Okay, so I will admit that I did have some gaps in my understanding of local and global register variables. This seems to be a GCC-specific extension and is not supported by the C language at all. However. I think it's not as straightforward as saying the compiler is **forced** to (1/n) - ThreadSky

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chimney7890.bsky.social • 10 days ago

Okay, so I will admit that I did have some gaps in my understanding of local and global register variables. This seems to be a GCC-specific extension and is not supported by the C language at all.

However.

I think it's not as straightforward as saying the compiler is **forced** to (1/n)

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chimney7890.bsky.social•10 days ago

use that register for that variable. I mean, okay, perhaps initially it starts out with that variable in that register, but subsequent optimizations can chuck it out of the register.

https://godbolt.org/z/ofedasGTe

I'm using a local register variable p and asking the compiler to store it in rcx. (2/n)

chimney7890.bsky.social•10 days ago

If you look at the generated assembly instructions, you'll see that the only place 1234 is stored is in esi. I imagine what happened was that at the start of the compilation, the compiler assigned rbx for variable p, but during the constant propagation optimization, it determined that the only (3/n)

chimney7890.bsky.social•10 days ago

place where p was used was as an argument to printf and some_function and therefore it just moved it to rsi optimized away rcx register usage completely.

But then, if I assume that p was in rcx all along and try to use it (mov %rdx, %rcx), it gets garbage (4/n)

chimney7890.bsky.social•10 days ago

In all of the cases of global register variables that I tested, it never "optimized away" the variable.

But, if you use a call-clobbered register such as rdi etc., you _will_ lose the variable value on a function call. https://godbolt.org/z/oq5cjPd34

GCC very kindly provides a warning here (5/n)

chimney7890.bsky.social•10 days ago

I still think your program shouldn't rely on certain variables being present in certain registers. This isn't something that is supported by the C standard and unless you're doing something really, really specific, you can usually do without making such assumptions about your registers. (6/n)

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