Dump C/C++ vtable & record layout information (clang + msvc + gcc)

Makefile

# Requirements:
#   clang - The classes/structs you want to dump must be used in code at least once, not just defined.
#   MSVC  - The classes/structs you want to dump must have "MEOW" in the name for "reportSingleClass" to work.
# Usage:
#   $ make dump_vtables file=test.cpp
dump_vtables:
  clang -cc1 -fdump-record-layouts -emit-llvm  $(file) > clang-vtable-layout-$(file).txt
  clang -cc1 -fdump-vtable-layouts -emit-llvm  $(file) > clang-record-layout-$(file).txt
  g++ -fdump-lang-class=$(file).txt $(file)
  cl.exe $(file) /d1reportSingleClassLayoutMEOW > msvc-single-class-vtable-layout-$(file).txt
  cl.exe $(file) /d1reportAllClassLayout > msvc-all-class-vtable-layout-$(file).txt

Hi, how to read the outputs of -fdump-record-layouts and -fdump-vtable-layouts -emit-llvm? I don't find any official doc about these two commands.

Heya,

On the lefthand side are the offsets from the start of the struct for each field, and on the right is the description of the field itself.
At the bottom is summary information:

*** Dumping AST Record Layout
   0 | class StarObject
   0 |   class SkyObject (primary base)
   0 |     class SkyPoint (primary base)
   0 |       (SkyPoint vtable pointer)
   0 |       (SkyPoint vftable pointer)
  16 |       long double lastPrecessJD
  32 |       class dms RA0
  32 |         double D
     |       [sizeof=8, dsize=8, align=8
     |        nvsize=8, nvalign=8]
  ...(snipped)...
 184 |   float B
 188 |   float V
     | [sizeof=192, dsize=192, align=16
     |  nvsize=192, nvalign=16]

One other useful bit of information is there is another form of this flag, -fdump-record-layouts-simple, which gives output like below:

Layout: <ASTRecordLayout
  Size:64
  DataSize:64
  Alignment:32
  FieldOffsets: [0, 32]>