[DebugInfo] Emit DWARF reference for DIVariable 'count' in DISubrange

[sdesmalen]

Summary

This patch implements the codegen of DWARF debug info for non-constant
'count' fields for DISubrange.

This is patch [2/3] in a series to extend LLVM's DISubrange Metadata
node to support debugging of C99 variable length arrays and vectors with
runtime length like the Scalable Vector Extension for AArch64. It is
also a first step towards representing more complex cases like arrays
in Fortran.

[sdesmalen]

Updated patch to use new interface for 'getCountVariableOrNull()'.

[sdesmalen]

Adjusted to use new interface for getCount()

[aprantl]

lib/CodeGen/AsmPrinter/DwarfUnit.cpp

1316

remove everything up until (and including) "since".

test/DebugInfo/Generic/disubrange.ll

5

For documentation, could you CHECK the TAG_variable first and also CHECK for its name?

[sdesmalen]

test/DebugInfo/Generic/disubrange.ll

5

I've done this by running llvm-dwarfdump once with -name=vla_expr to get the debug entry, and then a second llvm-dwarfdump cross references this entry with the value used for the count field. (this was needed because the DW_AT_name is on the next line after DW_TAG_variable).

[aprantl]

That's great. We should take advantage of the new dwarfdump features.

lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp

DIE *DwarfCompileUnit::constructVariableDIE(DbgVariable &DV, bool Abstract) {
   DV.setDIE(*D);
   return D;
 }
 
 DIE *DwarfCompileUnit::constructVariableDIEImpl(const DbgVariable &DV,
                                                 bool Abstract) {
   // Define variable debug information entry.
   auto VariableDie = DIE::get(DIEValueAllocator, DV.getTag());
+  insertDIE(DV.getVariable(), VariableDie);
 
   if (Abstract) {
     applyVariableAttributes(DV, *VariableDie);
     return VariableDie;
   }
 
   // Add variable address.
 

lib/CodeGen/AsmPrinter/DwarfUnit.cpp

void DwarfUnit::constructSubrangeDIE(DIE &Buffer, const DISubrange *SR,
   int64_t DefaultLowerBound = getDefaultLowerBound();
   int64_t Count = -1;
   if (auto *CI = SR->getCount().dyn_cast<ConstantInt*>())
     Count = CI->getSExtValue();
 
   if (DefaultLowerBound == -1 || LowerBound != DefaultLowerBound)
     addUInt(DW_Subrange, dwarf::DW_AT_lower_bound, None, LowerBound);
 
-  if (Count != -1)
+  if (auto *CV = SR->getCount().dyn_cast<DIVariable*>()) {
+    // I think we can assert() here that the DIE of the count variable has
+    // already been instantiated, since 'finishVariableDefinition' that
+    // creates the types for a variable is always called _after_ the DIEs
+    // for variables are created.
+    auto *CountVarDIE = getDIE(CV);
+    assert(CountVarDIE && "DIE for count is not yet instantiated");
+    addDIEEntry(DW_Subrange, dwarf::DW_AT_count, *CountVarDIE);
+  } else if (Count != -1)
     // FIXME: An unbounded array should reference the expression that defines
     // the array.
     addUInt(DW_Subrange, dwarf::DW_AT_count, None, Count);
 }
 
 DIE *DwarfUnit::getIndexTyDie() {
   if (IndexTyDie)
     return IndexTyDie;

test/DebugInfo/Generic/disubrange.ll

+; RUN: %llc_dwarf -O0 -filetype=obj < %s > %t
+; RUN: llvm-dwarfdump -verbose %t | FileCheck %s
+
+; CHECK-DAG: DW_AT_count [DW_FORM_ref4]  (cu + {{.*}} => {[[NODE:[0-9a-zA-Zx]+]]})
+; CHECK-DAG: [[NODE]]: DW_TAG_variable [4]
+
+define void @foo(i32 %n) !dbg !7 {
+entry:
+  call void @llvm.dbg.value(metadata i32 %n, i64 0, metadata !12, metadata !19), !dbg !20
+  ret void, !dbg !21
+}
+
+declare void @llvm.dbg.value(metadata, i64, metadata, metadata)
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4, !5}
+!llvm.ident = !{!6}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 5.0.1", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2)
+!1 = !DIFile(filename: "vla.c", directory: "/path/to")
+!2 = !{}
+!3 = !{i32 2, !"Dwarf Version", i32 4}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = !{i32 1, !"wchar_size", i32 4}
+!6 = !{!"clang version 5.0.1"}
+!7 = distinct !DISubprogram(name: "foo", scope: !1, file: !1, line: 20, type: !8, isLocal: false, isDefinition: true, scopeLine: 20, flags: DIFlagPrototyped, isOptimized: true, unit: !0, variables: !11)
+!8 = !DISubroutineType(types: !9)
+!9 = !{null, !10}
+!10 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!11 = !{!12, !13, !15}
+!12 = !DILocalVariable(name: "n", arg: 1, scope: !7, file: !1, line: 20, type: !10)
+!13 = !DILocalVariable(name: "vla_expr", scope: !7, file: !1, line: 21, type: !14)
+!14 = !DIBasicType(name: "long unsigned int", size: 64, encoding: DW_ATE_unsigned)
+!15 = !DILocalVariable(name: "vla", scope: !7, file: !1, line: 21, type: !16)
+!16 = !DICompositeType(tag: DW_TAG_array_type, baseType: !10, align: 32, elements: !17)
+!17 = !{!18, !18}
+!18 = !DISubrange(count: !13)
+!19 = !DIExpression()
+!20 = !DILocation(line: 20, column: 14, scope: !7)
+!21 = !DILocation(line: 22, column: 1, scope: !7)