AM (myname4rwt.delete@this.jee-male.com) on August 18, 2018 1:06 pm wrote:
> > > What they promised was 40% higher perf in the same power envelope as A75. And that for A76 on 7nm vs A75
> > > on 10nm.
> >
> > Considering that the difference between 7nm and 10nm is
> > less than 40% it does imply improved uArch efficiency.
>
> This is not true. E.g. TSMC has exactly 40% power reduction in its 7nm tech vs 10nm.


TSMC promising 40% power reduction in 7nm versus 10nm does not imply anything about performance. They say "20% better performance, 40% better power" but those types of things have always been one or the other in the past. i.e. you can either get the same performance at 40% less power or 20% more performance at the same power.

If ARM is promising 40% more performance, and get 20% from the process due to faster transistor switching speeds, they have to get the other 20% from greater IPC, assuming they aren't also increasing the power budget.

Of course, it would be helpful to know what ARM uses to do its measurements. It is possible to achieve huge improvements in SIMD heavy code by allowing the issue/retirement of more SIMD instructions per cycle. If you use (or assume) faster or lower latency RAM, you can get big improvements in FP. Getting improvements in twisty branchy integer code is the most difficult since it isn't just one change but a lot of them to get there. But presumably that's what they are targeting since SIMD and FP are less important on phones than integer code.

There's obviously ample room for ARM to make such improvements. Looking at the Galaxy S9 version using the SD845 w/A75 @ 2.8 GHz versus the iPhone X using the A11 @ 2.39 GHz, despite the 17% clock rate handicap the iPhone is ~70% faster on GB4 integer, and that holds even if you focus in on the more twisty branchy subtests like LLVM or HTML5 DOM. So basically double the IPC. What's surprising is not that ARM is claiming a 20% improvement, and further improvements in the future. What's surprising is that they fell that far behind.

So I don't think there's any reason to be skeptical about ARM getting a 20% improvement here, though that still leaves them at minimum 50% behind Apple - since Apple will get the same 20% improvement from TSMC, and presumably makes further IPC improvements in the A12. Very significant improvements, if you believe Charlie's claims about the A12 of 50% greater single thread performance... I'm kinda skeptical of that myself, but I guess we'll see next month. Almost makes me wonder if the numbers he saw were benchmarks of an A12X going into an iPad Pro, or possibly even a prototype of something targeted at laptops. 50% higher when they are only getting 20% from process would mean another huge IPC gain.