[LLVMdev] SwitchInst handling in backend

[Matt Johnson]

On 02/27/2012 02:52 PM, Nico wrote:
> Hi,
>
> if I want to know how switch instructions are handled in the backend, where do I have to look first?
> I'm not familiar with the backend framework and I couldn't figure out the interface between the LLVM instruction 'SwitchInst' and whatever there is in the backend.
LLVM IR gets passed to the backend as a SelectionDAG, which is 
constructed by peephole expansion of the IR (see 
http://llvm.org/releases/3.0/docs/CodeGenerator.html#selectiondag_intro 
).  The relevant code for 'select' instructions in IR is the 
visitSelect() function in 
lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp; a select gets expanded 
into a ISD::SELECT (for scalar types) or ISD::VECTOR (for vector types) 
for each select operand, plus a ISD::MERGE_VALUES to multiplex them all 
together.

For the MIPS backend handling code, see 
lib/Target/Mips/MipsISelLowering.cpp.  In particular, 
setOperation(ISD::SELECT, <typename>, Custom) means that the MIPS 
backend lowers SELECTS in C++ rather than a simple tablegen pattern.  In 
LowerSELECT() in that same file, you can see that MIPS will create a 
conditional move if the select condition is set by a floating point 
comparison, and the "return Op;" it does otherwise means that other 
cases should be handled by the default machinery rather than custom 
lowering code.

The "default machinery" requires quite a bit of background knowledge to 
understand, but the Code Generator doc gives a good broad overview, and 
the above is the extent of custom select-handling code in the MIPS backend.

-Matt

>
> I would be very happy about every hint where I have to look to find the entry point of switch instructions in the backend (in particular in the MIPS backend.
>
> Thanks a lot and kind regards,
> Nico
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