Re: Stealing a Great Idea from the 6600

EricP wrote:

MitchAlsup1 wrote:

 One of the things we found in Mc 88120 was that the compiler should
NEVER
be allowed to put unnecessary instructions in decode-execute slots that
were unused--and that almost invariable--the best code for the GBOoO machine was almost invariably the one with the fewest instructions, and
if several sequences had equally few instructions, it basically did not
matter.
 For example::
     for( i = 0; i < max, i++ )
         a[i] = b[i];
 was invariably faster than::
     for( ap = &a[0], bp = & b[0];, i = 0; i < max; i++ )
         *ap++ = *bp++;
 because the later has 3 ADDs in the loop wile the former has but 1.
Because of this, I altered my programming style and almost never end up
using ++ or -- anymore.

By the way:: The statement about Always applies to Mc 881x0's and a few
other RISC architectures with indexed address modes.

The 88000 had a scaled-indexed address mode on LD and ST.
Alpha didn't but had a scaled-indexed S8ADDQ (aka LEA) instruction.
ISA's that didn't used individual shifts and adds (like real RISC's
do!)
so for them an optimizing compiler converting to the ++ form is
optimal,
and might be folded into an auto-increment address mode.

No disagreement when other architectures made different choices getting
to where they got.