[bitcompost]

I recommend doing some experiments before considering no register allocation unbearingly slow. I once tried running Gentoo with everything compiled -O0 and the user experience with most software wasn't significantly different. The amount of performance critical C code on a modern PC is surprisingly low. Stuff like media decoding is usually done in assembly.

[cesarb]

> I recommend doing some experiments before considering no register allocation unbearingly slow. I once tried running Gentoo with everything compiled -O0

AFAIK, register allocation is one of the few optimization passes which are always enabled on all compilers, even with -O0, so your experiment proves nothing.

[bitcompost]

It's decided by function use_register_for_decl in gcc: https://github.com/gcc-mirror/gcc/blob/releases/gcc-12/gcc/f... With -g -O0 register is only used in special cases like using the register keyword.

The memory accesses are also easily visible by disassembling the compiled binary. Performance of resulting binary at -O0 is also rougly similar to performance of binary produced by Tiny C Compiler, which doesn't implement register allocation at all.