[llvm-dev] Questions about load/store incrementing address modes

Steve Montgomery via llvm-dev llvm-dev at lists.llvm.org
Fri Oct 30 06:46:50 PDT 2015

I’ve implemented something similar, though maybe not similar enough to be able to help you. My target supports increment and decrement, both pre and post, by an explicit constant in the range 1 to 8.

I didn’t write any custom lowering code, just used setIndexedLoadAction() and setIndexedStoreAction() in my ISelLowering to identify the types and actions, e.g.

	setIndexedLoadAction(ISD::PRE_INC, MVT::i16, Legal);

I didn’t think it was possible to match an indexed load or store using a TableGen pattern because it can’t match instructions with more than one output operand, though maybe things have changed and I’ve not noticed. So, I wrote custom code in my ISelDAGToDAG to match indexed loads and store and to select these instructions. That seems to work fine for constant increments/decrements. I don’t know whether the indexed load and store actions would deal with increment or decrement by a register.

I’ve also got an addressing mode similar to yours in which an address can be formed from a 16-bit register plus a zero-extended 8-bit register. I used a ComplexPattern to match the address expression and MIOperandInfo to specify the classes of the registers, e.g.

	def memR16R8 : Operand<i16> {
		let MIOperandInfo = (ops Reg16Class, Reg8Class);

Again, this seems to work OK for me.


> On 30 Oct 2015, at 10:43, Martin J. O'Riordan via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> I have a rudimentary implementation for load and store instructions, where the memory address operand is automatically post-incremented when the load or store instruction is issued.  However, this is currently coded using custom lowering, and explicit pattern matching in the ‘ISelDAGtoDAG’ implementation.  But it seems to me that I ought to be able to achieve this exclusively using TableGen with minimal custom C++ code, but I can’t quite get the patterns to work to achieve this.
> I have two types of post-increment:
> 1.      increment by an implied constant which is the size of the object being loaded or stored
> 2.      increment by the value contained in another register
> It should be possible to do these with the same underlying pattern, but providing a DAG fragment like ‘(i32 2)’ for an ‘i16’ load explicitly in the TableGen specialisation for the first form, and the actual register chain for the second.
> But I don’t seem to be quite get it to work, and in some cases the patterns I have attempted have crashed TableGen itself L  The incrementing store sort of works, but the incrementing load doesn’t (at least not using TableGen alone).
> The other form of addressing mode is where the address is a “base” plus signed “offset” register.  My patterns for this work okay, but I have a limitation.  The “offset” register is 16-bits, and in particular, it is the low-order 16-bits of a 32-bit actual register.  But even though I have explicitly stated in TableGen that the offset register is 16-bits and stated the appropriate register class, the code-generation still uses the instructions “as if” the offset register is 32-bits.
> Memory addresses in this architecture are 32-bits.
> Has anybody solved similar problems, and have advice or examples of how to do this?  Or is simply something that TableGen descriptions cannot completely describe?  I have looked at the other targets for inspiration, but they don’t quite seem to do what I need.
> Thanks,
>             Martin O’Riordan - Movidius Ltd.
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