On Wed, 12 May 2021, Linus Torvalds wrote: > Note that this might just be a random effect of inlining or other > register allocation pressure details. > > So it's possible that upstream builds mostly by luck. > > The "couldn't allocate output register" thing really does seem more > like a compiler issue than a kernel source code issue. Nope, `x' is the constraint for the multiply-divide unit/MDU accumulator register used for calculation output; there's only one, comprised of the HI and LO parts. This register was removed as from the MIPSr6 ISA, which I forgot that we support (unlike the microMIPSr6 ISA), in favour to using regular GPRs, in a slightly different manner. Rather than cluttering code with #ifdefs for the updated MIPSr6 divide and modulo instructions I chose to rewrite this piece in plain C, which actually makes pre-MIPSr6 code slightly better owing to better instruction scheduling (the pre-MIPSr6 MDU runs asynchronously and its output is only interlocked on read access to the accumulator register). NB I don't know if Clang actually supports the `x' constraint even with pre-MIPSr6 code; as it has turned out it has deficiencies compared to GCC with inline asm handling with the MIPS target. OTOH GCC has supported it since ~1991 if memory serves me, when MIPS support was initially added. Maciej
