Optimization

["Piotr Wyderski" <piotr dot wyderski at hoga dot pl>]

Hello,

I've just looked at the binary code produced by the GCC 3.2
C++ compiler and don't understand your new optimizer's
behaviour. Why don't you optimize the "jump to ret" construction?
I mean this:

    if (b)
        C;
    else
        D;

produces:

    cmp/test b
    je    L2
    C
L1:    ret

L2:  D
    jmp L1

instead of:

    cmp/test b
    je     L2
    C
    ret

L2:  D
    ret

I see that there's also a greedy stack frame allocator instead
of a lazy one. Using the previous example

    if (b)
        C;
    else
        D;

when C is short and can be computed using only the scratch
registers, i.e. eax, edx and ecx, and D is complex and needs
to save/restore the remaining GP registers, GCC generates:

    push max_register_usage(C,D);
    test b
    je L2
    C

L1:   pop max_register_usage(C,D);
    ret

L2: D
    jmp L1,

but it could do this:

    test b
    je L2
    C
    ret

L2:    push register_usage(D);
    D
    pop register_usage(D);
    ret

    Best regards
    Piotr Wyderski



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