From comments: > avoiding [...] errno Note that errno can be optimized by a sufficiently smart compiler, simply by treating it as a register, then annotating

From comments: > avoiding [...] errno Note that errno can be optimized by a sufficiently smart compiler, simply by treating it as a register, then annotating various functions with whether it is preserved, clobbered, or conditionally used for return. The libc boundary is quite annoying though; for this case in particular, that it doesn't expose the fact that errno is at a fixed offset from the TLS register. And generally, libc is vehemently opposed to the existence of smart compilers, since all libc calls are expected to be treated as opaque barriers.