[vm] Fix record instance layout with compressed pointers

After record shape representation changed in https://dart-review.googlesource.com/c/sdk/+/276201,
clang started to insert an extra alignment padding
between 'shape' and 'data' fields in record instances
in compressed pointers mode. This padding is not initialized
in the allocation stubs, but GC scans it as it falls between
'from' (shape) and 'to' (end of data). This change adjusts
layout of UntaggedRecord in compressed pointers mode to avoid
the implicit padding and GC visiting garbage.

TEST=Manual run of language/records/simple tests and
RecordCollections benchmark on x64c and simarm64c architectures.

Issue: #49719
Change-Id: I4b8a5b36c3ff757ccbbdb46c20d9ed8d12c53f96
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/276775
Commit-Queue: Alexander Markov <[email protected]>
Reviewed-by: Alexander Aprelev <[email protected]>

Author: alexmarkov

runtime/vm/compiler/runtime_offsets_extracted.h

@@ -2985,7 +2985,7 @@ static constexpr dart::compiler::target::word OneByteString_data_offset = 16;
 static constexpr dart::compiler::target::word PointerBase_data_offset = 8;
 static constexpr dart::compiler::target::word Pointer_type_arguments_offset =
     16;
-static constexpr dart::compiler::target::word Record_shape_offset = 8;
+static constexpr dart::compiler::target::word Record_shape_offset = 12;
 static constexpr dart::compiler::target::word
     SingleTargetCache_entry_point_offset = 16;
 static constexpr dart::compiler::target::word
@@ -3667,7 +3667,7 @@ static constexpr dart::compiler::target::word OneByteString_data_offset = 16;
 static constexpr dart::compiler::target::word PointerBase_data_offset = 8;
 static constexpr dart::compiler::target::word Pointer_type_arguments_offset =
     16;
-static constexpr dart::compiler::target::word Record_shape_offset = 8;
+static constexpr dart::compiler::target::word Record_shape_offset = 12;
 static constexpr dart::compiler::target::word
     SingleTargetCache_entry_point_offset = 16;
 static constexpr dart::compiler::target::word
@@ -8387,7 +8387,7 @@ static constexpr dart::compiler::target::word OneByteString_data_offset = 16;
 static constexpr dart::compiler::target::word PointerBase_data_offset = 8;
 static constexpr dart::compiler::target::word Pointer_type_arguments_offset =
     16;
-static constexpr dart::compiler::target::word Record_shape_offset = 8;
+static constexpr dart::compiler::target::word Record_shape_offset = 12;
 static constexpr dart::compiler::target::word
     SingleTargetCache_entry_point_offset = 16;
 static constexpr dart::compiler::target::word
@@ -9061,7 +9061,7 @@ static constexpr dart::compiler::target::word OneByteString_data_offset = 16;
 static constexpr dart::compiler::target::word PointerBase_data_offset = 8;
 static constexpr dart::compiler::target::word Pointer_type_arguments_offset =
     16;
-static constexpr dart::compiler::target::word Record_shape_offset = 8;
+static constexpr dart::compiler::target::word Record_shape_offset = 12;
 static constexpr dart::compiler::target::word
     SingleTargetCache_entry_point_offset = 16;
 static constexpr dart::compiler::target::word
@@ -13376,7 +13376,7 @@ static constexpr dart::compiler::target::word AOT_OneByteString_data_offset =
 static constexpr dart::compiler::target::word AOT_PointerBase_data_offset = 8;
 static constexpr dart::compiler::target::word
     AOT_Pointer_type_arguments_offset = 16;
-static constexpr dart::compiler::target::word AOT_Record_shape_offset = 8;
+static constexpr dart::compiler::target::word AOT_Record_shape_offset = 12;
 static constexpr dart::compiler::target::word
     AOT_SingleTargetCache_entry_point_offset = 16;
 static constexpr dart::compiler::target::word
@@ -14131,7 +14131,7 @@ static constexpr dart::compiler::target::word AOT_OneByteString_data_offset =
 static constexpr dart::compiler::target::word AOT_PointerBase_data_offset = 8;
 static constexpr dart::compiler::target::word
     AOT_Pointer_type_arguments_offset = 16;
-static constexpr dart::compiler::target::word AOT_Record_shape_offset = 8;
+static constexpr dart::compiler::target::word AOT_Record_shape_offset = 12;
 static constexpr dart::compiler::target::word
     AOT_SingleTargetCache_entry_point_offset = 16;
 static constexpr dart::compiler::target::word
@@ -18620,7 +18620,7 @@ static constexpr dart::compiler::target::word AOT_OneByteString_data_offset =
 static constexpr dart::compiler::target::word AOT_PointerBase_data_offset = 8;
 static constexpr dart::compiler::target::word
     AOT_Pointer_type_arguments_offset = 16;
-static constexpr dart::compiler::target::word AOT_Record_shape_offset = 8;
+static constexpr dart::compiler::target::word AOT_Record_shape_offset = 12;
 static constexpr dart::compiler::target::word
     AOT_SingleTargetCache_entry_point_offset = 16;
 static constexpr dart::compiler::target::word
@@ -19366,7 +19366,7 @@ static constexpr dart::compiler::target::word AOT_OneByteString_data_offset =
 static constexpr dart::compiler::target::word AOT_PointerBase_data_offset = 8;
 static constexpr dart::compiler::target::word
     AOT_Pointer_type_arguments_offset = 16;
-static constexpr dart::compiler::target::word AOT_Record_shape_offset = 8;
+static constexpr dart::compiler::target::word AOT_Record_shape_offset = 12;
 static constexpr dart::compiler::target::word
     AOT_SingleTargetCache_entry_point_offset = 16;
 static constexpr dart::compiler::target::word

runtime/vm/raw_object.h

@@ -3224,6 +3224,13 @@ COMPILE_ASSERT(sizeof(UntaggedFloat64x2) == 24);
 class UntaggedRecord : public UntaggedInstance {
   RAW_HEAP_OBJECT_IMPLEMENTATION(Record);
 
+#if defined(DART_COMPRESSED_POINTERS)
+  // This explicit padding avoids implict padding between [shape] and [data].
+  // Record allocation doesn't initialize the implicit padding but GC scans
+  // everything between 'from' (shape) and 'to' (end of data),
+  // so it would see garbage if implicit padding is inserted.
+  uint32_t padding_;
+#endif
   COMPRESSED_SMI_FIELD(SmiPtr, shape)
   VISIT_FROM(shape)
   // Variable length data follows here.