https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87733
--- Comment #12 from Rich Felker <bugdal at aerifal dot cx> --- > You can work around it on older GCC by simply not using a register var > for more than one asm operand, I think? Nope. Making a syscall inherently requires binding specific registers for all of the inputs/outputs, unless you want to spill everything to an explicit structure in memory and load them all explicitly in the asm block. So it really is a big deal. In particular, all mips variants need an earlyclobber constraint for the output register $2 because the old Linux kernel syscall contract was that, when restartable syscalls were interrupted, the syscall number passed in through $2 was lost, and the kernel returned to $pc-8 and expected a userspace instruction to reload $2 with the syscall number from an immediate or another register. If the input to load into $2 were itself passed in $2 (possible without earlyclobber), the reloading would be ineffective and restarting syscalls would execute the wrong syscall. The original mips port of musl had undocumented and seemingly useless "0"(r2) input constraints that were suppressing this bug, using the input to bind the register where the earlyclobber output failed to do so. After some recent changes broke compatibility with older kernels requiring the above contract, I manually reverted them (due to intervening conflicting diffs) and omitted the seemingly useless constraint, and it broke horribly. Eventually I found this bug searching the tracker. My plan for now is just to add back the "0"(r2) constraint, but since r2 is uninitialized, it's not clear that having it as an input constraint is even well-defined. Is this the best thing to do?
