Suppose a safe call: myCall(x,y,z) It is lowered into three unsafe calls in Cmm: r = suspendThread(...); myCall(x,y,z); resumeThread(r); Consider the following situation for myCall arguments: x = Sp[..] -- stack y = Hp[..] -- heap z = R1 -- global register r = suspendThread(...); myCall(x,y,z); resumeThread(r); The sink pass assumes that unsafe calls clobber memory (heap and stack), hence x and y assignments are not sunk after `suspendThread`. The sink pass also correctly handles global register clobbering for all unsafe calls, except `suspendThread`! `suspendThread` is special because it releases the capability the thread is running on. Hence the sink pass must also take into account global registers that are mapped into memory (in the capability). In the example above, we could get: r = suspendThread(...); z = R1 myCall(x,y,z); resumeThread(r); But this transformation isn't valid if R1 is (BaseReg->rR1) as BaseReg is invalid between suspendThread and resumeThread. This caused argument corruption at least with the C backend ("unregisterised") in #19237.