Different local stack variable policy (g++/clang)

different_stack_variable_policy.cpp

/*
g++ version : -W
struct size = 7
stack version addr diff = 16
array version addr diff = 7
new[] version addr diff = 7
struct size = 15
stack version addr diff = 16
array version addr diff = 15
new[] version addr diff = 15
struct size = 31
stack version addr diff = 32
array version addr diff = 31
new[] version addr diff = 31

clang++ version : -W
struct size = 7
stack version addr diff = -8
array version addr diff = 7
new[] version addr diff = 7
struct size = 15
stack version addr diff = -16
array version addr diff = 15
new[] version addr diff = 15
struct size = 31
stack version addr diff = -32
array version addr diff = 31
new[] version addr diff = 31

g++ version : -W -O3
struct size = 7
stack version addr diff = 16
array version addr diff = 7
new[] version addr diff = 7
struct size = 15
stack version addr diff = 16
array version addr diff = 15
new[] version addr diff = 15
struct size = 31
stack version addr diff = 32
array version addr diff = 31
new[] version addr diff = 31

clang++ version : -W -O3
struct size = 7
stack version addr diff = -32
array version addr diff = 7
new[] version addr diff = 7
struct size = 15
stack version addr diff = -32
array version addr diff = 15
new[] version addr diff = 15
struct size = 31
stack version addr diff = -32
array version addr diff = 31
new[] version addr diff = 31
*/

#include <cstdio>
#include <cassert>

template<int N>
class Sample {
  char padding[N];
};

template<typename T>
void stack_version() 
{
  T a;
  T b;
  int diff = (unsigned long)&b - (unsigned long)&a;
  printf("stack version addr diff = %d\n", diff);
}

template<typename T>
void array_version()
{
  T arr[2];
  int diff = (unsigned long)&arr[1] - (unsigned long)&arr[0];
  printf("array version addr diff = %d\n", diff);
}

template<typename T>
void new_array_version()
{
  T *data = new T[2];
  int diff = (unsigned long)(data+1) - (unsigned long)data;
  printf("new[] version addr diff = %d\n", diff);
  delete[](data);
}

template<typename T>
void test_main()
{
  printf("struct size = %d\n", (int)sizeof(T));
  stack_version<T>();
  array_version<T>();
  new_array_version<T>();
}

int main()
{
  test_main< Sample<7> >();
  test_main< Sample<15> >();
  test_main< Sample<31> >();

  return 0;
}

Makefile

CXX_FLAGS	= -W
SRC	= different_stack_variable_policy.cpp

all:
	@make gcc
	@echo ""
	@make clang

gcc:
	@g++ $(SRC) $(CXX_FLAGS)
	@echo "g++ version : $(CXX_FLAGS)"
	@./a.out

clang:
	@clang++ $(SRC) $(CXX_FLAGS)
	@echo "clang++ version : $(CXX_FLAGS)"
	@./a.out

http://stackoverflow.com/questions/1102049/order-of-local-variable-allocation-on-the-stack