Optimization for Area has higher Fmax than optimization for Speed?

[Altera_Forum]

Hi everyone, 

 Has anybody encountered this problem? I was running some compilations on the altera floating point megafunctions and I tested the optimization for area and speed and the optimization for area not only gave me a compilation result of lesser logic elements, but also higher fmax, with shorter delays, as compared to the optimization for speed option. Is this normal? Thanks guys.  Best regards, Chris

[Altera_Forum]

The synthesis options, Speed, Balanced, and Area give said results over a suite of designs. Area tends to be pretty accurate, as synthesis is a good job of how big a design will be. But performance is dependent on many factors AFTER synthesis, and so it's really just doing what it thinks its best. It doesn't happen a lot, but synthesizing for Balanced or Area certainly can give faster performance. 

(On a similar note, Balanced tends to be the best of both worlds, i.e. it's not much bigger than synthesizing for Area, and it's not much slower than synthesizing for Speed, which is why it's the default. Changing the settings to Area and Speed tend to throw all the algorithms at those goals, regardless of how they affect other things..)

[Altera_Forum]

--- Quote Start ---  The synthesis options, Speed, Balanced, and Area give said results over a suite of designs. Area tends to be pretty accurate, as synthesis is a good job of how big a design will be. But performance is dependent on many factors AFTER synthesis, and so it's really just doing what it thinks its best. It doesn't happen a lot, but synthesizing for Balanced or Area certainly can give faster performance. (On a similar note, Balanced tends to be the best of both worlds, i.e. it's not much bigger than synthesizing for Area, and it's not much slower than synthesizing for Speed, which is why it's the default. Changing the settings to Area and Speed tend to throw all the algorithms at those goals, regardless of how they affect other things..)  --- Quote End ---    Hi,  Thanks for replying. Does this mean that I have to test for all parameters(speed, area and balanced) even if I want to optimise for speed? A bit weird isn't it?  Or does it mean that the design might perform faster, even if it has a lower Fmax?  Best regards, Chris

[Altera_Forum]

1) Synthesis occurs before fitting, so there just isn't enough information. It does it's best, but it's just impossible to do. 

2) Synthesis settings really control a number of other settings, and to be honest, there are probably hundreds of other levers being controlled behing the scenes. Most of them are not black-and-white on whether they help performance either. If you were to take all of these into account, it would be impossible to run through all combinations and determine what's "best".  For the most part, they work as described.  If you have the time, try a few different settings. The Tools -> Design Space Explorer can run sweeps of different settings to find different things.  For the most part, if you're just barely missing timing and can't do it with code changes or more manual things, then it may be worth trying, but otherwise it's probably not worth the time besides a few basic things.

[Altera_Forum]

Thanks a lot Rysc for your replies, you've been really helpful! Appreciate it. 

 Best regards, Chris