By: David Kanter (dkanter.delete@this.realworldtech.com), August 9, 2014 9:55 am
Room: Moderated Discussions
> > > Well, obviously they're well documented inside Intel and AMD nowadays, and that's all that really matters.
> > > That's not the cost of implementation. The cost is in the jet engines required to make the pig fly.
> >
> > Yeah... The problem for that argument that there are no jet engines. There are no special
> > things that only Intel can do and only does it to make the x86 ISA competitive.
>
> Yes there is. Big decoders, high performance microcode modes, complex decoded instruction caches,
> big and capable store forwarding, memory disambiguation, stack tracking, memory speculation, etc.
>
> Many of these things are good regardless of ISA, but also other ISAs did not have to implement them. POWER6
> had no store forwarding, POWER7 (I believe) did not have memory disambiguation, none of uop caches.
No store forwarding is just retarded, and the POWER6 was not a good design.
> > Look at the reality: x86 processors are among the highest performing with the lowest cost to comparative
> > performance alternatives.
>
> I would ask you to do the same thing: x86 processors are among the highest performing in space they
> have been targeting for the past several decades. From somewhere around Atom space to all the way down
> to ~10,000 gate microcontrollers, x86 is anywhere from uncompetitive to completely impossible.
Yes that's true. But for something that is 2-issue or wider and targeted at high performance, x86 simply isn't a major barrier.
I know a number of architects who have worked on high-performance x86 and ARM cores, and they all say the same thing. The difference between the two ISAs is not that significant. x86 has more complexity, but it's in the noise compared to the difficulty of designing a good memory subsystem, etc.
> > It the ISA has overheads
>
> Oh, so it does have overheads now?
x86 overheads are noticeable and depend on the target market. As you pointed out, it's not very attractive for microcontrollers at all...but for a high performance core, the difference is perhaps 5-10% of the core area. And in most SoCs, cores are around 25-40% of the total area. 10% of 40% is 4% - and that's best case.
David
> > > That's not the cost of implementation. The cost is in the jet engines required to make the pig fly.
> >
> > Yeah... The problem for that argument that there are no jet engines. There are no special
> > things that only Intel can do and only does it to make the x86 ISA competitive.
>
> Yes there is. Big decoders, high performance microcode modes, complex decoded instruction caches,
> big and capable store forwarding, memory disambiguation, stack tracking, memory speculation, etc.
>
> Many of these things are good regardless of ISA, but also other ISAs did not have to implement them. POWER6
> had no store forwarding, POWER7 (I believe) did not have memory disambiguation, none of uop caches.
No store forwarding is just retarded, and the POWER6 was not a good design.
> > Look at the reality: x86 processors are among the highest performing with the lowest cost to comparative
> > performance alternatives.
>
> I would ask you to do the same thing: x86 processors are among the highest performing in space they
> have been targeting for the past several decades. From somewhere around Atom space to all the way down
> to ~10,000 gate microcontrollers, x86 is anywhere from uncompetitive to completely impossible.
Yes that's true. But for something that is 2-issue or wider and targeted at high performance, x86 simply isn't a major barrier.
I know a number of architects who have worked on high-performance x86 and ARM cores, and they all say the same thing. The difference between the two ISAs is not that significant. x86 has more complexity, but it's in the noise compared to the difficulty of designing a good memory subsystem, etc.
> > It the ISA has overheads
>
> Oh, so it does have overheads now?
x86 overheads are noticeable and depend on the target market. As you pointed out, it's not very attractive for microcontrollers at all...but for a high performance core, the difference is perhaps 5-10% of the core area. And in most SoCs, cores are around 25-40% of the total area. 10% of 40% is 4% - and that's best case.
David
