Non constant size and offset in DWARF (#141106)

In Ada, a record type can have a non-constant size, and a field can
appear at a non-constant bit offset in a record.

To support this, this patch changes DIType to record the size and offset
using metadata, rather than plain integers. In addition to a constant
offset, both DIVariable and DIExpression are now supported here.

One thing of note in this patch is the choice of how exactly to
represent a non-constant bit offset, with the difficulty being that
DWARF 5 does not support this. DWARF 3 did have a way to support a
non-constant byte offset, combined with a constant bit offset within the
byte, but this was deprecated in DWARF 4 and removed from DWARF 5.

This patch takes a simple approach: a DWARF extension allowing the use
of an expression with DW_AT_data_bit_offset. There is a corresponding
DWARF issue, see https://dwarfstd.org/issues/250501.1.html. The main
reason for this approach is that it keeps API simplicity: just a single
value is needed, rather than having separate data describing the byte
offset and the bit within the byte.

1 files changed, 46 insertions, 15 deletions

diff --git a/llvm/lib/Bitcode/Reader/MetadataLoader.cpp b/llvm/lib/Bitcode/Reader/MetadataLoader.cpp
index 1cd1797..a9467d1 100644
--- a/llvm/lib/Bitcode/Reader/MetadataLoader.cpp
+++ b/llvm/lib/Bitcode/Reader/MetadataLoader.cpp
@@ -1287,6 +1287,14 @@ Error MetadataLoader::MetadataLoaderImpl::parseOneMetadata(
     return MetadataList.upgradeTypeRef(getMDOrNull(ID));
   };
 
+  auto getMetadataOrConstant = [&](bool IsMetadata,
+                                   uint64_t Entry) -> Metadata * {
+    if (IsMetadata)
+      return getMDOrNull(Entry);
+    return ConstantAsMetadata::get(
+        ConstantInt::get(Type::getInt64Ty(Context), Entry));
+  };
+
 #define GET_OR_DISTINCT(CLASS, ARGS)                                           \
   (IsDistinct ? CLASS::getDistinct ARGS : CLASS::get ARGS)
 
@@ -1525,15 +1533,18 @@ Error MetadataLoader::MetadataLoaderImpl::parseOneMetadata(
     if (Record.size() < 6 || Record.size() > 8)
       return error("Invalid record");
 
-    IsDistinct = Record[0];
+    IsDistinct = Record[0] & 1;
+    bool SizeIsMetadata = Record[0] & 2;
     DINode::DIFlags Flags = (Record.size() > 6)
                                 ? static_cast<DINode::DIFlags>(Record[6])
                                 : DINode::FlagZero;
     uint32_t NumExtraInhabitants = (Record.size() > 7) ? Record[7] : 0;
 
+    Metadata *SizeInBits = getMetadataOrConstant(SizeIsMetadata, Record[3]);
+
     MetadataList.assignValue(
         GET_OR_DISTINCT(DIBasicType,
-                        (Context, Record[1], getMDString(Record[2]), Record[3],
+                        (Context, Record[1], getMDString(Record[2]), SizeInBits,
                          Record[4], Record[5], NumExtraInhabitants, Flags)),
         NextMetadataNo);
     NextMetadataNo++;
@@ -1543,9 +1554,12 @@ Error MetadataLoader::MetadataLoaderImpl::parseOneMetadata(
     if (Record.size() < 11)
       return error("Invalid record");
 
-    IsDistinct = Record[0];
+    IsDistinct = Record[0] & 1;
+    bool SizeIsMetadata = Record[0] & 2;
     DINode::DIFlags Flags = static_cast<DINode::DIFlags>(Record[6]);
 
+    Metadata *SizeInBits = getMetadataOrConstant(SizeIsMetadata, Record[3]);
+
     size_t Offset = 9;
 
     auto ReadWideInt = [&]() {
@@ -1565,7 +1579,7 @@ Error MetadataLoader::MetadataLoaderImpl::parseOneMetadata(
 
     MetadataList.assignValue(
         GET_OR_DISTINCT(DIFixedPointType,
-                        (Context, Record[1], getMDString(Record[2]), Record[3],
+                        (Context, Record[1], getMDString(Record[2]), SizeInBits,
                          Record[4], Record[5], Flags, Record[7], Record[8],
                          Numerator, Denominator)),
         NextMetadataNo);
@@ -1576,17 +1590,21 @@ Error MetadataLoader::MetadataLoaderImpl::parseOneMetadata(
     if (Record.size() > 9 || Record.size() < 8)
       return error("Invalid record");
 
-    IsDistinct = Record[0];
+    IsDistinct = Record[0] & 1;
+    bool SizeIsMetadata = Record[0] & 2;
     bool SizeIs8 = Record.size() == 8;
     // StringLocationExp (i.e. Record[5]) is added at a later time
     // than the other fields. The code here enables backward compatibility.
     Metadata *StringLocationExp = SizeIs8 ? nullptr : getMDOrNull(Record[5]);
     unsigned Offset = SizeIs8 ? 5 : 6;
+    Metadata *SizeInBits =
+        getMetadataOrConstant(SizeIsMetadata, Record[Offset]);
+
     MetadataList.assignValue(
         GET_OR_DISTINCT(DIStringType,
                         (Context, Record[1], getMDString(Record[2]),
                          getMDOrNull(Record[3]), getMDOrNull(Record[4]),
-                         StringLocationExp, Record[Offset], Record[Offset + 1],
+                         StringLocationExp, SizeInBits, Record[Offset + 1],
                          Record[Offset + 2])),
         NextMetadataNo);
     NextMetadataNo++;
@@ -1615,15 +1633,20 @@ Error MetadataLoader::MetadataLoaderImpl::parseOneMetadata(
         PtrAuthData.emplace(Record[14]);
     }
 
-    IsDistinct = Record[0];
+    IsDistinct = Record[0] & 1;
+    bool SizeIsMetadata = Record[0] & 2;
     DINode::DIFlags Flags = static_cast<DINode::DIFlags>(Record[10]);
+
+    Metadata *SizeInBits = getMetadataOrConstant(SizeIsMetadata, Record[7]);
+    Metadata *OffsetInBits = getMetadataOrConstant(SizeIsMetadata, Record[9]);
+
     MetadataList.assignValue(
         GET_OR_DISTINCT(DIDerivedType,
                         (Context, Record[1], getMDString(Record[2]),
                          getMDOrNull(Record[3]), Record[4],
                          getDITypeRefOrNull(Record[5]),
-                         getDITypeRefOrNull(Record[6]), Record[7], Record[8],
-                         Record[9], DWARFAddressSpace, PtrAuthData, Flags,
+                         getDITypeRefOrNull(Record[6]), SizeInBits, Record[8],
+                         OffsetInBits, DWARFAddressSpace, PtrAuthData, Flags,
                          getDITypeRefOrNull(Record[11]), Annotations)),
         NextMetadataNo);
     NextMetadataNo++;
@@ -1633,13 +1656,17 @@ Error MetadataLoader::MetadataLoaderImpl::parseOneMetadata(
     if (Record.size() != 13)
       return error("Invalid record");
 
-    IsDistinct = Record[0];
+    IsDistinct = Record[0] & 1;
+    bool SizeIsMetadata = Record[0] & 2;
     DINode::DIFlags Flags = static_cast<DINode::DIFlags>(Record[7]);
+
+    Metadata *SizeInBits = getMetadataOrConstant(SizeIsMetadata, Record[5]);
+
     MetadataList.assignValue(
         GET_OR_DISTINCT(DISubrangeType,
                         (Context, getMDString(Record[1]),
                          getMDOrNull(Record[2]), Record[3],
-                         getMDOrNull(Record[4]), Record[5], Record[6], Flags,
+                         getMDOrNull(Record[4]), SizeInBits, Record[6], Flags,
                          getDITypeRefOrNull(Record[8]), getMDOrNull(Record[9]),
                          getMDOrNull(Record[10]), getMDOrNull(Record[11]),
                          getMDOrNull(Record[12]))),
@@ -1654,18 +1681,18 @@ Error MetadataLoader::MetadataLoaderImpl::parseOneMetadata(
     // If we have a UUID and this is not a forward declaration, lookup the
     // mapping.
     IsDistinct = Record[0] & 0x1;
-    bool IsNotUsedInTypeRef = Record[0] >= 2;
+    bool IsNotUsedInTypeRef = Record[0] & 2;
+    bool SizeIsMetadata = Record[0] & 4;
     unsigned Tag = Record[1];
     MDString *Name = getMDString(Record[2]);
     Metadata *File = getMDOrNull(Record[3]);
     unsigned Line = Record[4];
     Metadata *Scope = getDITypeRefOrNull(Record[5]);
     Metadata *BaseType = nullptr;
-    uint64_t SizeInBits = Record[7];
     if (Record[8] > (uint64_t)std::numeric_limits<uint32_t>::max())
       return error("Alignment value is too large");
     uint32_t AlignInBits = Record[8];
-    uint64_t OffsetInBits = 0;
+    Metadata *OffsetInBits = nullptr;
     uint32_t NumExtraInhabitants = (Record.size() > 22) ? Record[22] : 0;
     DINode::DIFlags Flags = static_cast<DINode::DIFlags>(Record[10]);
     Metadata *Elements = nullptr;
@@ -1712,7 +1739,9 @@ Error MetadataLoader::MetadataLoaderImpl::parseOneMetadata(
         TemplateParams = getMDOrNull(Record[14]);
     } else {
       BaseType = getDITypeRefOrNull(Record[6]);
-      OffsetInBits = Record[9];
+
+      OffsetInBits = getMetadataOrConstant(SizeIsMetadata, Record[9]);
+
       Elements = getMDOrNull(Record[11]);
       VTableHolder = getDITypeRefOrNull(Record[13]);
       TemplateParams = getMDOrNull(Record[14]);
@@ -1740,6 +1769,8 @@ Error MetadataLoader::MetadataLoaderImpl::parseOneMetadata(
     if (Record.size() > 24 && Record[24] != dwarf::DW_APPLE_ENUM_KIND_invalid)
       EnumKind = Record[24];
 
+    Metadata *SizeInBits = getMetadataOrConstant(SizeIsMetadata, Record[7]);
+
     DICompositeType *CT = nullptr;
     if (Identifier)
       CT = DICompositeType::buildODRType(