Conditional JSR and RTS

[DanScott]

So imagine I have this code:

blah
blah
tst.w d0
beq.s .DoNotJsr1
jsr DoSomething
.DoNotJsr1
blah
blah
tst.w d0
beq.s .DoNotJsr2
jsr DoSomething
.DoNotJsr2
blah
blah


DoSomething:

blah
blah
blah
rts


just to be clear, blah = some instructions

Is there a quicker way of doing this conditional check / jsr / rts combo?

The JSR can be BSR if that makes it easier....

[jotd]

looks fine to me (but hey I haven't written major Amiga titles). Maybe you want to analyse the frequency of the branch vs the non-branch. If it branches often, maybe use a bne and a bra so you're not breaking the instruction pipeline/prefetch.

[DanScott]

Originally Posted by jotd

looks fine to me (but hey I haven't written major Amiga titles). Maybe you want to analyse the frequency of the branch vs the non-branch. If it branches often, maybe use a bne and a bra so you're not breaking the instruction pipeline/prefetch.

it can vary between always taking the branch and never taking the branch depending on the input bit stream.. it is also part of a recursive routine, that can iterate up to possibly 1280 times (minimum 20, max 1280)

Obviously, I need to make sure input data keeps iterations to a minimum.

It's for vanilla 68k / A500

[Thomas Richter]

That depends on the statistics of the branches. An alternative approach would be to store the branch destination in an address register that either points to the real target, or to a single RTS. This is quicker in case you often take the subroutine and rarely jump over it.

However, typically: It does not matter. If your function is reasonably sized, I doubt that this makes a lot of difference and isn't worth the trouble. Profile first, then decide.

[Jobbo]

Presuming DoSomething doesn't depend on the prior results.
Could you combine 16 test results into a word something like:

moveq #0,d1 // d1 = results bitfield

tst.w d0
add.w d1,d1 // shift one bit left
sub.w d0,d1 // add our tst bit (sub because tst returns -1)

Do that 16x and then you can jump have DoSomething unrolled 16x and jump to the appropriate place.

That or something like it might reduce the number of jsr rts uses.

[DanScott]

I am now thinking of putting this all into nested macros, so that I don't have to do any JSR/RTS

[Jobbo]

Inlining DoSomething would seem like the simplest solution if there is room.

[Knocker]

See http://eab.abime.net/showthread.php?...72#post1104172 for ideas for macro(s).

[a/b]

Not enough info.. What is the break/exit condition and where is it checked? Are blahs the same unrolled code or different code?
Could you write it in this form, gives you better branching and gets rid of jsrs...

  bra.b ENTRY
do_something:
  blah
  blah
  blah
ENTRY:
  blah
  blah
  test.w d0
  bne.b do_something
  blah
  blah
  test.w d0
  bne.b do_something
  blah
  blah
  test.w d0
  bne.b do_something
...

[h0ffman]

Originally Posted by DanScott

I am now thinking of putting this all into nested macros, so that I don't have to do any JSR/RTS

Macros and relative labels ftw id say. Not writing to the stack potentially 1280 times will be a massive win.