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Code Generation Notes for MSA

Intrinsics are lowered to SelectionDAG nodes where possible in order to enable
optimisation, reduce the size of the ISel matcher, and reduce repetition in
the implementation. In a small number of cases, this can cause different
(semantically equivalent) instructions to be used in place of the requested
instruction, even when no optimisation has taken place.


This section describes any quirks of instruction selection for MSA. For
example, two instructions might be equally valid for some given IR and one is
chosen in preference to the other.

        It is not possible to emit bclri.b since andi.b covers exactly the
        same cases. andi.b should use fractionally less power than bclri.b in
        most hardware implementations so it is used in preference to bclri.b.

        It is not possible to emit vshf.w when the shuffle description is
        constant since shf.w covers exactly the same cases. shf.w is used
        instead. It is also impossible for the shuffle description to be
        unknown at compile-time due to the definition of shufflevector in
        LLVM IR.

        When the shuffle description describes a splat operation, splat.[bhwd]
        instructions will be selected instead of vshf.[bhwd]. Unlike the ilv*,
        and pck* instructions, this is matched from MipsISD::VSHF instead of
        a special-case MipsISD node.

ilvl.d, pckev.d:
        It is not possible to emit ilvl.d, or pckev.d since ilvev.d covers the
        same shuffle. ilvev.d will be emitted instead.

ilvr.d, ilvod.d, pckod.d:
        It is not possible to emit ilvr.d, or pckod.d since ilvod.d covers the
        same shuffle. ilvod.d will be emitted instead.

        The intrinsic will work as expected. However, unlike other intrinsics
        it lowers directly to MipsISD::VSHF instead of using common IR.

        It is not possible to emit splati.w since shf.w covers the same cases.
        shf.w will be emitted instead.

        On MIPS32, the copy_u.d intrinsic will emit this instruction instead of
        copy_u.w. This is semantically equivalent since the general-purpose
        register file is 32-bits wide.

binsri.[bhwd],  binsli.[bhwd]:
        These two operations are equivalent to each other with the operands
        swapped and condition inverted. The compiler may use either one as
        Furthermore, the compiler may use bsel.[bhwd] for some masks that do
        not survive the legalization process (this is a bug and will be fixed).

bmnz.v, bmz.v, bsel.v:
        These three operations differ only in the operand that is tied to the
        result and the order of the operands.
        It is (currently) not possible to emit bmz.v, or bsel.v since bmnz.v is
        the same operation and will be emitted instead.
        In future, the compiler may choose between these three instructions
        according to register allocation.
        These three operations can be very confusing so here is a mapping
        between the instructions and the vselect node in one place:
                bmz.v  wd, ws, wt/i8 -> (vselect wt/i8, wd, ws)
                bmnz.v wd, ws, wt/i8 -> (vselect wt/i8, ws, wd)
                bsel.v wd, ws, wt/i8 -> (vselect wd, wt/i8, ws)

bmnzi.b, bmzi.b:
        Like their non-immediate counterparts, bmnzi.v and bmzi.v are the same
        operation with the operands swapped. bmnzi.v will (currently) be emitted
        for both cases.

        Unlike the non-immediate versions, bseli.v is distinguishable from
        bmnzi.b and bmzi.b and can be emitted.