🚀 go-pugleaf

In article <1992Nov19.204729.24714@crl.dec.com> payne@crl.dec.com (Andrew Payne) writes:
>>The last sentence is key here. If branches can be predicted with decent
>>accuracy then it is hard to obtain benefits from conditional moves
>>over using conditional branches. 
>
>Do you have any data to back up this claim?

Sure do. If branches can be predicted with 100% accuracy then the
version with conditional branches may run up to twice as fast. :-)
Seriously, the silly example above should nonetheless show that if
there are many instructions to guard, then potentially only half of the 
functional units will accomplish useful work while running the code
surrounded with conditional moves. 

As a previous poster mentioned, it would be nice to get some
numbers on the impact of conditional moves on a superscalar processor
with good branch prediction. Not on a 10 line function but on a large
integer program (some of the SPECint92 programs).

>Don't forget that conditional move sequences can be shorter than the 
>corresponding branch sequence.  In other words, the two instruction
>sequence branch/op can sometimes be replaced with a single conditional move 
>instruction.

True. I have seen some great little examples where conditional moves
perform very well [e.g. if (a>b) then c=3 else c=5] or finding max (a,b).
Notice that in most examples, either the guarded code is very small
and/or the branches could not be predicted well.
 
>Second, I believe the second order effect of providing larger basic blocks
>is the conditional move's greatest benefit.  Larger basic blocks offer
>more scheduling opportunities.  I don't have any data (yet), but I've 
>certainly seen it happen.

Since conditional moves won't replace all of the conditional branches,
a good compiler will have to handle moving instructions across
conditional branches anyway, otherwise performance will be low.
Large basic blocks can be created through several methods
(e.g.superblocks from Illinois) that basically group basic blocks together.
Conditional moves will help wherever branches were hard to predict
or where the trace did not show any definite direction for the branch
(once again, hard to predict branches).

Finally, as you may know, SPARC V9 (64-bit) does include conditional moves
and we do think that we can get some benefits out of them.

- Marc Tremblay.
Sun Microsystems.