[LLVMdev] Physical register definition removed by MachineCSE

[Mikael Holmén]

Hi,

My target has a special configuration register that many instructions 
read implicitly, configuring for example if an add instruction should 
saturate at over/underflow or not.

Now, I have a problem where the MachineCSE removes a setting of this 
configuration register in a basic block, because 
MachineCSE::isPhysDefTriviallyDead can't find any uses of it in the 
basic block where it's defined.

Similar to other special physical registers, e.g the stack pointer, this 
configuration register is marked as "reserved".

Is there anything else I need to do for the MachineCSE to leave this 
register and not remove it just because it can't find any uses in the 
current basic block?

Best regards,
Mikael Holmén