[Bug c++/47658] New: -Os generates bigger code than -O2/3 for many small inline functions (objects)

[Kicer86 at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47658

           Summary: -Os generates bigger code than -O2/3 for many small
                    inline functions (objects)
           Product: gcc
           Version: 4.5.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassig...@gcc.gnu.org
        ReportedBy: kice...@gmail.com


I've noticed that, when I use a class which is a wrapper for a pointer to
volatile variable too often, then the -Os generates code bigger than -O2/-O3.

here's a big example:


class A
{
  volatile int * const ptr;

public:
  inline A(int * a):ptr(a)
  {}

  inline int read() const
  {
    return *ptr;
  }

  inline void write(int v) const
  {
    *ptr=v;
  }
};


class B
{
  const A a1,a2,a3;

public:
  inline B(int *a):a1(a),a2(a-2),a3(a-1)
  {}

  inline int operator()() const
  {
    return a2.read();
  }

  inline const B &operator=(int v) const
  {
    a1.write(v);
    return *this;
  }

  inline void foo(int v) const
  {
    a3.write(v);
  }
};

template<int *addr, int *addr2>
class C
{
public:
  C()
  {
    B foo(addr),bar(addr2);
    foo=56;
    foo.foo(67);

    bar=58;
    bar.foo(345);    
  };

  void foo1()
  {
    B foo(addr),bar(addr2);

    foo=bar();
    foo.foo(12);
    bar.foo(foo());

    foo2(2);
  }

  void foo2(int v)
  {
    B foo(addr),bar(addr2);

    bar=foo();
    bar.foo(34*v);
    foo.foo(bar());
  }

  void foo3()
  {
    B foo(addr),bar(addr2);

    for (int i=0;i<128; i++)
    {
      foo.foo(i);
      foo=bar();
      bar.foo(i+1);
    }
  }

};


template<int *addr, int *addr2>
class D
{
public:
  D()
  {
    B foo(addr),bar(addr2);
    foo=56;
    foo.foo(67);

    bar=58;
    bar.foo(345);
  };

  void foo1()
  {
    B foo(addr),bar(addr2);

    foo=bar();
    foo.foo(12);
    bar.foo(foo());

    foo2(2);
  }

  void foo2(int v)
  {
    B foo(addr),bar(addr2);

    bar=foo();
    bar.foo(34*v);
    foo.foo(bar());
  }

  void foo3()
  {
    B foo(addr),bar(addr2);

    for (int i=0;i<128; i++)
    {
      foo.foo(i);
      foo=bar();
      bar.foo(i+1);
    }
  }
};

template<int *addr, int *addr2>
class E
{
public:
  E()
  {
    B foo(addr),bar(addr2);
    foo=56;
    foo.foo(67);

    bar=58;
    bar.foo(345);
  };

  void foo1()
  {
    B foo(addr),bar(addr2);

    foo=bar();
    foo.foo(12);
    bar.foo(foo());

    foo2(2);
  }

  void foo2(int v)
  {
    B foo(addr),bar(addr2);

    bar=foo();
    bar.foo(34*v);
    foo.foo(bar());
  }

  void foo3()
  {
    B foo(addr),bar(addr2);

    for (int i=0;i<128; i++)
    {
      foo.foo(i);
      foo=bar();
      bar.foo(i+1);
    }
  }
};

int a,b;
int a2,b2;
int a3,b3;

void f() __attribute__((used));
void f()
{
  C<&a,&b> c;
  D<&a2,&b2> d;
  E<&a3,&b3> e;
  c.foo3();
  c.foo1();
  d.foo3();
  d.foo1();
  e.foo3();
  e.foo1();
}


int main()
{
  C<&a,&b> c;
  D<&a2,&b2> d;
  E<&a3,&b3> e;
  c.foo1();
  c.foo3();
  d.foo1();
  d.foo3();
  e.foo1();
  e.foo3();
  return 1;
}

class A is a wrapper, class B expands it. Then classes C and D are using class
B. While the program is small (just comment function void f()), all calls to
class' A and class' B functions are inlined and code is small (as all those
functions are simple movs). But then the code is big (like above) g++ expands
all functions to standalone ones (when -Os is used) and code is getting bigger.
-O2 keeps all functions to be inlined.

So the problem is that g++ thinks that many inlines of same functions are
bigger than calling non-inlined versions of them (which is wrong in this case).

[michal@Kicer michal]$ g++ -v
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-mandriva-linux-gnu/4.5.2/lto-wrapper
Target: x86_64-mandriva-linux-gnu
Configured with: ../gcc-4.5.2/configure --host=x86_64-mandriva-linux-gnu
--build=x86_64-mandriva-linux-gnu --program-prefix= --prefix=/usr
--exec-prefix=/usr --bindir=/usr/bin --sbindir=/usr/sbin --sysconfdir=/etc
--datadir=/usr/share --includedir=/usr/include --libdir=/usr/lib64
--libexecdir=/usr/libexec --localstatedir=/usr/var --sharedstatedir=/usr/com
--mandir=/usr/share/man --infodir=/usr/share/info --enable-checking=release
--enable-languages=c,c++,objc,obj-c++,fortran --with-system-zlib
--enable-threads=posix --enable-shared --enable-long-long --enable-__cxa_atexit
--disable-libunwind-exceptions --enable-clocale=gnu --enable-gtk-cairo
--disable-libjava-multilib --enable-ssp --disable-libssp --disable-werror
--enable-lto --program-suffix=-4.5.2 CFLAGS= CXXFLAGS=
Thread model: posix
gcc version 4.5.2 (GCC) 
[michal@Kicer michal]$