Uninitialized garbage on ia64 can be deadly (2004)

Early x86-64 Linux had a similar problem. The x86-64 ABI uses registers for the first 6 arguments. To support variable number of arguments (like printf) requires passing the number of arguments in an extra register (RAX), so that the callee can save the registers to memory for va_arg() and friends. Doing this for every call is too expensive, so it's only done when the prototype is marked as stdarg.

Now the initial gcc implemented this saving to memory with a kind of duffs device, with a computed jump into a block of register saving instructions to only save the needed registers. There was no boundary check, so if the no argument register (RAX) was not initialized correctly it would jump randomly based on the junk, and cause very confusing bug reports.

This bit quite some software which didn't use correct prototypes, calling stdarg functions without indicating that in the prototype. On 32bit code which didn't use register arguments this wasn't a problem.

Later compiler versions switched to saving all registers unconditionally.

Raymond Chen has a whole "Introduction to IA-64" series of posts on his blog, by the way. It's such an unconventional ISA that I am baffled that Intel seriously thought they would've been able to persuade anyone to switch to it from x86: it's very poorly suited for general-purpose computations. Number crunching, sure, but anything more freeform, and you stare at the specs and wonder how the hell the designers supposed this thing to be programmed and used.

> The ia64 is a very demanding architecture. In tomorrow’s entry, I’ll talk about some other ways the ia64 will make you pay the penalty when you take shortcuts in your code and manage to skate by on the comparatively error-forgiving i386.

https://devblogs.microsoft.com/oldnewthing/20040120-00/?p=40... "ia64 – misdeclaring near and far data"

https://devblogs.microsoft.com/oldnewthing/2004/01

Pretty surprising. So IA64 registers were 65 bit, with the extra bit describing whether the register contains garbage or not. If NaT (Not a Thing) is set, the register contents are invalid and that can cause "fun" things to happen...

Not that this matters to anyone anymore. IA64 utterly failed long ago.

Yet another reason IA64 was a design disaster.

VLIW architectures still live on in GPUs and special purpose (parallel) processors, where these sorts of constraints are more reasonable.