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604f8d3d8b
analogous to commit ddc7c4f936c7a90781072f10dbaa122007e939d0 for mips64 and n32, remove the hack to load the syscall number into $2 via asm, and use a constraint to let the compiler load it instead. now, only $4, $5, and $6 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. as before, $8, $9, and $10 are conditionally input-output registers for 5-, 6-, and 7-argument syscalls. their role in input is carrying in the values that will be stored on the stack for arguments 5-7. their role in output is carrying back whatever the kernel has clobbered them with, so that the compiler cannot assume they still contain the input values.