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Comment by moonchild | Hacker News Reader

I'm not sure what he was talking about, but I can make a guess.

According to the sysv abi, every function has to have a stack frame; so for strict conformance (e.g. for unwinding the stack), you should have a stack frame for every function. In practice, this is kinda dumb. Gcc/clang will do TCO (and won't even generate a stack frame if you pass -fomit-frame-pointer). There's no particular reason why the generated code should abide internally by the sysv abi, only that it be able to call external code written according to said abi (and pass callbacks to said external code, but you'd need thunks for that anyway to account for closures).

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I just wanted to record for posterity that after 14 years of reading it as fomit frame pointer this is the post where I finally realized that it is -f omit frame pointer. What are are fomit pointers and what do they do!? Well now I know.

And now I wonder what -f orce are

(for context: -fomit-frame-pointer causes the FP to not be saved on the stack, stack unwinding then relies on DWARF or compiler-internal information to unwind the stack.)

I don't see how that would have an effect on TCO, as the storing or not of the FP (beyond in a register) is not part of the calling convention AFAIK. - Only externally linked functions have to conform, so I can understand if TCO does not apply to mutually recursive external functions. Perhaps with LTO it can? - Even an external function that calls itself in tail-position should be TCO-able under -fno-omit-frame-pointer. The compiler reuses the same frame and generates a jump instead of a full function call.

> I don't see how that would have an effect on TCO, as the storing or not of the FP (beyond in a register) is not part of the calling convention AFAIK

Sysv abi[1], section 3.2.2:

> In addition to registers, each function has a frame on the run-time stack

So it is part of the calling convention. But again, this is IMO pedantry that doesn't really matter.

> Even an external function that calls itself in tail-position should be TCO-able under -fno-omit-frame-pointer. The compiler reuses the same frame and generates a jump instead of a full function call

Yes, I agree. Just interpreting what I think he meant.

1. https://raw.githubusercontent.com/wiki/hjl-tools/x86-psABI/x...

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