[LLVMdev] Emulate i64 add with 3 instructions

[Johannes Birgmeier]

Hello!

Is there a way to tell LLVM how to emulate an instruction with multiple 
others? Specifically, in our processor, there is no instruction for 
adding two i64s; it has to be done like this

dst_high32:dst_low32 = src1_low32 + src2_low32 (unsigned add; dst_high 
might contain the overflow bit)
dst_high32 = dst_high32 + src1_high32
dst_high32 = dst_high32 + src2_high32

I tried it with patterns like the following in InstrInfo.td (this is 
obviously wrong, but you have to start somewhere):

def : Pattern<(set LLRegs:$dst, (add LLRegs:$src1, LLRegs:$src2)), [(set 
LLRegs:$dst, (anyext (add (trunc LLRegs:$src1), (trunc LLRegs:$src2))))]>;

... but tablegen complains: "Cannot use 'add' in an output pattern!". Is 
this the right way to do it - that is, defining a pattern for an 
operation? ... Or better: What is the right way to do it?

Any help appreciated!

Thanks,
Johannes