[LLDB][NativePDB] Estimate symbol sizes (llvm#165727)

Nerixyz · Debadri Basak · commit 99afa70ddc3f · 2025-11-03T11:35:26.000Z
In llvm#165604, a test was skipped on Windows, because the native PDB plugin didn't set sizes on symbols. While the test isn't compiled with debug info, it's linked with `-gdwarf`, causing a PDB to be created on Windows. This PDB will only contain the public symbols (written by the linker) and section information. The symbols themselves don't have a size, however the DIA SDK sets a size for them. It seems like, for these data symbols, the size given from DIA is the distance to the next symbol (or the section end). This PR implements the naive approach for the native plugin. The main difference is in function/code symbols. There, DIA searches for a corresponding `S_GPROC32` which have a "code size" that is sometimes slightly smaller than the difference to the next symbol.
diff --git a/lldb/source/Plugins/SymbolFile/NativePDB/SymbolFileNativePDB.cpp b/lldb/source/Plugins/SymbolFile/NativePDB/SymbolFileNativePDB.cpp
@@ -1130,16 +1130,47 @@ void SymbolFileNativePDB::AddSymbols(Symtab &symtab) {
   if (!section_list)
     return;
 
-  for (auto pid : m_index->publics().getPublicsTable()) {
+  PublicSym32 last_sym;
+  size_t last_sym_idx = 0;
+  lldb::SectionSP section_sp;
+
+  // To estimate the size of a symbol, we use the difference to the next symbol.
+  // If there's no next symbol or the section/segment changed, the symbol will
+  // take the remaining space. The estimate can be too high in case there's
+  // padding between symbols. This similar to the algorithm used by the DIA
+  // SDK.
+  auto finish_last_symbol = [&](const PublicSym32 *next) {
+    if (!section_sp)
+      return;
+    Symbol *last = symtab.SymbolAtIndex(last_sym_idx);
+    if (!last)
+      return;
+
+    if (next && last_sym.Segment == next->Segment) {
+      assert(last_sym.Offset <= next->Offset);
+      last->SetByteSize(next->Offset - last_sym.Offset);
+    } else {
+      // the last symbol was the last in its section
+      assert(section_sp->GetByteSize() >= last_sym.Offset);
+      assert(!next || next->Segment > last_sym.Segment);
+      last->SetByteSize(section_sp->GetByteSize() - last_sym.Offset);
+    }
+  };
+
+  // The address map is sorted by the address of a symbol.
+  for (auto pid : m_index->publics().getAddressMap()) {
     PdbGlobalSymId global{pid, true};
     CVSymbol sym = m_index->ReadSymbolRecord(global);
     auto kind = sym.kind();
     if (kind != S_PUB32)
       continue;
     PublicSym32 pub =
         llvm::cantFail(SymbolDeserializer::deserializeAs<PublicSym32>(sym));
+    finish_last_symbol(&pub);
+
+    if (!section_sp || last_sym.Segment != pub.Segment)
+      section_sp = section_list->FindSectionByID(pub.Segment);
 
-    auto section_sp = section_list->FindSectionByID(pub.Segment);
     if (!section_sp)
       continue;
 
@@ -1148,20 +1179,24 @@ void SymbolFileNativePDB::AddSymbols(Symtab &symtab) {
         (pub.Flags & PublicSymFlags::Code) != PublicSymFlags::None)
       type = eSymbolTypeCode;
 
-    symtab.AddSymbol(Symbol(/*symID=*/pid,
-                            /*name=*/pub.Name,
-                            /*type=*/type,
-                            /*external=*/true,
-                            /*is_debug=*/true,
-                            /*is_trampoline=*/false,
-                            /*is_artificial=*/false,
-                            /*section_sp=*/section_sp,
-                            /*value=*/pub.Offset,
-                            /*size=*/0,
-                            /*size_is_valid=*/false,
-                            /*contains_linker_annotations=*/false,
-                            /*flags=*/0));
-  }
+    last_sym_idx =
+        symtab.AddSymbol(Symbol(/*symID=*/pid,
+                                /*name=*/pub.Name,
+                                /*type=*/type,
+                                /*external=*/true,
+                                /*is_debug=*/true,
+                                /*is_trampoline=*/false,
+                                /*is_artificial=*/false,
+                                /*section_sp=*/section_sp,
+                                /*value=*/pub.Offset,
+                                /*size=*/0,
+                                /*size_is_valid=*/false,
+                                /*contains_linker_annotations=*/false,
+                                /*flags=*/0));
+    last_sym = pub;
+  }
+
+  finish_last_symbol(nullptr);
 }
 
 size_t SymbolFileNativePDB::ParseFunctions(CompileUnit &comp_unit) {
diff --git a/lldb/test/API/functionalities/multiple-slides/TestMultipleSlides.py b/lldb/test/API/functionalities/multiple-slides/TestMultipleSlides.py
@@ -12,10 +12,6 @@
 class MultipleSlidesTestCase(TestBase):
     NO_DEBUG_INFO_TESTCASE = True
 
-    # The intermediate object main.o is compiled without debug info, but
-    # a.out is linked with `-gdwarf` on Windows. This creates a PDB.
-    # However, in the native PDB plugin, the symbols don't have a size.
-    @expectedFailureWindows
     def test_mulitple_slides(self):
         """Test that a binary can be slid multiple times correctly."""
         self.build()
@@ -33,10 +29,13 @@ def test_mulitple_slides(self):
             first_sym.GetEndAddress().GetOffset()
             - first_sym.GetStartAddress().GetOffset()
         )
+        int_size = target.FindFirstType("int").GetByteSize()
+        self.assertGreaterEqual(first_size, 2048 * int_size)
         second_size = (
             second_sym.GetEndAddress().GetOffset()
             - second_sym.GetStartAddress().GetOffset()
         )
+        self.assertGreaterEqual(second_size, 2048 * int_size)
 
         # View the first element of `first` and `second` while
         # they have no load address set.
diff --git a/lldb/test/Shell/SymbolFile/NativePDB/symtab.cpp b/lldb/test/Shell/SymbolFile/NativePDB/symtab.cpp
@@ -42,18 +42,18 @@ int main(int argc, char **argv) {
   return ns::a_function() + b.b_func();
 }
 
-// CHECK-DAG: Code {{.*}} main
-// CHECK-DAG: Code {{.*}} ?b_func@?$B@F@ns@@QEBAHXZ
-// CHECK-DAG: Code {{.*}} ?something@A@@QEAAXXZ
-// CHECK-DAG: Code {{.*}} ??_GDyn@ns@@UEAAPEAXI@Z
-// CHECK-DAG: Code {{.*}} ??2@YAPEAX_K@Z
-// CHECK-DAG: Code {{.*}} ??3@YAXPEAX_K@Z
-// CHECK-DAG: Code {{.*}} ?static_fn@C@?$B@H@ns@@SAHXZ
-// CHECK-DAG: Code {{.*}} ?a_function@ns@@YAHXZ
-// CHECK-DAG: Code {{.*}} ?static_fn@C@?$B@_N@ns@@SAHXZ
-// CHECK-DAG: Code {{.*}} ??1Dyn@ns@@UEAA@XZ
-// CHECK-DAG: Code {{.*}} ??0Dyn@ns@@QEAA@XZ
-// CHECK-DAG: Data {{.*}} ?global_int@@3HA
-// CHECK-DAG: Data {{.*}} ??_7Dyn@ns@@6B@
-// CHECK-DAG: Data {{.*}} ?global_a@@3UA@@A
-// CHECK-DAG: Data {{.*}} ?global_c@@3UC@?$B@_J@ns@@A
+// CHECK-DAG: Code 0x{{[0-9a-f]+}} 0x{{0*[1-9a-f][0-9a-f]*}} 0x00000000 main
+// CHECK-DAG: Code 0x{{[0-9a-f]+}} 0x{{0*[1-9a-f][0-9a-f]*}} 0x00000000 ?b_func@?$B@F@ns@@QEBAHXZ
+// CHECK-DAG: Code 0x{{[0-9a-f]+}} 0x{{0*[1-9a-f][0-9a-f]*}} 0x00000000 ?something@A@@QEAAXXZ
+// CHECK-DAG: Code 0x{{[0-9a-f]+}} 0x{{0*[1-9a-f][0-9a-f]*}} 0x00000000 ??_GDyn@ns@@UEAAPEAXI@Z
+// CHECK-DAG: Code 0x{{[0-9a-f]+}} 0x{{0*[1-9a-f][0-9a-f]*}} 0x00000000 ??2@YAPEAX_K@Z
+// CHECK-DAG: Code 0x{{[0-9a-f]+}} 0x{{0*[1-9a-f][0-9a-f]*}} 0x00000000 ??3@YAXPEAX_K@Z
+// CHECK-DAG: Code 0x{{[0-9a-f]+}} 0x{{0*[1-9a-f][0-9a-f]*}} 0x00000000 ?static_fn@C@?$B@H@ns@@SAHXZ
+// CHECK-DAG: Code 0x{{[0-9a-f]+}} 0x{{0*[1-9a-f][0-9a-f]*}} 0x00000000 ?a_function@ns@@YAHXZ
+// CHECK-DAG: Code 0x{{[0-9a-f]+}} 0x{{0*[1-9a-f][0-9a-f]*}} 0x00000000 ?static_fn@C@?$B@_N@ns@@SAHXZ
+// CHECK-DAG: Code 0x{{[0-9a-f]+}} 0x{{0*[1-9a-f][0-9a-f]*}} 0x00000000 ??1Dyn@ns@@UEAA@XZ
+// CHECK-DAG: Code 0x{{[0-9a-f]+}} 0x{{0*[1-9a-f][0-9a-f]*}} 0x00000000 ??0Dyn@ns@@QEAA@XZ
+// CHECK-DAG: Data 0x{{[0-9a-f]+}} 0x{{0*[1-9a-f][0-9a-f]*}} 0x00000000 ?global_int@@3HA
+// CHECK-DAG: Data 0x{{[0-9a-f]+}} 0x{{0*[1-9a-f][0-9a-f]*}} 0x00000000 ??_7Dyn@ns@@6B@
+// CHECK-DAG: Data 0x{{[0-9a-f]+}} 0x{{0*[1-9a-f][0-9a-f]*}} 0x00000000 ?global_a@@3UA@@A
+// CHECK-DAG: Data 0x{{[0-9a-f]+}} 0x{{0*[1-9a-f][0-9a-f]*}} 0x00000000 ?global_c@@3UC@?$B@_J@ns@@A
