[llvm-dev] HW loads wider than int

[Simon Dardis via llvm-dev]

This sounds quite similar to MIPS. MIPS64 supersets MIPS32 with additional rules
on how to extend 32 bit results for a 64 bit register.

For the MIPS64 backend in LLVM, we consider i64 and i32 legal types. This requires our
register classes to be 64 bit with 32 bit sub registers as Tom N. briefly described. I.E.
we have V0_64 which is the 64 bit register for returning a result, and V0, the
corresponding 32-bit subregister.

For the example you've given--which is familiar for MIPS64--by defining i32s as a legal
types and supplying tablegen instruction definitions that describe an i32 add which
takes a 32 bit register operands and returns a 32 bit result, LLVM can pattern match
the addition for the 32 bit case. Load and store instructions for a 32 bit value would
also be required.

For 64 bit operations, you'd want to define a register class that is 64 bit and supply
the corresponding instruction definitions. For your 32 bit sign extending load, you'd
want to define a load which has a dag pattern along the lines of:
(set GPR64Opnd:$rt, (sextloadi32 addr:$addr)) which maps to the same instruction
as your 32 bit load for a 32 bit register.

In short:
* Although your target is 64 bit, describe it has having 64 bit registers which have 32 bit
subregisters.
* Describe operations which operate on the lower 32 bits as only operating the
subregister.
* Multiple instruction definitions will be required in select cases, e.g. MIPS has LW and
LW64--both describe the same instruction, but have different dag patterns and register
inputs/outputs.

Thanks,
Simon

> -----Original Message-----
> From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of Davis,
> Alan via llvm-dev
> Sent: 11 January 2017 17:09
> To: llvm-dev at lists.llvm.org
> Subject: [llvm-dev] HW loads wider than int
> 
> I am trying to prototype a back end for a new processor. It has a 64-bit
> datapath, so all registers are 64 bits and load instructions always extend to 64
> bits. But the type 'int' is 32 bits, and arithmetic instructions have variants that
> operate on only the lower 32 bits of each register.
> 
> So for a basic 'a = b + c' example, we get
>   %0 = load i32, i32* @b, align 4, !tbaa !1
>   %1 = load i32, i32* @c, align 4, !tbaa !1
>   %add = add nsw i32 %1, %0
>   store i32 %add, i32* @a, align 4, !tbaa !1
> 
> And we'd want to generate
>  ldw %r0, at b     ; load b (32 bits) from memory with sign extension to 64 bits
>  ldw %r1, at c      ; load c (32 bits) from memory with sign extension to 64 bits
>  addw %r2,%r0,%r1   ; add lower 32 bits of r0 and r1
>  stw @a,%r2   ; store lower 32 bits of r2 to a
> 
> If I define the ldw instruction faithfully according to the HW, that is,
> extending to 64 bits, it won't match the load i32. Does that mean I will need
> to define both 32 and 64 bit versions (via a multiclass perhaps)? Or would I
> just define the true (64-bit) version and use a Pattern to map 32-bit loads to
> the true instruction? Or is there something that would be done in lowering? I
> tried this lowering action:
>    setOperationAction(ISD::LOAD, MVT::i32, Promote); but got an assertion
> failure: "Can only promote loads to same size type"
> 
> Please forgive the elementary level of the question; we are just getting
> started and finding ISel a bit of a tough nut to crack.
> 
> -Alan  Davis
> 
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