HANDLE_SPECIALIZED_MDNODE_LEAF_UNIQUABLE
 #undef HANDLE_SPECIALIZED_MDNODE_BRANCH

lib/AsmParser/LLParser.cpp

#define VISIT_MD_FIELDS(OPTIONAL, REQUIRED)                                    \
   PARSE_MD_FIELDS();
 #undef VISIT_MD_FIELDS
 
   Result = GET_OR_DISTINCT(DILexicalBlockFile,
                            (Context, scope.Val, file.Val, discriminator.Val));
   return false;
 }
 
+/// ParseDICommonBlock:
+///   ::= !DICommonBlock(scope: !0, file: !2, name: "COMMON name", line: 9)
+bool LLParser::ParseDICommonBlock(MDNode *&Result, bool IsDistinct) {
+#define VISIT_MD_FIELDS(OPTIONAL, REQUIRED)                                    \
+  REQUIRED(scope, MDField, );                                                  \
+  OPTIONAL(declaration, MDField, );                                            \
+  OPTIONAL(name, MDStringField, );                                             \
+  OPTIONAL(file, MDField, );                                                   \
+  OPTIONAL(line, LineField, );                   
+  PARSE_MD_FIELDS();
+#undef VISIT_MD_FIELDS
+
+  Result = GET_OR_DISTINCT(DICommonBlock,
+                           (Context, scope.Val, declaration.Val, name.Val,
+                            file.Val, line.Val));
+  return false;
+}
+
 /// ParseDINamespace:
 ///   ::= !DINamespace(scope: !0, file: !2, name: "SomeNamespace", line: 9)
 bool LLParser::ParseDINamespace(MDNode *&Result, bool IsDistinct) {
 #define VISIT_MD_FIELDS(OPTIONAL, REQUIRED)                                    \
   REQUIRED(scope, MDField, );                                                  \
   OPTIONAL(name, MDStringField, );                                             \
   OPTIONAL(exportSymbols, MDBoolField, );
   PARSE_MD_FIELDS();

lib/Bitcode/Reader/MetadataLoader.cpp

case BitstreamEntry::Record: {
       case bitc::METADATA_SUBROUTINE_TYPE:
       case bitc::METADATA_MODULE:
       case bitc::METADATA_FILE:
       case bitc::METADATA_COMPILE_UNIT:
       case bitc::METADATA_SUBPROGRAM:
       case bitc::METADATA_LEXICAL_BLOCK:
       case bitc::METADATA_LEXICAL_BLOCK_FILE:
       case bitc::METADATA_NAMESPACE:
+      case bitc::METADATA_COMMON_BLOCK:
       case bitc::METADATA_MACRO:
       case bitc::METADATA_MACRO_FILE:
       case bitc::METADATA_TEMPLATE_TYPE:
       case bitc::METADATA_TEMPLATE_VALUE:
       case bitc::METADATA_GLOBAL_VAR:
       case bitc::METADATA_LOCAL_VAR:
       case bitc::METADATA_LABEL:
       case bitc::METADATA_EXPRESSION:
case bitc::METADATA_LEXICAL_BLOCK_FILE: {
     MetadataList.assignValue(
         GET_OR_DISTINCT(DILexicalBlockFile,
                         (Context, getMDOrNull(Record[1]),
                          getMDOrNull(Record[2]), Record[3])),
         NextMetadataNo);
     NextMetadataNo++;
     break;
   }
+  case bitc::METADATA_COMMON_BLOCK: {
+    IsDistinct = Record[0] & 1;
+    MetadataList.assignValue(
+        GET_OR_DISTINCT(DICommonBlock,
+                        (Context, getMDOrNull(Record[1]),
+                         getMDOrNull(Record[2]), getMDString(Record[3]),
+                         getMDOrNull(Record[4]), Record[5])),
+        NextMetadataNo);
+    NextMetadataNo++;
+    break;
+  }
   case bitc::METADATA_NAMESPACE: {
     // Newer versions of DINamespace dropped file and line.
     MDString *Name;
     if (Record.size() == 3)
       Name = getMDString(Record[2]);
     else if (Record.size() == 5)
       Name = getMDString(Record[3]);
     else

lib/Bitcode/Writer/BitcodeWriter.cpp

void writeDICompileUnit(const DICompileUnit *N,
                           SmallVectorImpl<uint64_t> &Record, unsigned Abbrev);
   void writeDISubprogram(const DISubprogram *N,
                          SmallVectorImpl<uint64_t> &Record, unsigned Abbrev);
   void writeDILexicalBlock(const DILexicalBlock *N,
                            SmallVectorImpl<uint64_t> &Record, unsigned Abbrev);
   void writeDILexicalBlockFile(const DILexicalBlockFile *N,
                                SmallVectorImpl<uint64_t> &Record,
                                unsigned Abbrev);
+  void writeDICommonBlock(const DICommonBlock *N,
+                          SmallVectorImpl<uint64_t> &Record, unsigned Abbrev);
   void writeDINamespace(const DINamespace *N, SmallVectorImpl<uint64_t> &Record,
                         unsigned Abbrev);
   void writeDIMacro(const DIMacro *N, SmallVectorImpl<uint64_t> &Record,
                     unsigned Abbrev);
   void writeDIMacroFile(const DIMacroFile *N, SmallVectorImpl<uint64_t> &Record,
                         unsigned Abbrev);
   void writeDIModule(const DIModule *N, SmallVectorImpl<uint64_t> &Record,
                      unsigned Abbrev);
void ModuleBitcodeWriter::writeDILexicalBlockFile(
   Record.push_back(VE.getMetadataOrNullID(N->getScope()));
   Record.push_back(VE.getMetadataOrNullID(N->getFile()));
   Record.push_back(N->getDiscriminator());
 
   Stream.EmitRecord(bitc::METADATA_LEXICAL_BLOCK_FILE, Record, Abbrev);
   Record.clear();
 }
 
+void ModuleBitcodeWriter::writeDICommonBlock(const DICommonBlock *N,
+                                             SmallVectorImpl<uint64_t> &Record,
+                                             unsigned Abbrev) {
+  Record.push_back(N->isDistinct());
+  Record.push_back(VE.getMetadataOrNullID(N->getScope()));
+  Record.push_back(VE.getMetadataOrNullID(N->getDecl()));
+  Record.push_back(VE.getMetadataOrNullID(N->getRawName()));
+  Record.push_back(VE.getMetadataOrNullID(N->getFile()));
+  Record.push_back(N->getLineNo());
+
+  Stream.EmitRecord(bitc::METADATA_COMMON_BLOCK, Record, Abbrev);
+  Record.clear();
+}
+
 void ModuleBitcodeWriter::writeDINamespace(const DINamespace *N,
                                            SmallVectorImpl<uint64_t> &Record,
                                            unsigned Abbrev) {
   Record.push_back(N->isDistinct() | N->getExportSymbols() << 1);
   Record.push_back(VE.getMetadataOrNullID(N->getScope()));
   Record.push_back(VE.getMetadataOrNullID(N->getRawName()));
 
   Stream.EmitRecord(bitc::METADATA_NAMESPACE, Record, Abbrev);

lib/CodeGen/AsmPrinter/DwarfCompileUnit.h

struct GlobalExpr {
     const DIExpression *Expr;
   };
 
   /// Get or create global variable DIE.
   DIE *
   getOrCreateGlobalVariableDIE(const DIGlobalVariable *GV,
                                ArrayRef<GlobalExpr> GlobalExprs);
 
+  DIE *getOrCreateCommonBlock(const DICommonBlock *CB,
+                              ArrayRef<GlobalExpr> GlobalExprs);
+
+  void addLocationAttribute(DIE *ToDIE, const DIGlobalVariable *GV,
+                            ArrayRef<GlobalExpr> GlobalExprs);
+
   /// addLabelAddress - Add a dwarf label attribute data and value using
   /// either DW_FORM_addr or DW_FORM_GNU_addr_index.
   void addLabelAddress(DIE &Die, dwarf::Attribute Attribute,
                        const MCSymbol *Label);
 
   /// addLocalLabelAddress - Add a dwarf label attribute data and value using
   /// DW_FORM_addr only.
   void addLocalLabelAddress(DIE &Die, dwarf::Attribute Attribute,

lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp

unsigned DwarfCompileUnit::getOrCreateSourceID(const DIFile *File) {
   unsigned CUID = Asm->OutStreamer->hasRawTextSupport() ? 0 : getUniqueID();
   if (!File)
     return Asm->OutStreamer->EmitDwarfFileDirective(0, "", "", nullptr, None, CUID);
   return Asm->OutStreamer->EmitDwarfFileDirective(
       0, File->getDirectory(), File->getFilename(), getMD5AsBytes(File),
       File->getSource(), CUID);
 }
 
-DIE *DwarfCompileUnit::getOrCreateGlobalVariableDIE(
-    const DIGlobalVariable *GV, ArrayRef<GlobalExpr> GlobalExprs) {
-  // Check for pre-existence.
-  if (DIE *Die = getDIE(GV))
-    return Die;
-
-  assert(GV);
-
-  auto *GVContext = GV->getScope();
-  auto *GTy = DD->resolve(GV->getType());
-
-  // Construct the context before querying for the existence of the DIE in
-  // case such construction creates the DIE.
-  DIE *ContextDIE = getOrCreateContextDIE(GVContext);
-
-  // Add to map.
-  DIE *VariableDIE = &createAndAddDIE(GV->getTag(), *ContextDIE, GV);
-  DIScope *DeclContext;
-  if (auto *SDMDecl = GV->getStaticDataMemberDeclaration()) {
-    DeclContext = resolve(SDMDecl->getScope());
-    assert(SDMDecl->isStaticMember() && "Expected static member decl");
-    assert(GV->isDefinition());
-    // We need the declaration DIE that is in the static member's class.
-    DIE *VariableSpecDIE = getOrCreateStaticMemberDIE(SDMDecl);
-    addDIEEntry(*VariableDIE, dwarf::DW_AT_specification, *VariableSpecDIE);
-    // If the global variable's type is different from the one in the class
-    // member type, assume that it's more specific and also emit it.
-    if (GTy != DD->resolve(SDMDecl->getBaseType()))
-      addType(*VariableDIE, GTy);
-  } else {
-    DeclContext = GV->getScope();
-    // Add name and type.
-    addString(*VariableDIE, dwarf::DW_AT_name, GV->getDisplayName());
-    addType(*VariableDIE, GTy);
-
-    // Add scoping info.
-    if (!GV->isLocalToUnit())
-      addFlag(*VariableDIE, dwarf::DW_AT_external);
-
-    // Add line number info.
-    addSourceLine(*VariableDIE, GV);
-  }
-
-  if (!GV->isDefinition())
-    addFlag(*VariableDIE, dwarf::DW_AT_declaration);
-  else
-    addGlobalName(GV->getName(), *VariableDIE, DeclContext);
-
-  if (uint32_t AlignInBytes = GV->getAlignInBytes())
-    addUInt(*VariableDIE, dwarf::DW_AT_alignment, dwarf::DW_FORM_udata,
-            AlignInBytes);
-
-  if (MDTuple *TP = GV->getTemplateParams())
-    addTemplateParams(*VariableDIE, DINodeArray(TP));
-
-  // Add location.
+void DwarfCompileUnit::addLocationAttribute(
+    DIE *ToDIE, const DIGlobalVariable *GV, ArrayRef<GlobalExpr> GlobalExprs) {
   bool addToAccelTable = false;
   DIELoc *Loc = nullptr;
   std::unique_ptr<DIEDwarfExpression> DwarfExpr;
   for (const auto &GE : GlobalExprs) {
     const GlobalVariable *Global = GE.Var;
     const DIExpression *Expr = GE.Expr;
 
     // For compatibility with DWARF 3 and earlier,
     // DW_AT_location(DW_OP_constu, X, DW_OP_stack_value) becomes
     // DW_AT_const_value(X).
     if (GlobalExprs.size() == 1 && Expr && Expr->isConstant()) {
       addToAccelTable = true;
-      addConstantValue(*VariableDIE, /*Unsigned=*/true, Expr->getElement(1));
+      addConstantValue(*ToDIE, /*Unsigned=*/true, Expr->getElement(1));
       break;
     }
 
     // We cannot describe the location of dllimport'd variables: the
     // computation of their address requires loads from the IAT.
     if (Global && Global->hasDLLImportStorageClass())
       continue;
 
for (const auto &GE : GlobalExprs) {
     // It would be better if this were unconditional, but malformed input that
     // mixes non-fragments and fragments for the same variable is too expensive
     // to detect in the verifier.
     if (DwarfExpr->isUnknownLocation())
       DwarfExpr->setMemoryLocationKind();
     DwarfExpr->addExpression(Expr);
   }
   if (Loc)
-    addBlock(*VariableDIE, dwarf::DW_AT_location, DwarfExpr->finalize());
+    addBlock(*ToDIE, dwarf::DW_AT_location, DwarfExpr->finalize());
 
   if (DD->useAllLinkageNames())
-    addLinkageName(*VariableDIE, GV->getLinkageName());
+    addLinkageName(*ToDIE, GV->getLinkageName());
 
   if (addToAccelTable) {
-    DD->addAccelName(*CUNode, GV->getName(), *VariableDIE);
+    DD->addAccelName(*CUNode, GV->getName(), *ToDIE);
 
     // If the linkage name is different than the name, go ahead and output
     // that as well into the name table.
     if (GV->getLinkageName() != "" && GV->getName() != GV->getLinkageName() &&
         DD->useAllLinkageNames())
-      DD->addAccelName(*CUNode, GV->getLinkageName(), *VariableDIE);
-  }
+      DD->addAccelName(*CUNode, GV->getLinkageName(), *ToDIE);
+  }
 
+DIE *DwarfCompileUnit::getOrCreateCommonBlock(
+    const DICommonBlock *CB, ArrayRef<GlobalExpr> GlobalExprs) {
+  // Construct the context before querying for the existence of the DIE in case
+  // such construction creates the DIE.
+  DIE *ContextDIE = getOrCreateContextDIE(CB->getScope());
+
+  if (DIE *NDie = getDIE(CB))
+    return NDie;
+  DIE &NDie = createAndAddDIE(dwarf::DW_TAG_common_block, *ContextDIE, CB);
+  StringRef Name = CB->getName().empty() ? "_BLNK_" : CB->getName();
+  addString(NDie, dwarf::DW_AT_name, Name);
+  addGlobalName(Name, NDie, CB->getScope());
+  if (CB->getFile())
+    addSourceLine(NDie, CB->getLineNo(), CB->getFile());
+  if (DIGlobalVariable *V = CB->getDecl())
+    getCU().addLocationAttribute(&NDie, V, GlobalExprs);
+  return &NDie;
+}
+
+DIE *DwarfCompileUnit::getOrCreateGlobalVariableDIE(
+    const DIGlobalVariable *GV, ArrayRef<GlobalExpr> GlobalExprs) {
+  // Check for pre-existence.
+  if (DIE *Die = getDIE(GV))
+    return Die;
+
+  assert(GV);
+
+  auto *GVContext = GV->getScope();
+  auto *GTy = DD->resolve(GV->getType());
+
+  // Construct the context before querying for the existence of the DIE in
+  // case such construction creates the DIE.
+  auto *CB = dyn_cast<DICommonBlock>(GVContext);
+  DIE *ContextDIE = CB ? getOrCreateCommonBlock(CB, GlobalExprs)
+    : getOrCreateContextDIE(GVContext);
+
+  // Add to map.
+  DIE *VariableDIE = &createAndAddDIE(GV->getTag(), *ContextDIE, GV);
+  DIScope *DeclContext;
+  if (auto *SDMDecl = GV->getStaticDataMemberDeclaration()) {
+    DeclContext = resolve(SDMDecl->getScope());
+    assert(SDMDecl->isStaticMember() && "Expected static member decl");
+    assert(GV->isDefinition());
+    // We need the declaration DIE that is in the static member's class.
+    DIE *VariableSpecDIE = getOrCreateStaticMemberDIE(SDMDecl);
+    addDIEEntry(*VariableDIE, dwarf::DW_AT_specification, *VariableSpecDIE);
+    // If the global variable's type is different from the one in the class
+    // member type, assume that it's more specific and also emit it.
+    if (GTy != DD->resolve(SDMDecl->getBaseType()))
+      addType(*VariableDIE, GTy);
+  } else {
+    DeclContext = GV->getScope();
+    // Add name and type.
+    addString(*VariableDIE, dwarf::DW_AT_name, GV->getDisplayName());
+    addType(*VariableDIE, GTy);
+
+    // Add scoping info.
+    if (!GV->isLocalToUnit())
+      addFlag(*VariableDIE, dwarf::DW_AT_external);
+
+    // Add line number info.
+    addSourceLine(*VariableDIE, GV);
+  }
+
+  if (!GV->isDefinition())
+    addFlag(*VariableDIE, dwarf::DW_AT_declaration);
+  else
+    addGlobalName(GV->getName(), *VariableDIE, DeclContext);
+
+  // Add location.
+  addLocationAttribute(VariableDIE, GV, GlobalExprs);
+
   return VariableDIE;
 }
 
 void DwarfCompileUnit::addRange(RangeSpan Range) {
   bool SameAsPrevCU = this == DD->getPrevCU();
   DD->setPrevCU(this);
   // If we have no current ranges just add the range and return, otherwise,
   // check the current section and CU against the previous section and CU we

lib/CodeGen/AsmPrinter/DwarfUnit.cpp

if (auto *CT = dyn_cast<DICompositeType>(Ty)) {
       // A runtime language of 0 actually means C/C++ and that any
       // non-negative value is some version of Objective-C/C++.
       IsImplementation = CT->getRuntimeLang() == 0 || CT->isObjcClassComplete();
     }
     unsigned Flags = IsImplementation ? dwarf::DW_FLAG_type_implementation : 0;
     DD->addAccelType(*CUNode, Ty->getName(), TyDIE, Flags);
 
     if (!Context || isa<DICompileUnit>(Context) || isa<DIFile>(Context) ||
-        isa<DINamespace>(Context))
+        isa<DINamespace>(Context) || isa<DICommonBlock>(Context))
       addGlobalType(Ty, TyDIE, Context);
   }
 }
 
 void DwarfUnit::addType(DIE &Entity, const DIType *Ty,
                         dwarf::Attribute Attribute) {
   assert(Ty && "Trying to add a type that doesn't exist?");
   addDIEEntry(Entity, Attribute, DIEEntry(*getOrCreateTypeDIE(Ty)));

lib/IR/AsmWriter.cpp

static void writeDINamespace(raw_ostream &Out, const DINamespace *N,
   Out << "!DINamespace(";
   MDFieldPrinter Printer(Out, TypePrinter, Machine, Context);
   Printer.printString("name", N->getName());
   Printer.printMetadata("scope", N->getRawScope(), /* ShouldSkipNull */ false);
   Printer.printBool("exportSymbols", N->getExportSymbols(), false);
   Out << ")";
 }
 
+static void writeDICommonBlock(raw_ostream &Out, const DICommonBlock *N,
+                               TypePrinting *TypePrinter, SlotTracker *Machine,
+                               const Module *Context) {
+  Out << "!DICommonBlock(";
+  MDFieldPrinter Printer(Out, TypePrinter, Machine, Context);
+  Printer.printMetadata("scope", N->getRawScope(), false);
+  Printer.printMetadata("declaration", N->getRawDecl(), false);
+  Printer.printString("name", N->getName());
+  Printer.printMetadata("file", N->getRawFile());
+  Printer.printInt("line", N->getLineNo());
+  Out << ")";
+}
+
 static void writeDIMacro(raw_ostream &Out, const DIMacro *N,
                          TypePrinting *TypePrinter, SlotTracker *Machine,
                          const Module *Context) {
   Out << "!DIMacro(";
   MDFieldPrinter Printer(Out, TypePrinter, Machine, Context);
   Printer.printMacinfoType(N);
   Printer.printInt("line", N->getLine());
   Printer.printString("name", N->getName());

lib/IR/DIBuilder.cpp

auto *SP = getSubprogram(
       isDefinition ? CUNode : nullptr, TParams, nullptr, nullptr, ThrownTypes);
 
   if (isDefinition)
     AllSubprograms.push_back(SP);
   trackIfUnresolved(SP);
   return SP;
 }
 
+DICommonBlock *DIBuilder::createCommonBlock(
+    DIScope *Scope, DIGlobalVariable *Decl, StringRef Name, DIFile *File,
+    unsigned LineNo) {
+  return DICommonBlock::get(
+      VMContext, Scope, Decl, Name, File, LineNo);
+}
+
 DINamespace *DIBuilder::createNameSpace(DIScope *Scope, StringRef Name,
                                         bool ExportSymbols) {
 
   // It is okay to *not* make anonymous top-level namespaces distinct, because
   // all nodes that have an anonymous namespace as their parent scope are
   // guaranteed to be unique and/or are linked to their containing
   // DICompileUnit. This decision is an explicit tradeoff of link time versus
   // memory usage versus code simplicity and may get revisited in the future.

lib/IR/DebugInfoMetadata.cpp

if (auto *SP = dyn_cast<DISubprogram>(this))
     return SP->getScope();
 
   if (auto *LB = dyn_cast<DILexicalBlockBase>(this))
     return LB->getScope();
 
   if (auto *NS = dyn_cast<DINamespace>(this))
     return NS->getScope();
 
+  if (auto *CB = dyn_cast<DICommonBlock>(this))
+    return CB->getScope();
+
   if (auto *M = dyn_cast<DIModule>(this))
     return M->getScope();
 
   assert((isa<DIFile>(this) || isa<DICompileUnit>(this)) &&
          "Unhandled type of scope.");
   return nullptr;
 }
 
 StringRef DIScope::getName() const {
   if (auto *T = dyn_cast<DIType>(this))
     return T->getName();
   if (auto *SP = dyn_cast<DISubprogram>(this))
     return SP->getName();
   if (auto *NS = dyn_cast<DINamespace>(this))
     return NS->getName();
+  if (auto *CB = dyn_cast<DICommonBlock>(this))
+    return CB->getName();
   if (auto *M = dyn_cast<DIModule>(this))
     return M->getName();
   assert((isa<DILexicalBlockBase>(this) || isa<DIFile>(this) ||
           isa<DICompileUnit>(this)) &&
          "Unhandled type of scope.");
   return "";
 }
 
DINamespace *DINamespace::getImpl(LLVMContext &Context, Metadata *Scope,
                                   StorageType Storage, bool ShouldCreate) {
   assert(isCanonical(Name) && "Expected canonical MDString");
   DEFINE_GETIMPL_LOOKUP(DINamespace, (Scope, Name, ExportSymbols));
   // The nullptr is for DIScope's File operand. This should be refactored.
   Metadata *Ops[] = {nullptr, Scope, Name};
   DEFINE_GETIMPL_STORE(DINamespace, (ExportSymbols), Ops);
 }
 
+DICommonBlock *DICommonBlock::getImpl(LLVMContext &Context, Metadata *Scope,
+                                      Metadata *Decl, MDString *Name,
+                                      Metadata *File, unsigned LineNo,
+                                      StorageType Storage, bool ShouldCreate) {
+  assert(isCanonical(Name) && "Expected canonical MDString");
+  DEFINE_GETIMPL_LOOKUP(DICommonBlock, (Scope, Decl, Name, File, LineNo));
+  // The nullptr is for DIScope's File operand. This should be refactored.
+  Metadata *Ops[] = {Scope, Decl, Name, File};
+  DEFINE_GETIMPL_STORE(DICommonBlock, (LineNo), Ops);
+}
+
 DIModule *DIModule::getImpl(LLVMContext &Context, Metadata *Scope,
                             MDString *Name, MDString *ConfigurationMacros,
                             MDString *IncludePath, MDString *ISysRoot,
                             StorageType Storage, bool ShouldCreate) {
   assert(isCanonical(Name) && "Expected canonical MDString");
   DEFINE_GETIMPL_LOOKUP(
       DIModule, (Scope, Name, ConfigurationMacros, IncludePath, ISysRoot));
   Metadata *Ops[] = {Scope, Name, ConfigurationMacros, IncludePath, ISysRoot};

lib/IR/LLVMContextImpl.h

return Scope == RHS->getRawScope() && Name == RHS->getRawName() &&
            ExportSymbols == RHS->getExportSymbols();
   }
 
   unsigned getHashValue() const {
     return hash_combine(Scope, Name);
   }
 };
 
+template <> struct MDNodeKeyImpl<DICommonBlock> {
+  Metadata *Scope;
+  Metadata *Decl;
+  MDString *Name;
+  Metadata *File;
+  unsigned LineNo;
+
+  MDNodeKeyImpl(Metadata *Scope, Metadata *Decl, MDString *Name,
+                Metadata *File, unsigned LineNo)
+      : Scope(Scope), Decl(Decl), Name(Name), File(File), LineNo(LineNo) {}
+  MDNodeKeyImpl(const DICommonBlock *N)
+      : Scope(N->getRawScope()), Decl(N->getRawDecl()), Name(N->getRawName()),
+        File(N->getRawFile()), LineNo(N->getLineNo()) {}
+
+  bool isKeyOf(const DICommonBlock *RHS) const {
+    return Scope == RHS->getRawScope() && Decl == RHS->getRawDecl() &&
+      Name == RHS->getRawName() && File == RHS->getRawFile() &&
+      LineNo == RHS->getLineNo();
+  }
+
+  unsigned getHashValue() const {
+    return hash_combine(Scope, Decl, Name, File, LineNo);
+  }
+};
+
 template <> struct MDNodeKeyImpl<DIModule> {
   Metadata *Scope;
   MDString *Name;
   MDString *ConfigurationMacros;
   MDString *IncludePath;
   MDString *ISysRoot;
 
   MDNodeKeyImpl(Metadata *Scope, MDString *Name, MDString *ConfigurationMacros,

lib/IR/Verifier.cpp

void Verifier::visitDILexicalBlock(const DILexicalBlock &N) {
   AssertDI(N.getLine() || !N.getColumn(),
            "cannot have column info without line info", &N);
 }
 
 void Verifier::visitDILexicalBlockFile(const DILexicalBlockFile &N) {
   visitDILexicalBlockBase(N);
 }
 
+void Verifier::visitDICommonBlock(const DICommonBlock &N) {
+  AssertDI(N.getTag() == dwarf::DW_TAG_common_block, "invalid tag", &N);
+  if (auto *S = N.getRawScope())
+    AssertDI(isa<DIScope>(S), "invalid scope ref", &N, S);
+  if (auto *S = N.getRawDecl())
+    AssertDI(isa<DIGlobalVariable>(S), "invalid declaration", &N, S);
+}
+
 void Verifier::visitDINamespace(const DINamespace &N) {
   AssertDI(N.getTag() == dwarf::DW_TAG_namespace, "invalid tag", &N);
   if (auto *S = N.getRawScope())
     AssertDI(isa<DIScope>(S), "invalid scope ref", &N, S);
 }
 
 void Verifier::visitDIMacro(const DIMacro &N) {
   AssertDI(N.getMacinfoType() == dwarf::DW_MACINFO_define ||

test/DebugInfo/DICommonBlock.ll

+; ModuleID = 'none.f90'
+; RUN: %llc_dwarf -O0 -filetype=obj < %s > %t
+; RUN: llvm-dwarfdump -v -debug-info %t | FileCheck %s
+; CHECK: DW_TAG_common_block
+; CHECK-NOT: {{DW_TAG|NULL}}
+; CHECK: DW_AT_name
+; CHECK: DW_AT_location
+
+target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-pc-linux-gnu"
+
+@common_a = common global [32 x i8] zeroinitializer, align 8, !dbg !13, !dbg !15
+
+define i32 @subr() !dbg !9 {
+    %1 = getelementptr inbounds [32 x i8], [32 x i8]* @common_a, i64 0, i32 8
+    %2 = bitcast i8* %1 to i32*
+    %3 = load i32, i32* %2
+    ret i32 %3
+}
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!6, !7}
+!llvm.ident = !{!8}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_Fortran90, file: !1, producer: "PGI Fortran", isOptimized: false, runtimeVersion: 2, emissionKind: FullDebug, retainedTypes: !14, globals: !17)
+!1 = !DIFile(filename: "none.f90", directory: "/not/here/")
+!2 = !{}
+!3 = !{}
+!4 = distinct !DIGlobalVariable(name: "common /a/", scope: !5, file: !1, line: 4, isLocal: false, isDefinition: true, type: !12)
+!5 = !DICommonBlock(scope: !9, declaration: !4, name: "a", file: !1, line: 4)
+!6 = !{i32 2, !"Dwarf Version", i32 4}
+!7 = !{i32 2, !"Debug Info Version", i32 3}
+!8 = !{!"PGI Fortran"}
+!9 = distinct !DISubprogram(name: "subrtn", scope: !0, file: !1, line: 1, type: !10, isLocal: false, isDefinition: true, unit: !0)
+!10 = !DISubroutineType(types: !11)
+!11 = !{!12, !12}
+!12 = !DIBasicType(name: "int", size: 32)
+!13 = !DIGlobalVariableExpression(var: !4, expr: !DIExpression())
+!14 = !{!12, !10}
+!15 = !DIGlobalVariableExpression(var: !16, expr: !DIExpression(DW_OP_plus_uconst, 4))
+!16 = distinct !DIGlobalVariable(scope: !5, name: "c", file: !1, type: !12, isDefinition: true)
+!17 = !{!13, !15}

test/DebugInfo/DICommonBlock2.ll

+; ModuleID = 'none.f90'
+; RUN: llvm-as < %s | llvm-dis | llvm-as | llvm-dis | FileCheck %s
+; CHECK: !DICommonBlock({{.*}}name: "a"
+
+target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-pc-linux-gnu"
+
+@common_a = common global [32 x i8] zeroinitializer, align 8, !dbg !13, !dbg !15
+
+define i32 @subr() !dbg !9 {
+    %1 = getelementptr inbounds [32 x i8], [32 x i8]* @common_a, i64 0, i32 8
+    %2 = bitcast i8* %1 to i32*
+    %3 = load i32, i32* %2
+    ret i32 %3
+}
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!6, !7}
+!llvm.ident = !{!8}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_Fortran90, file: !1, producer: "PGI Fortran", isOptimized: false, runtimeVersion: 2, emissionKind: FullDebug, retainedTypes: !14, globals: !17)
+!1 = !DIFile(filename: "none.f90", directory: "/not/here/")
+!2 = !{}
+!3 = !{}
+!4 = distinct !DIGlobalVariable(name: "common /a/", scope: !5, file: !1, line: 4, isLocal: false, isDefinition: true, type: !12)
+!5 = !DICommonBlock(scope: !9, declaration: !4, name: "a", file: !1, line: 4)
+!6 = !{i32 2, !"Dwarf Version", i32 4}
+!7 = !{i32 2, !"Debug Info Version", i32 3}
+!8 = !{!"PGI Fortran"}
+!9 = distinct !DISubprogram(name: "subrtn", scope: !0, file: !1, line: 1, type: !10, isLocal: false, isDefinition: true, unit: !0)
+!10 = !DISubroutineType(types: !11)
+!11 = !{!12, !12}
+!12 = !DIBasicType(name: "int", size: 32)
+!13 = !DIGlobalVariableExpression(var: !4, expr: !DIExpression())
+!14 = !{!12, !10}
+!15 = !DIGlobalVariableExpression(var: !16, expr: !DIExpression(DW_OP_plus_uconst, 4))
+!16 = distinct !DIGlobalVariable(scope: !5, name: "c", file: !1, type: !12, isDefinition: true)
+!17 = !{!13, !15}