Date: Wed, 28 Jun 95 11:22 MDT
From: mat AT ardi DOT com (Mat Hostetter)
To: "George C. Moschovitis" <gmoscho AT alexander DOT cc DOT ece DOT ntua DOT gr>
Subject: Re: optimisation...
References: <199506280836 DOT LAA02806 AT skorpios DOT cc DOT ece DOT ntua DOT gr >
Cc: djgpp AT sun DOT soe DOT clarkson DOT edu

>>>>> "George" == George C Moschovitis <gmoscho AT alexander DOT cc DOT ece DOT ntua DOT gr> writes:

    George> is there anyway i can rearange the c++ code for the
    George> compiler to produce better output ? or any switch i should
    George> use ?
  
Returning structs is often slow in C, depending on the calling
convention used by the compiler.  Why not pass in a pointer to the
struct you want to fill in?  That's the standard C idiom.  For
example:

inline void
__dpmi_allocate_low_memory(int size, __dpmi_memblock *m)
{
  m->l.segment = __dpmi_allocate_dos_memory((size>>4)+1,&m->l.selector);
  m->l.size    = size;
}

By the way, "(size + 15) >> 4" will do the right thing and also waste
less memory.

Who cares how slow your function to allocate DOS memory is anyway?
It's got so much other overhead involved that a few cycles won't
matter.  Don't fall into the trap of wasting all your time optimizing
irrelevant code...I've seen too many programmers make that mistake.

    George> From the (admitedly) limited asm outputs i
    George> have seen gcc doesnt seem to use register passing that
    George> much :( On this topic how can i tell the compiler to which
    George> registers to pass the parameters ?

On the x86 gcc doesn't pass arguments in registers, it passes them on
the stack.  If you inline a function, of course, arguments won't get
passed at all.

-Mat