[llvm][DebugInfo] Allow DIDerivedType as a bound in DISubrangeType (#165880)

Consider this Ada type:

```
   type Array_Type is array (Natural range <>) of Integer;
   type Record_Type (L1, L2 : Natural) is record
      I1 : Integer;
      A1 : Array_Type (1 .. L1);
      I2 : Integer;
      A2 : Array_Type (1 .. L2);
      I3 : Integer;
   end record;
```

Here, the array fields have lengths that depend on the discriminants of
the record type. However, in this case the array lengths cannot be
expressed as DWARF location expressions, with the issue being that "A2"
has a non-constant offset, but an expression involving
DW_OP_push_object_address will push the address of the field -- with no
way to find the location of "L2".

In a case like this, I believe the correct DWARF is to emit the array
ranges using a direct reference to the discriminant, like:

```
 <3><1156>: Abbrev Number: 1 (DW_TAG_member)
    <1157>   DW_AT_name        : l1
...
 <3><1177>: Abbrev Number: 6 (DW_TAG_array_type)
    <1178>   DW_AT_name        : (indirect string, offset: 0x1a0b): vla__record_type__T4b
    <117c>   DW_AT_type        : <0x1287>
    <1180>   DW_AT_sibling     : <0x118e>
 <4><1184>: Abbrev Number: 7 (DW_TAG_subrange_type)
    <1185>   DW_AT_type        : <0x1280>
    <1189>   DW_AT_upper_bound : <0x1156>
```

(FWIW this is what GCC has done for years.)

This patch makes this possible in LLVM, by letting a DISubrangeType
refer to a DIDerivedType. gnat-llvm can then arrange for the DIE
reference to be correct by setting the array type's scope to be the
record.

Author: tromey

llvm/include/llvm/IR/DebugInfoMetadata.h

@@ -1520,7 +1520,9 @@ inline bool operator!=(DIDerivedType::PtrAuthData Lhs,
 /// is also a DIType.
 class DISubrangeType : public DIType {
 public:
-  typedef PointerUnion<ConstantInt *, DIVariable *, DIExpression *> BoundType;
+  typedef PointerUnion<ConstantInt *, DIVariable *, DIExpression *,
+                       DIDerivedType *>
+      BoundType;
 
 private:
   friend class LLVMContextImpl;

llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp

@@ -1598,6 +1598,9 @@ void DwarfUnit::constructSubrangeDIE(DIE &DW_Subrange, const DISubrangeType *SR,
     if (auto *BV = dyn_cast_if_present<DIVariable *>(Bound)) {
       if (auto *VarDIE = getDIE(BV))
         addDIEEntry(DW_Subrange, Attr, *VarDIE);
+    } else if (auto *DT = dyn_cast_if_present<DIDerivedType *>(Bound)) {
+      if (auto *DTDIE = getDIE(DT))
+        addDIEEntry(DW_Subrange, Attr, *DTDIE);
     } else if (auto *BE = dyn_cast_if_present<DIExpression *>(Bound)) {
       addBlock(DW_Subrange, Attr, BE);
     } else if (auto *BI = dyn_cast_if_present<ConstantInt *>(Bound)) {

llvm/lib/IR/DebugInfoMetadata.cpp

@@ -839,7 +839,7 @@ DISubrangeType::convertRawToBound(Metadata *IN) const {
     return BoundType();
 
   assert(isa<ConstantAsMetadata>(IN) || isa<DIVariable>(IN) ||
-         isa<DIExpression>(IN));
+         isa<DIExpression>(IN) || isa<DIDerivedType>(IN));
 
   if (auto *MD = dyn_cast<ConstantAsMetadata>(IN))
     return BoundType(cast<ConstantInt>(MD->getValue()));
@@ -850,6 +850,9 @@ DISubrangeType::convertRawToBound(Metadata *IN) const {
   if (auto *MD = dyn_cast<DIExpression>(IN))
     return BoundType(MD);
 
+  if (auto *DT = dyn_cast<DIDerivedType>(IN))
+    return BoundType(DT);
+
   return BoundType();
 }
 

llvm/lib/IR/Verifier.cpp

@@ -1187,13 +1187,17 @@ void Verifier::visitDISubrangeType(const DISubrangeType &N) {
   CheckDI(!BaseType || isType(BaseType), "BaseType must be a type");
   auto *LBound = N.getRawLowerBound();
   CheckDI(!LBound || isa<ConstantAsMetadata>(LBound) ||
-              isa<DIVariable>(LBound) || isa<DIExpression>(LBound),
-          "LowerBound must be signed constant or DIVariable or DIExpression",
+              isa<DIVariable>(LBound) || isa<DIExpression>(LBound) ||
+              isa<DIDerivedType>(LBound),
+          "LowerBound must be signed constant or DIVariable or DIExpression or "
+          "DIDerivedType",
           &N);
   auto *UBound = N.getRawUpperBound();
   CheckDI(!UBound || isa<ConstantAsMetadata>(UBound) ||
-              isa<DIVariable>(UBound) || isa<DIExpression>(UBound),
-          "UpperBound must be signed constant or DIVariable or DIExpression",
+              isa<DIVariable>(UBound) || isa<DIExpression>(UBound) ||
+              isa<DIDerivedType>(UBound),
+          "UpperBound must be signed constant or DIVariable or DIExpression or "
+          "DIDerivedType",
           &N);
   auto *Stride = N.getRawStride();
   CheckDI(!Stride || isa<ConstantAsMetadata>(Stride) ||

llvm/test/DebugInfo/X86/derived-in-subrange.ll

@@ -0,0 +1,88 @@
+; RUN: llvm-as < %s | llvm-dis | llc -mtriple=x86_64 -O0 -filetype=obj -o - | llvm-dwarfdump -v -debug-info - | FileCheck %s
+
+; A test to verify the use of a DIDerivedType as a bound of a
+; DISubrangeType.
+
+; CHECK: DW_TAG_array_type
+; CHECK-NEXT: DW_AT_type [DW_FORM_ref4] {{.*}}
+; CHECK: DW_TAG_subrange_type {{.*}}
+; CHECK-NEXT: DW_AT_type [DW_FORM_ref4] {{.*}}
+; CHECK-NEXT: DW_AT_upper_bound [DW_FORM_ref4] {{.*}}
+
+; ModuleID = 'vla.ads'
+source_filename = "vla.ads"
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+%vla__array_type_FP = type { ptr, ptr }
+%vla__record_type_I = type <{ i32 }>
+
+@vla_E = dso_local global i16 0, align 2, !dbg !0
+
+; Function Attrs: inlinehint
+define dso_local void @vla__array_typeIP(%vla__array_type_FP %_init) #0 !dbg !10 {
+entry:
+  %0 = extractvalue %vla__array_type_FP %_init, 1, !dbg !15
+  %1 = call ptr @llvm.invariant.start.p0(i64 8, ptr %0), !dbg !15
+  ret void, !dbg !15
+}
+
+; Function Attrs: nocallback nofree nosync nounwind willreturn memory(argmem: readwrite)
+declare ptr @llvm.invariant.start.p0(i64 immarg, ptr nocapture) #1
+
+; Function Attrs: inlinehint
+define dso_local void @vla__record_typeIP(ptr noalias nocapture nonnull %_init, i32 %l1) #0 !dbg !16 {
+entry:
+    #dbg_declare(ptr %_init, !31, !DIExpression(), !32)
+    #dbg_value(i32 %l1, !33, !DIExpression(), !34)
+  %0 = getelementptr inbounds %vla__record_type_I, ptr %_init, i32 0, i32 0, !dbg !32
+  store i32 %l1, ptr %0, align 4, !dbg !32, !tbaa !35
+  ret void, !dbg !32
+}
+
+attributes #0 = { inlinehint }
+attributes #1 = { nocallback nofree nosync nounwind willreturn memory(argmem: readwrite) }
+
+!llvm.module.flags = !{!6, !7, !8, !9}
+!llvm.dbg.cu = !{!2}
+
+!0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())
+!1 = distinct !DIGlobalVariable(name: "vla_E", scope: !2, file: !3, line: 16, type: !5, isLocal: false, isDefinition: true, align: 16)
+!2 = distinct !DICompileUnit(language: DW_LANG_Ada95, file: !3, producer: "GNAT/LLVM", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, globals: !4, splitDebugInlining: false)
+!3 = !DIFile(filename: "vla.ads", directory: "")
+!4 = !{!0}
+!5 = !DIBasicType(name: "short_integer", size: 16, encoding: DW_ATE_signed)
+!6 = !{i32 8, !"PIC Level", i32 2}
+!7 = !{i32 7, !"PIE Level", i32 2}
+!8 = !{i32 2, !"Debug Info Version", i32 3}
+!9 = !{i32 2, !"Dwarf Version", i32 4}
+!10 = distinct !DISubprogram(name: "vla__array_typeIP", scope: !3, file: !3, line: 17, type: !11, scopeLine: 17, spFlags: DISPFlagDefinition, unit: !2)
+!11 = !DISubroutineType(types: !12)
+!12 = !{null, !13}
+!13 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !14, size: 64, align: 64, dwarfAddressSpace: 0)
+!14 = !DIBasicType(tag: DW_TAG_unspecified_type, name: "vla__array_type")
+!15 = !DILocation(line: 17, column: 9, scope: !10)
+!16 = distinct !DISubprogram(name: "vla__record_typeIP", scope: !3, file: !3, line: 18, type: !17, scopeLine: 18, spFlags: DISPFlagDefinition, unit: !2, retainedNodes: !30)
+!17 = !DISubroutineType(types: !18)
+!18 = !{null, !19, !23}
+!19 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !20, size: 64, align: 64, dwarfAddressSpace: 0)
+!20 = !DICompositeType(tag: DW_TAG_structure_type, name: "vla__record_type", file: !3, line: 18, size: !DIExpression(DW_OP_push_object_address, DW_OP_deref_size, 4, DW_OP_constu, 32, DW_OP_mul, DW_OP_constu, 32, DW_OP_plus), align: 32, elements: !21, identifier: "vla__record_type")
+!21 = !{!22, !26}
+!22 = !DIDerivedType(tag: DW_TAG_member, name: "l1", file: !3, line: 18, baseType: !23)
+!23 = !DISubrangeType(name: "natural", file: !24, line: 1, size: 32, align: 32, baseType: !25, lowerBound: i64 0, upperBound: i64 2147483647)
+!24 = !DIFile(filename: "system.ads", directory: "/home/tromey/AdaCore/gnat-llvm/llvm-interface//lib/gnat-llvm/x86_64-unknown-linux-gnu/rts-native/adainclude/")
+!25 = !DIBasicType(name: "integer", size: 32, encoding: DW_ATE_signed)
+!26 = !DIDerivedType(tag: DW_TAG_member, name: "a1", file: !3, line: 19, baseType: !27, offset: 32)
+!27 = !DICompositeType(tag: DW_TAG_array_type, scope: !20, file: !3, line: 19, baseType: !25, align: 32, elements: !28)
+!28 = !{!29}
+!29 = !DISubrangeType(baseType: !25, lowerBound: i64 1, upperBound: !22)
+!30 = !{}
+!31 = !DILocalVariable(name: "_init", arg: 1, scope: !16, file: !3, line: 18, type: !20, flags: DIFlagArtificial)
+!32 = !DILocation(line: 18, column: 9, scope: !16)
+!33 = !DILocalVariable(name: "l1", arg: 2, scope: !16, file: !3, line: 18, type: !23, flags: DIFlagArtificial)
+!34 = !DILocation(line: 18, column: 22, scope: !16)
+!35 = !{!36, !36, i64 0, i64 4}
+!36 = !{!37, i64 4, !"natural#T5"}
+!37 = !{!38, i64 4, !"natural#TN"}
+!38 = !{!39, i64 4, !"integerB#TN"}
+!39 = !{!"Ada Root"}