From 98fd1f9c47fc442f179c0f104865b352583780ca Mon Sep 17 00:00:00 2001
From: Jesse Ruderman <jruderman@gmail.com>
Date: Sun, 11 Jul 2021 07:45:55 -0700
Subject: [PATCH] Use offsetof to reduce UB

---
 Makefile      | 1 +
 locations.cpp | 5 ++---
 mymake.cpp    | 2 +-
 3 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/Makefile b/Makefile
index d74aade6..59744a49 100644
--- a/Makefile
+++ b/Makefile
@@ -84,6 +84,7 @@ ifeq (${TOOLCHAIN},clang)
   CXXFLAGS_EARLY += -march=native -fPIC
   CXXFLAGS_EARLY += -W -Wall -Wextra -Werror -pedantic
   CXXFLAGS_EARLY += -Wno-unused-parameter -Wno-implicit-fallthrough -Wno-maybe-uninitialized -Wno-unknown-warning-option
+  CXXFLAGS_EARLY += -Wno-invalid-offsetof
 endif
 
 ifeq (${TOOLCHAIN},gcc)
diff --git a/locations.cpp b/locations.cpp
index e4f564a3..904110fd 100644
--- a/locations.cpp
+++ b/locations.cpp
@@ -134,7 +134,7 @@ template<class T> struct connection_table {
   unsigned char *spintable() { return (unsigned char*) (&move_table[full()->degree()]); }
 
   /** \brief get the full T from the pointer to this connection table */
-  T* full() { T* x = (T*) this; return (T*)((char*)this - ((char*)(&(x->c)) - (char*)x)); }
+  T* full() { return (T*)((char*)this - offsetof(T, c)); }
   /** \brief for the edge d, set the `spin` and `mirror` attributes */
   void setspin(int d, int spin, bool mirror) { 
     unsigned char& c = spintable() [d];
@@ -178,10 +178,9 @@ template<class T> struct connection_table {
  */
 
 template<class T> T* tailored_alloc(int degree) {
-  const T* sample = nullptr;
   T* result;
 #ifndef NO_TAILORED_ALLOC
-  int b = (char*)&sample->c.move_table[degree] + degree - (char*) sample;
+  int b = offsetof(T, c) + offsetof(connection_table<T>, move_table) + sizeof(T*) * degree + degree;
   result = (T*) new char[b];
   new (result) T();
 #else
diff --git a/mymake.cpp b/mymake.cpp
index 9d089e62..f29bb853 100644
--- a/mymake.cpp
+++ b/mymake.cpp
@@ -75,7 +75,7 @@ void set_linux() {
 
 void set_mac() {
   preprocessor = "g++ -E";
-  compiler = "g++ -march=native -W -Wall -Wextra -pedantic -Wno-unused-parameter -Wno-implicit-fallthrough -c";
+  compiler = "g++ -march=native -W -Wall -Wextra -pedantic -Wno-unused-parameter -Wno-implicit-fallthrough -Wno-invalid-offsetof -c";
   linker = "g++ -o hyper";
   opts = "-DMAC -I/usr/local/include";
   libs = " -L/usr/local/lib -framework AppKit -framework OpenGL -lSDL -lSDLMain -lSDL_gfx -lSDL_mixer -lSDL_ttf -lpng -lpthread -lz";