What is the purpose of MipsEmulateBranch function?

[John Baldwin]

On Thursday, April 26, 2018 12:01:11 PM Jan Mazur wrote:
> Hi
> 
> I'm wondering why FreeBSD has to simulate branches when the faulting
> instruction is in the branch delay slot.
> I'm curious about MipsEmulateBranch function, which is defined in
> http://bxr.su/FreeBSD/sys/mips/mips/trap.c#MipsEmulateBranch
> 
> Why can't you just restart execution from the branch instruction?

For single-stepping via PT_STEP you need to place the next breakpoint
instruction at the target of the branch, so you don't emulate the branch,
but you need to know what it will do so you know where to place the
breakpoint.

For cpu_fetch_syscall_args, we don't want to return to the existing
instruction and retrigger the system call unless the system call fails
with ERESTART, so we need to manually advance PC to the next
instruction.

The handler for 'trap' instructions similarly needs to advance PC past
the instruction rather than restarting it and staying in an infinite
loop.

The code to handle unaligned stores wants to simulate the load or store
and advance PC rather than retrying the instruction.

-- 
John Baldwin