[Info-vax] SOBGEQ vs SOBGTR

[Arne Vajhøj]

On 2/23/2024 10:29 PM, Arne Vajhøj wrote:
> On 2/23/2024 10:14 PM, Lawrence D'Oliveiro wrote:
>>          ... set up loop count in Rn ...
>>          BR      9000$
>> 1000$:
>>          ... body of loop ...
>> 9000$:  SOBGEQ  Rn, 1000$
>>
>> Why branch and use SOBGEQ instead of SOBGTR? So that, if the loop
>> count is initially zero, the branch falls right through without
>> executing the body of the loop.
> 
> With the details provided then it will be pure
> speculation.
> 
>      n = whatever
> loop:
>      if n <= 0 goto endloop
>      ...
>      n = n - 1
>      goto loop
> endloop:
> 
> has more instructions than:
> 
>      n = whatever
> loop:
>      ...
>      n = n - 1
>      if n > 0 goto loop
> 
> but the second has the problem of not handling the n = 0
> case properly.
> 
>      n = whatever
>      goto checkfirst
> loop:
>      ...
>      n = n - 1
> checkfirst:
>      if n > 0 goto loop
> 
> looks like a hack to fix that problem.

The entire thing reminds me of Fortran 66 vs 77
on DO loops.

Arne