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R> Why is this dependent on __mips_abs2008?

There is also __mips_nan2008 (always set for hard-float R6 and -mnan=2008).

Binaries built with this option (implicit or not) are unusable on -mnan=legacy system, this is enforced by kernel (unless booted with some debugging option).

The fabs code could be changed to also depend on __mips_nan2008 (since these ISA features are paired) to prevent -mabs=2008 musl on -mabs=legacy system (rather unrealistic).

Why is it wrong to depend on fine-grained ISA features after all?
Why is it wrong to explicitly depend e.g. on __mips_dsp in the strchr code fearing improper usage on a system without DSP ASE?

powerpc64, s390x have similar ifdefs in their math code and IIUC nothing prevents running (until SIGILL) statically-linked _ARCH_PWR5X binary on an _ARCH_PWR5 system.

Or some powerpc64 code depends on __VSX__. Is it wrong to depend on __mips_msa?

What is different with mips here?