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0 pointsmFixman2y ago0 comments

Could programs compiled in architectures with 16 general purpose registers fail in one with 15?

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4 comments · 2 top-level

Well, for a start you probably mean compiled for an arch with 16 registers. It doesn't actually matter what arch the compiler ran on (assuming modern cross-compiler like GCC).

If a program uses 16 registers then it needs 16 registers. But note it's the program itself that decides to use a frame pointer, it's not being reserved by the operating system or something. Programs don't even have to use the stack pointer as a stack pointer, they could use all 16 as general purpose, but in practice almost all programs use a call stack (I guess all C programs must do, but you might be able to disable it if you make no function calls?)

mFixmanOP2y ago

Ohh, I misunderstood what Ubuntu was doing.

So the only change is that GCC and its toolkit will compile programs using a register as frame pointer by default? That seems like a very reasonable change. If having an extra general purpose register is critical for performance of then this can be disabled in that program's the makefile.

brancz2y ago

Correct and there are already known exceptions such as the python interpreter in which the “interpret function” function actually falls into this case and so for the foreseeable future python is going to continue to be compiled omitting frame pointers. But by default this destructive micro optimization is off by default until proven a performance bottleneck, just like any performance issue should be!

kelnos2y ago

Not really? I mean, there are architectures with fewer than 16 GP registers (IA32 is one of them); if you can compile some C code on that architecture, then surely you can also compile it on x86_64 with 15 (well, 14, really, due to the stack pointer also being reserved) rather than 16 (15).

If someone is writing in assembly, then they've already decided if they're going to allocate a register for the frame pointer, and Ubuntu's change isn't going to affect that, as this is about compiled code, not assembled assembly.

The only real issue is performance: if a program has a particular hot-path function (or just many functions overall) that really benefit from having that extra register available, and would otherwise have to spill data into memory, then this change could have a big negative impact. But that's not really a big deal; the packager can decide to omit frame pointers just for that particular app or library.

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