#133 Z80: faster/smaller _mullong

None
closed-fixed
z80 port (10)
5
2013-07-18
2009-04-14
No

The best should be to write a (small) asm replacement, but I do not know exactly how to do it (I can write the mullong.s, but how to convince the compiler to use it?)

Meanwhile I cleaned the C code for the long integer product (32x32->32), it uses a smaller local variable and replaces six 8x8->16 products (four of them should be 8x8->8, because the high byte is discarder) with two 16x16->16 multiplication.

The C code obviously works on all the ports, but it was developed with Z80 limitations in mind, that's why an #ifdef limits its use to the z80 environment. On the other ports this code MAY show a small gain in code size, but it SHOULD NOT give any speed improvement (or may be it slow down the library), that's why it is disabled.

Discussion

  • Marco Bodrato

    Marco Bodrato - 2009-04-24
    • labels: --> z80 port
     
  • b-s-a

    b-s-a - 2010-11-11

    This code is not looks like optimal.
    On this site is more optimal code on z80 assembler: http://baze.au.com/misc/z80bits.html

     
  • alvin

    alvin - 2010-11-11

    The optimal one is in z88dk, a native 32-bit multiply.

     
  • b-s-a

    b-s-a - 2012-03-12

    as I understand this algo (pointed by aralbrec) is not used because it changes alternative register set. Why this restriction? Or need to port it?

     
  • Marco Bodrato

    Marco Bodrato - 2012-03-15

    I never claimed that this code was optimal! :-)

    I repeat, "a (small) asm replacement" is the best improvement, but the proposed patch was better than the previous code.

    Meanwile, I notice that z88dk introduced some GPL'd code in their library...
    http://www.z88dk.org/forum/viewtopic.php?id=4195
    Did they take care of the fact that the "special exception" is valid for SDCC?

     
  • Philipp Klaus Krause

    • status: open --> closed-fixed
    • assigned_to: Philipp Klaus Krause
    • Group: -->
     
  • Philipp Klaus Krause

    I don't like having special cases and separate code for different ports, even if that provides a small advantage. There is a cost in maintaining the code after all.
    But both the z80 and gbz80 ports would profit from this, and once RFE #372 is implemented, the r2k and r3ka ports would profit as well. On the other hand, z180, hc08, s08 and stm8 would suffer. So I enabled the use of this multiplication for the z80, gbz80, r2k and r3ka.

    Patch applied in revision #8767.

    Philipp

     

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