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> If they'd packed everything into one HAL, single-processor systems would have to take the performance hit of all the synchronization code even though it wasn't necessary. Memory usage would be higher too.

Linux also used to be like this, but these days has unified MP/UP kernels; on single-CPU systems (or if you give nosmp), the extra code is patched away at boot time. It wouldn't have been an unheard of technique at the time.

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I actually would love this to be built in to a language/compiler. A lot of times when I’m building a single-threaded program but I’m using libraries written by other people. These libraries don’t know whether they are being incorporated into programs with single thread or not. So they either take the performance penalty of assuming multi-threaded (the approach by std::shared_ptr) or they give callers choice by making two implementations (Rust Arc and Rc). But the latter doesn’t actually work because this needs to be a global setting, not just a decision made at a local call site. It won’t work if such a library is a transitive dependency.

Zig supports this. If you compile with -fsingle-threaded, operations on mutexes turn into nops, atomics become simple loads/stores, etc.

Glibc has a bunch of tests throughout the codebase where it checks if there have been any threads started besides the main one. I don’t really know how effective they are from a performance perspective. (In principle, turning fgetc into getc_unlocked, for instance, could be quite beneficial.) Microsoft used to have a single-threaded C runtime, but it was done away with some time ago, I’m guessing because they started putting things into the platform that would start and manage random threads outside the programmer’s control.

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