[Lldb-commits] [lldb] [lldb][DWARFASTParserClang] Prevent unnamed bitfield creation in the presence of overlapping fields (PR #108343)

[Michael Buch via lldb-commits]

https://github.com/Michael137 created https://github.com/llvm/llvm-project/pull/108343

This bug surfaced after https://github.com/llvm/llvm-project/pull/105865 (currently reverted, but blocked on this to be relanded).

Because Clang doesn't emit `DW_TAG_member`s for unnamed bitfields, LLDB has to make an educated guess about whether they existed in the source. It does so by checking whether there is a gap between where the last field began and the currently parsed field starts. In the example test case, the empty field `padding` was folded into the storage of `data`. Because the `bit_offset` of `padding` is `0x0` and its `DW_AT_byte_size` is `0x1`, LLDB thinks the field ends at `0x1` (not quite because we first round the size to a word size, but this is an implementation detail), erroneously deducing that there's a gap between `flag` and `padding`.

This patch adds the notion of "effective field end", which accounts for fields that share storage. Its value is the end of the storage that the two fields occupy. Then we use this to check for gaps in the unnamed bitfield creation logic.

>From 51bca430ecb8be09ccacef5e616ea4ed9d85ab30 Mon Sep 17 00:00:00 2001
From: Michael Buch <michaelbuch12 at gmail.com>
Date: Thu, 12 Sep 2024 09:07:16 +0100
Subject: [PATCH] [lldb][DWARFASTParserClang] Prevent unnamed bitfield creation
 in the presence of overlapping fields

---
 .../SymbolFile/DWARF/DWARFASTParserClang.cpp  | 14 ++++++---
 .../SymbolFile/DWARF/DWARFASTParserClang.h    | 31 +++++++++++++++++++
 .../no_unique_address-with-bitfields.cpp      |  6 ----
 3 files changed, 41 insertions(+), 10 deletions(-)

diff --git a/lldb/source/Plugins/SymbolFile/DWARF/DWARFASTParserClang.cpp b/lldb/source/Plugins/SymbolFile/DWARF/DWARFASTParserClang.cpp
index 5b9de6f1566b05..88511e036fd8c2 100644
--- a/lldb/source/Plugins/SymbolFile/DWARF/DWARFASTParserClang.cpp
+++ b/lldb/source/Plugins/SymbolFile/DWARF/DWARFASTParserClang.cpp
@@ -2932,14 +2932,20 @@ void DWARFASTParserClang::ParseSingleMember(
     last_field_info = this_field_info;
     last_field_info.SetIsBitfield(true);
   } else {
-    last_field_info.bit_offset = field_bit_offset;
+    FieldInfo this_field_info{.is_bitfield = false};
+    this_field_info.bit_offset = field_bit_offset;
 
+    // TODO: we shouldn't silently ignore the bit_size if we fail
+    //       to GetByteSize.
     if (std::optional<uint64_t> clang_type_size =
             member_type->GetByteSize(nullptr)) {
-      last_field_info.bit_size = *clang_type_size * character_width;
+      this_field_info.bit_size = *clang_type_size * character_width;
     }
 
-    last_field_info.SetIsBitfield(false);
+    if (this_field_info.GetFieldEnd() < last_field_info.GetFieldEnd())
+      this_field_info.SetEffectiveFieldEnd(last_field_info.GetFieldEnd());
+
+    last_field_info = this_field_info;
   }
 
   // Don't turn artificial members such as vtable pointers into real FieldDecls
@@ -3738,7 +3744,7 @@ void DWARFASTParserClang::AddUnnamedBitfieldToRecordTypeIfNeeded(
     const FieldInfo &current_field) {
   // TODO: get this value from target
   const uint64_t word_width = 32;
-  uint64_t last_field_end = previous_field.bit_offset + previous_field.bit_size;
+  uint64_t last_field_end = previous_field.GetEffectiveFieldEnd();
 
   if (!previous_field.IsBitfield()) {
     // The last field was not a bit-field...
diff --git a/lldb/source/Plugins/SymbolFile/DWARF/DWARFASTParserClang.h b/lldb/source/Plugins/SymbolFile/DWARF/DWARFASTParserClang.h
index 3809ee912cec6f..4ff464ce26c548 100644
--- a/lldb/source/Plugins/SymbolFile/DWARF/DWARFASTParserClang.h
+++ b/lldb/source/Plugins/SymbolFile/DWARF/DWARFASTParserClang.h
@@ -258,9 +258,27 @@ class DWARFASTParserClang : public lldb_private::plugin::dwarf::DWARFASTParser {
 
 private:
   struct FieldInfo {
+    ///< Size in bits that this field occopies. Can but
+    ///< need not be the DW_AT_bit_size of the field.
     uint64_t bit_size = 0;
+
+    ///< Offset of this field in bits from the beginning
+    ///< of the containing struct. Can but need not
+    ///< be the DW_AT_data_bit_offset of the field.
     uint64_t bit_offset = 0;
+
+    ///< In case this field is folded into the storage
+    ///< of a previous member's storage (for example
+    ///< with [[no_unique_address]]), the effective field
+    ///< end is the offset in bits from the beginning of
+    ///< the containing struct where the field we were
+    ///< folded into ended.
+    std::optional<uint64_t> effective_field_end;
+
+    ///< Set to 'true' if this field is a bit-field.
     bool is_bitfield = false;
+
+    ///< Set to 'true' if this field is DW_AT_artificial.
     bool is_artificial = false;
 
     FieldInfo() = default;
@@ -276,6 +294,19 @@ class DWARFASTParserClang : public lldb_private::plugin::dwarf::DWARFASTParser {
       // bit offset than any previous bitfield + size.
       return (bit_size + bit_offset) <= next_bit_offset;
     }
+
+    /// Returns the offset in bits of where the storage this field
+    /// occupies ends.
+    uint64_t GetFieldEnd() const { return bit_size + bit_offset; }
+
+    void SetEffectiveFieldEnd(uint64_t val) { effective_field_end = val; }
+
+    /// If this field was folded into storage of a previous field,
+    /// returns the offset in bits of where that storage ends. Otherwise,
+    /// returns the regular field end (see \ref GetFieldEnd).
+    uint64_t GetEffectiveFieldEnd() const {
+      return effective_field_end.value_or(GetFieldEnd());
+    }
   };
 
   /// Parsed form of all attributes that are relevant for parsing type members.
diff --git a/lldb/test/Shell/SymbolFile/DWARF/no_unique_address-with-bitfields.cpp b/lldb/test/Shell/SymbolFile/DWARF/no_unique_address-with-bitfields.cpp
index 1c9cc36a711b47..b1672a384c9fe4 100644
--- a/lldb/test/Shell/SymbolFile/DWARF/no_unique_address-with-bitfields.cpp
+++ b/lldb/test/Shell/SymbolFile/DWARF/no_unique_address-with-bitfields.cpp
@@ -1,7 +1,3 @@
-// LLDB currently erroneously adds an unnamed bitfield
-// into the AST when an overlapping no_unique_address
-// field precedes a bitfield.
-
 // RUN: %clang --target=x86_64-apple-macosx -c -gdwarf -o %t %s
 // RUN: %lldb %t \
 // RUN:   -o "target var global" \
@@ -12,8 +8,6 @@
 // CHECK:      CXXRecordDecl {{.*}} struct Foo definition
 // CHECK:      |-FieldDecl {{.*}} data 'char[5]'
 // CHECK-NEXT: |-FieldDecl {{.*}} padding 'Empty'
-// CHECK-NEXT: |-FieldDecl {{.*}} 'int'
-// CHECK-NEXT: | `-IntegerLiteral {{.*}} 'int' 8
 // CHECK-NEXT: `-FieldDecl {{.*}} sloc> flag 'unsigned long'
 // CHECK-NEXT:   `-IntegerLiteral {{.*}} 'int' 1