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
ever since inline syscalls were added for (o32) mips in commit 328810d32524e4928fec50b57e37e1bf330b2e40, the asm has nonsensically loaded the syscall number, rather than taking $2 as an input constraint to let the compiler load it. commit cfc09b1ecf0c6981494fd73dffe234416f66af10 improved on this somewhat by allowing a constant syscall number to propagate into an immediate, but missed that the whole operation made no sense. now, only $4, $5, $6, $8, and $9 are potential input-only registers. $2 is always input and output, and $7 is both when it's an argument, otherwise output-only. previously, $7 was treated as an input (with a "1" constraint matching its output position) even when it was not an input, which was arguably undefined behavior (asm input from indeterminate value). this is corrected.