17.0.1 released, and jawing about the WiiUCPU

17.0.1 release candidate is available. Near as I can tell from releng, Mozilla plans to release this tomorrow. Please test it today. Changesets are available.

I am still stymied by the locking problems we still have on some sites, where the browser spins its wheels in semaphore wait. It's only a minority of sites, thank goodness, but it even occurs on some pure HTML/CSS sites with no JavaScript at all implying that the hangup is within layout. Once I get a debug build of Fx19 mounted, I want to review this problem in detail; it seems there should be a simple solution.

The fanboi boards and the usual blogosphere suspects are aflutter with "WiiU CPU sux" based on a couple of tweets made by self-proclaimed Nintendo WiiU hacker Hector Martin. The triple-core PowerPC CPU in the WiiU, codenamed "Espresso," is according to Martin's measurements clocked at only around 1.24GHz per core, compared with around 1.6GHz (when multithreaded; 3.2GHz max) for the Xbox 360's tri-core PowerPC-based Xenon. Given that apparently dismal clock speed, is this another case of the megahertz myth reborn?

Both the preceding Nintendo GameCube's Gekko and the Wii's Broadway were evolved forms of the G3, specifically modifications of the PowerPC 750CXe and 750CL. IBM customized Gekko's FPU with SIMD instructions (sort of an "AltiVec Lite") to facilitate media processing and built a fast path from Gekko to the GameCube's Flipper GPU, and then took that same basic design and essentially cranked up the clock to generate Broadway. The systems run at 485MHz and 729MHz respectively. Even to this day IBM continues to make custom versions of the venerable G3; it's cheap to produce, simple to modify and can be ground out in quantity from their secret underground base in Fishkill.

Despite this, IBM can and has made other kinds of application-specific PowerPC designs. The Cell in the Playstation 3 is the most obvious example with its PowerPC "PPE" core and satellite SPEs, and the Xenon in the Xbox 360 is three PPE cores stuck together; Xenon even has modified AltiVec instructions ("VMX128"). As if to confirm IBM was trying something new with Nintendo, during the WiiU's hype-y-moon Nintendo marketing billed the processor as "the same processor technology found in Watson." We assumed this to be a POWER7 derivative, given that Watson was IBM's POWER7-based know-it-all computer cluster that pretended to win Jeopardy.

However, Martin claims that the WiiU CPU is also, once again, another morph of our old friend the PowerPC 750 with some additional cache. Well, I'm dubious, and the reason is that the 750 and its ancestor, the 603e, were never designed for multicore environments (and Espresso has three). The only multi-CPU 603 I ever met was the BeBox, and the BeBox's design hobbled its two CPUs because it had to overcome the 603's incomplete cache coherency with glue logic (the 603 and the G3 do not support all five MERSI states necessary for multiprocessing; they are only MEI). I've never met a multiprocessor G3, and Apple certainly never made one. In fact, early 7400 G4s have the same limitation.

Furthermore, Martin also admits that the CPU is out-of-order, which the 750 never was either. POWER7, interestingly enough, is. On the other hand, "big" POWER like the POWER7 has significantly different execution characteristics than "little" POWER. We know this personally from this project, because the G5 acts like a "big POWER" CPU and requires "big POWER" optimizations that are different from what a G3 or G4 would require. The G5 can run G3 and G4 code, but it certainly doesn't do so as well as its own, which was why early G5s weren't really all that much faster than the MDD G4. The fact that the WiiU cores implement the same instruction set as Broadway, including the modified SIMD FPU instructions -- because, other than emulation, how else could it still run Wii games acceptably? -- make it pretty unlikely it really is a POWER7 derivative, and Espresso also lacks POWER7's hardware threads. However, to work in a multicore environment, whatever is in there cannot be a garden-variety 750 (unless the system board situation is as tricky as it was with the BeBox). The pipeline is certainly not a regular 750's to have OOOE.

In the final analysis what we're looking at is a new design, inspired by the 750, but not a 750 (just like the 7400 G4 was more than a G3 with an AltiVec unit bolted on); there are too many fundamental changes in its operation to just call Espresso merely an evolutionary variant. Martin should know this and I'm a bit disappointed in his simplistic analysis, even though I salute his technical skills. It also means that, like the megahertz myth of the PowerPC vs x86 days, the clock of these cores is probably not at all comparable to the more deeply-pipelined PPE in Xenon and Cell. And that's why fanbois suck.