A Method

[=?gb2312?B?1cXUxrfl?= <kzyf76 at sohu dot com>]

I want to provide a method to let GCC optimaze more than before.

For example,if I input

{

    int a=1,b=2,c=3,d=4;

    int x=c+a+d;

    int y=b+a;

    int z=a+c+d+b;

}

GCC will assemble them to six addition instruction

{

    mov x,c    add x,a    add x,d

    mov y,b    add y,a

    mov z,a    add z,c    add z,d    add z,b

}

but GCC can do more better,use four addition instruction other than six

{

    mov y(temp1),a        add y,b

    mov temp2,c           add temp2,d

    mov x,a               add x,temp2

    mov z,y(temp1)        add z,temp2

}or{

    int y(temp1)=a+b;

    int temp2=c+d;

    int x=a+temp2;

    int z=temp2+y(temp1);

}

It's easy.I can provide an optimal method--Optimize Expression by Sorting by Variable(OESV) to solved it easily.

OESV:The first,sort all variables by dictionary,and then search all repeat expression and store them to temp variables,then optimize.

For example,optimize above

First sort all variables:

{

    int x=a+c+d;

    int y=a+b;

    int z=a+b+c+d;

}

Now,search all repeat expression about 'a',we will find 'a+b' is used twice,so y=temp1=a+b,this we get a varible--temp1=y,

repeat continueouly we will find an other varible temp2=c+d,in a word...it's easy to computer

With y(temp1) and temp2 help,we can optimaze more than before,isn't it?

A Note:(about function)

{

    int x=a+myfun(m+n);

    int y=b+myfun(m+n)+a;

    int myfun(k){return int k+ret<<stdin;}//notice myfun implenments by stdin

}

If we product an optimazing temp=a+myfun(m+n),it isn't always correct!Because myfun(k)!=myfun(k)!!(due to stdin,it's uncertain when call them twice)

but the optimal method above is correct in the bulk cases if we set up a table about <cmath> and sign all function which fun(x)==fun(x),most programers will benefit from my works.

That's all.If you have problems,email me!

gcc:2.95

I hope found OESV in GCC3.0

                                    yfzhang

                                    real name:Zhang Yunfeng(China)