Also that example, heh. I tried to go back gcc versions to see if there was a case where it didn't do TCO - nope. Also, I like how returning 0 is "xor eax, eax".
Is this the same thing as, or related to, the calling conventions that used to be used in Microsoft DOS and Windows native language / C apps some years ago - things like "long far pascal" as function declaration /definition qualifiers, and things like the fact that C pushes function arguments onto the stack (before the call) from left to right, and Pascal does it from right to left (or the other way around)? (Did some of that stuff, but it's been a while).
I did read the surrounding comments to this one, and saw that some of the topic was about registers, not the stack.
So the choice of registers cannot be arbitrary, unless the compiler knows the function is only used within an object file.
The registers are predetermined by a convention unless you use the 'static' keyword to signal that the function is only used internally to a module, so the compiler has complete freedom to choose registers.
Ah, interesting, I figured that the generated object files would just store some metadata on that basically.
By using information kept with the function, or perhaps even encoded into the function name itself (as already happens when distinguishing between different calling conventions, or in the case of C++ name mangling)?
Coming from an Asm background, where there basically is no one "calling convention", and programmers would document which registers (almost always registers, rarely the stack --- and that can make for some great efficiency gains) are for what, I've always wondered why that idea didn't seem to go far.
How would you do that with dynamically linked code, inspect functions you're calling at runtime before laying out your arguments?
> perhaps even encoded into the function name itself
That would mean name mangling in C and assembly.
> Coming from an Asm background, where there basically is no one "calling convention"
Right, because you can lay out memory however you want since you're at the assembly level. Higher-level code (C up) can't do that, so instead you've got standard calling conventions for inter-library call (inside a compilation unit, the compiler is free to use alternate calling conventions since it has complete control over both sides of the callsite, that's also how it can inline calls entirely).
> programmers would document which registers (almost always registers, rarely the stack --- and that can make for some great efficiency gains)
Some standard CC (though not the old CDECL) also use registers, so far as they can, depending on the arch. The SystemV AMD64 ABI uses 6 x86_64 registers for integer/pointer arguments and 8 SSE registers for FP arguments, with the rest on the stack.
(And realistically, for all but the most trivial functions, having one convention is probably a highly reasonable default. Trivial functions should probably be made available to the compiler for inlining anyway. Note also that GCC allows you to override the number of arguments passed on the stack via an annotation on 32-bit x86, if you insist.)
Why is it so different with different optimisation levels? The default emits quite a bit of code, -O1 is `mov eax, 0`
A proficient human coder, on the other hand, writes assembler that is partly optimized by default.
But few humans could write code like a seriously optimizing compiler, esp. on modern pipelined architectures - that stuff is unintelligible. Which is as it should be, because modern processors are not designed to be programmed directly by humans.
Often you would not want the flags updated when simply clearing a register - you're hardly likely to test the Zero flag having just set something to zero, because it's obvious, and more importantly you may want to set something to zero while preserving the flags from a previous operation.
But often you don't care about the flags so you can use the slightly shorter and/or faster XOR operation. It used to generally be shorter and faster because the MOV instruction had the initial step of an immediate load of the zero from memory.
And that's why it changes with different optimisation levels - the compiler knows when the flags need to be preserved, and if they don't it can get away with using XOR.
> -O1 is `mov eax, 0`
Simply because it is shorter: On x86-64 (and x86-32)
xor eax,eax
31h C0h or 33h C0h
mov eax,0x0
B8h 00h 00h 00h 00h
Having privately analyzed some 256b demos I cannot even imagine how one could even come to the idea to use `mov r32, imm32` for zeroing a register (except for the reason that people don't want to understand how the assembly code is internally encoded) - the canonical way to use is `xor` (`sub` also works in principle, but `xor` is the way that is recommended by Intel).
EDIT: Here is an article about that topic: https://randomascii.wordpress.com/2012/12/29/the-surprising-...
If the condition flags have to be preserved, you are right. But otherwise, read the linked article (https://randomascii.wordpress.com/2012/12/29/the-surprising-...):
"On Sandybridge this gets even better. The register renamer detects certain instructions (xor reg, reg and sub reg, reg and various others) that always zero a register. In addition to realizing that these instructions do not really have data dependencies, the register renamer also knows how to execute these instructions – it can zero the registers itself. It doesn’t even bother sending the instructions to the execution engine, meaning that these instructions use zero execution resources, and have zero latency! See section 2.1.3.1 of Intel’s optimization manual where it talks about dependency breaking idioms. It turns out that the only thing faster than executing an instruction is not executing it."