Slightly improve memory usage by packing some structs used in the die cache

CMakeLists.txt

@@ -223,6 +223,10 @@ target_compile_features(
 
 target_compile_definitions(${target_name} PRIVATE NOMINMAX)
 
+if(HAS_ATTRIBUTE_PACKED)
+  target_compile_definitions(${target_name} PRIVATE HAS_ATTRIBUTE_PACKED)
+endif()
+
 if(NOT CPPTRACE_STD_FORMAT)
   target_compile_definitions(${target_name} PUBLIC CPPTRACE_NO_STD_FORMAT)
 endif()

cmake/Autoconfig.cmake

@@ -15,6 +15,10 @@ if(NOT CMAKE_CXX_COMPILER_ID STREQUAL "MSVC")
   check_support(HAS_CXXABI has_cxxabi.cpp "" "" "")
 endif()
 
+if(NOT CMAKE_CXX_COMPILER_ID STREQUAL "MSVC")
+  check_support(HAS_ATTRIBUTE_PACKED has_attribute_packed.cpp "" "" "")
+endif()
+
 if(NOT WIN32)
   check_support(HAS_UNWIND has_unwind.cpp "" "" "")
   check_support(HAS_EXECINFO has_execinfo.cpp "" "" "")

cmake/has_attribute_packed.cpp

@@ -0,0 +1,6 @@
+struct __attribute__((packed)) foo {
+    int i;
+    double d;
+};
+
+int main() {}

src/symbols/dwarf/dwarf_resolver.cpp

@@ -21,6 +21,7 @@
 #include <cstdint>
 #include <cstdio>
 #include <functional>
+#include <limits>
 #include <memory>
 #include <string>
 #include <type_traits>
@@ -44,19 +45,15 @@ namespace libdwarf {
     class die_cache {
     public:
         struct die_handle {
-            std::size_t die_index;
+            std::uint32_t die_index;
         };
     private:
-        struct basic_range_entry {
+        struct PACKED basic_range_entry {
             die_handle die;
             Dwarf_Addr low;
             Dwarf_Addr high;
         };
-        static_assert(
-            sizeof(basic_range_entry) == 3 * sizeof(void*),
-            "Expected basic_range_entry to be smaller (this is memory critical)"
-        );
-        struct annotated_range_entry {
+        struct PACKED annotated_range_entry {
             die_handle die;
             Dwarf_Addr low;
             Dwarf_Addr high;
@@ -72,7 +69,8 @@ namespace libdwarf {
     public:
         die_handle add_die(die_object&& die) {
             dies.push_back(std::move(die));
-            return die_handle{dies.size() - 1};
+            VERIFY(dies.size() < std::numeric_limits<std::uint32_t>::max());
+            return die_handle{static_cast<std::uint32_t>(dies.size() - 1)};
         }
         template<typename Void = void>
         auto insert(die_handle die, Dwarf_Addr low, Dwarf_Addr high)

src/utils/common.hpp

@@ -30,6 +30,13 @@
  #define MSVC_CDECL
 #endif
 
+// support is pretty good https://godbolt.org/z/djTqv7WMY, checked in cmake during config
+#ifdef HAS_ATTRIBUTE_PACKED
+ #define PACKED __attribute__((packed))
+#else
+ #define PACKED
+#endif
+
 namespace cpptrace {
 namespace detail {
     static const stacktrace_frame null_frame {