Athlon 64: How AMD turned the tables on Intel

Youngsters today don't remember it; x86 was fucking dead according to the press; it really wasn't until Athlon 64 came out (which gave a huge bump to Linux as it was one of the first OSes to fully support it - one of the reasons I went to Gentoo early on was to get that sweet 64 bit compilation!) that everyone started to admit the Itanium was a turd.

The key to the whole thing was that it was a great 32 bit processor; the 64 bit stuff was gravy for many, later.

Apple did something similar with its CPU changes - now three - they only swap when the old software runs better on the new chip even if emulated than it did on the old.

AMD64 was also well thought out; it wasn't just a simple "have two more bytes" slapped on 32 bit. Doubling the number of general purpose registers was noticeable - you took a performance hit going to 64 bit early on because all the memory addresses were wider, but the extra registers usually more than made up for it.

This is also where the NX bit entered.

Itanium wasn’t a turd. It was just not compatible with x86. And that was enough to sink it.

I acquired a copy of the Itanium manuals, and in flicking through it, you can barely get through a page before going "you did WHAT?" over some feature.

Example example example example must see examples!

Some of the examples:

* Itanium has register windows.

* Itanium has register rotations, so that you can modulo-schedule a loop.

* Itanium has so many registers that a context switch is going to involve spilling several KB of memory.

* The main registers have "Not-a-Thing" values to be able to handle things like speculative loads that would have trapped. Handling this for register spills (or context switches!) appears to be "fun."

* It's a bi-endian architecture.

* The way you pack instructions in the EPIC encoding is... fun.

* The rules of how you can execute instructions mean that you kind of have branch delay slots, but not really.

* There are four floating-point environments because why not.

* Also, Itanium is predicated.

* The hints, oh god the hints. It feels like every time someone came up with an idea for a hint that might be useful to the processor, it was thrown in there. How is a compiler supposed to be able to generate all of these hints?

* It's an architecture that's complicated enough that you need to handwrite assembly to get good performance, but the assembly has enough arcane rules that handwriting assembly is unnecessarily difficult.