Can register class effect ISel pattern?

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1

Hi,
Can register class effect Instruction selection prefer in ISel pattern?

Example:
Same LLVM IR add instruction, same data type i32, different asm
instruction, different register class GPR, SGPR
if one operator comes from GPR register class, it selects GPR "add" instruction
if all operators come from SGPR register class, it select SGPR "add" instruction

2

Hi Yu,

I don’t think that you can make LLVM to do it for you automatically based on the tablegen files alone, but you should be able to implement this in yourTargetDAGToDAGISel::Select function. Check for the ISD::ADD instruction to come, then replace it by your 'gpr-add', or ‘sgpr-add' instruction depending on its operands by calling CurDAG->SelectNodeTo.

Joan

3

Hi Nancy,

No, in general register classes haven't been assigned yet when instruction selection is trying to match the SelectionDAG representation. When you specify a register class in a pattern you're actually specifying the types that can be stored in that register class.

GlobalISel is slightly different though. GlobalISel adds a 'register bank' concept which is available to the instruction selector when it tries to match. In this case specifying a register class in a pattern specifies the types that can be stored in the class as well as the bank the class belongs to. There's another pass called Register Bank Allocator that assigns register banks in advance of running instruction selection.

Hope that helps