Note [Register parameter passing]

On certain architectures, some registers are utilized for parameter
passing in the C calling convention.  For example, in x86-64 Linux
convention, rdi, rsi, rdx and rcx (as well as r8 and r9) may be used for
argument passing.  These are registers R3-R6, which our generated
code may also be using; as a result, it's necessary to save these
values before doing a foreign call.  This is done during initial
code generation in callerSaveVolatileRegs in GHC.StgToCmm.Utils.

However, one result of doing this is that the contents of these registers may
mysteriously change if referenced inside the arguments.  This is dangerous, so
you'll need to disable inlining much in the same way is done in GHC.Cmm.Sink
currently.  We should fix this!