factorial: let's get ahead of jhc! :)

[Bulat Ziganshin]

Hello Bulat,

Friday, February 24, 2006, 6:37:42 PM, you wrote:

SPJ>> Perhaps you may consider doing this transformation on the C-- data type
SPJ>> only, without involving the (already very complicated) STG -> C-- code
SPJ>> generator?

i have found my investiations in this area. that is the C-- code
generated for fac worker:

Fac_zdwfac_entry() {
    c1C0:
        _s1BD = I32[Sp + 0];
        if (_s1BD != 1) goto c1C4;
        R1 = I32[Sp + 4];
        Sp = Sp + 8;
        jump (I32[Sp + 0]);
    c1C4:
        _s1BI = _s1BD * I32[Sp + 4];
        _s1BF = _s1BD - 1;
        I32[Sp + 4] = _s1BI;
        I32[Sp + 0] = _s1BF;
        jump c1C0;
}

first, we convert jump to the explicit loop:

        _s1BD = I32[Sp + 0];
        while (_s1BD != 1) {
            _s1BI = _s1BD * I32[Sp + 4];
            _s1BF = _s1BD - 1;
            I32[Sp + 4] = _s1BI;
            I32[Sp + 0] = _s1BF;
            _s1BD = I32[Sp + 0];
        }
        R1 = I32[Sp + 4];
        Sp = Sp + 8;
        jump (I32[Sp + 0]);

then, we cache contents of sp[*] variables in the local ones:

        sp4 = I32[Sp + 4];
        sp0 = I32[Sp + 0];
        _s1BD = sp0;
        while (_s1BD != 1) {
            _s1BI = _s1BD * sp4;
            _s1BF = _s1BD - 1;
            sp4 = _s1BI;
            sp0 = _s1BF;
            _s1BD = sp0;
        }
        I32[Sp + 4] = sp4;
        I32[Sp + 0] = sp0;
        R1 = I32[Sp + 4];
        Sp = Sp + 8;
        jump (I32[Sp + 0]);

and then we wipe out all the superfluous variables:

        sp4 = I32[Sp + 4];
        sp0 = I32[Sp + 0];
        while (sp0 != 1) {
            sp4 = sp0 * sp4;
            sp0 = sp0 - 1;
        }
        R1 = sp4;
        Sp = Sp + 8;
        jump (I32[Sp + 0]);

and it is even for simple fac() function!!! instead of straightforward
generation of code that we need we should make all these nontrivial
studying and hard transformations on already generated code. on the
other side, we can just generate the following code right from the
STG:

int fac () {
        sp4 = I32[Sp + 4];
        sp0 = I32[Sp + 0];
        while (sp0 != 1) {
            sp4 = sp0 * sp4;
            sp0 = sp0 - 1;
        }
        R1 = sp4;
        Sp = Sp + 8;
        jump (I32[Sp + 0]);
}

i think that my way is 100x simpler
        
-- 
Best regards,
 Bulat                            mailto:Bulat.Ziganshin at gmail.com