Add jit loop invariant improve

[Dimoner]

This PR improves loop invariant code motion opportunities for integer arithmetic by making loop-invariant subexpressions easier to recognize and hoist.
For loops like:

private static int Sum(int x, int y, int z, int x1)
{
    int res = 0;
    for (int i = 0; i < z; i++)
    {
        res += i + x + y + x1;
    }
    return res;
}

the JIT currently keeps the expression as a chain of ADDs where loop-variant and loop-invariant terms are interleaved. As a result, the invariant part (x + y + x1) is not formed as a standalone subtree, so PHASE Hoist loop code cannot hoist it.

Generated code therefore performs redundant additions of x, y, and x1 inside the loop body, even though they are invariant.

STORE_LCL_VAR int V05 loc0
\--* ADD int
    +--* LCL_VAR int V05 loc0      (last use)
    \--* ADD int
        +--* ADD int
        |   +--* ADD int
        |   |   +--* LCL_VAR int V06 loc1
        |   |   \--* LCL_VAR int V00 arg0
        |   \--* LCL_VAR int V01 arg1
        \--* LCL_VAR int V03 arg3

Asm

G_M22598_IG03:
    add     w4, w4, w5     ; res += i
    add     w4, w4, w0     ; res += x
    add     w4, w4, w1     ; res += y
    add     w4, w4, w3     ; res += x1
    add     w5, w5, #1     ; i++
    cmp     w5, w2
    blt     G_M22598_IG03

During morphing, we reassociate commutative/associative integer arithmetic trees (starting with GT_ADD) into a more canonical form that groups loop-invariant operands together.

* STORE_LCL_VAR int V05 loc0
\--* ADD int
    +--* ADD int
    |   +--* LCL_VAR int V05 loc0     (last use)
    |   \--* LCL_VAR int V06 loc1
    \--* ADD int
        +--* LCL_VAR int V00 arg0
        \--* ADD int
            +--* LCL_VAR int V01 arg1
            \--* LCL_VAR int V03 arg3

Asm

G_M22598_IG03:                ;; offset=0x0018
    add     w1, w1, w3        ; y += x1
    add     w0, w1, w0        ; x = x + (y + x1)
    align   [0 bytes for IG04]
    align   [0 bytes]
    align   [0 bytes]
    align   [0 bytes]

G_M22598_IG04:                ;; offset=0x0020
    add     w4, w4, w5        ; res += i
    add     w4, w4, w0        ; res += (x + y + x1)
    add     w5, w5, #1        ; i++
    cmp     w5, w2
    blt     G_M22598_IG04

This optimization could potentially be extended beyond integer arithmetic.
In particular, a similar reassociation approach may be applicable to certain
string concatenation patterns, where loop-invariant fragments could be
identified and computed outside the loop.

The current implementation intentionally targets a narrow scenario and
handles only a single reassociation case for integer addition. If this
direction is considered valid, the transformation can be generalized to
cover additional arithmetic patterns and operators in follow-up changes.

Bench for string

public class ConcatBench
{
    private string _x = "abcdef";
    private string _y = "ghijkl";
    private int _x1 = 42;

    [Params(1, 4, 16, 64, 256, 1024)]
    public int Z;

    [Benchmark(Baseline = true)]
    public string Original() => InlineDemo.SumTrash2_Original(_x, _y, Z, _x1);

    [Benchmark]
    public string PreConcat() => InlineDemo.SumTrash2_PreConcat(_x, _y, Z, _x1);
}

public static class InlineDemo
{
    [MethodImpl(MethodImplOptions.NoInlining)]
    public static string SumTrash2_Original(string x, string y, int z, int x1)
    {
        var res = "";

        for (int i = 0; i < z; i++)
        {
            res += i + x + y + x1;
        }

        return res;
    }

    [MethodImpl(MethodImplOptions.NoInlining)]
    public static string SumTrash2_PreConcat(string x, string y, int z, int x1)
    {
        var res = "";

        var data = x + y + x1; 

        for (int i = 0; i < z; i++)
        {
            res += i + data;
        }

        return res;
    }
}

Result

Method	Z	Mean	Error	StdDev	Ratio	RatioSD	Gen0	Gen1	Allocated	Alloc Ratio
Original	1	27.66 ns	0.179 ns	0.167 ns	1.00	0.01	0.0143	-	120 B	1.00
PreConcat	1	18.38 ns	0.086 ns	0.076 ns	0.66	0.00	0.0134	-	112 B	0.93
Original	4	127.27 ns	1.347 ns	1.260 ns	1.00	0.01	0.0782	-	656 B	1.00
PreConcat	4	58.02 ns	0.205 ns	0.181 ns	0.46	0.00	0.0545	-	456 B	0.70
Original	16	652.67 ns	3.237 ns	2.870 ns	1.00	0.01	0.6657	-	5568 B	1.00
PreConcat	16	329.46 ns	1.934 ns	1.714 ns	0.50	0.00	0.5498	0.0005	4600 B	0.83
Original	64	4,746.60 ns	28.205 ns	26.383 ns	1.00	0.01	8.5144	0.0381	71232 B	1.00
PreConcat	64	3,385.09 ns	18.963 ns	16.810 ns	0.71	0.01	8.0299	0.0229	67192 B	0.94
Original	256	52,401.48 ns	451.811 ns	422.625 ns	1.00	0.01	130.9204	2.5635	1095192 B	1.00
PreConcat	256	47,885.65 ns	493.215 ns	437.222 ns	0.91	0.01	129.0283	2.5635	1078864 B	0.99
Original	1024	715,410.96 ns	12,209.280 ns	10,195.302 ns	1.00	0.02	2115.2344	162.1094	17743112 B	1.00
PreConcat	1024	685,020.04 ns	6,550.896 ns	5,807.199 ns	0.96	0.02	2107.4219	162.1094	17677632 B	1.00

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[EgorBo]

The idea is good (I'm sure there are open issues/ideas for it already here), but presumably it should be better than just relying on lvIsParam which is way too conservative. It should detect all truly loop invariant operands, I'm not sure what would be the best phase for that. Maybe as part of loop clonning.

[AndyAyersMS]

Loop cloning? Maybe. It has a simple model for loop invariance.

Once we have SSA we have a better model, and one that's easier to reason about.

[EgorBo]

Loop cloning? Maybe. It has a simple model for loop invariance.

Once we have SSA we have a better model, and one that's easier to reason about.

But unfortunately Loop Hoisting happens before that 😞 (if that is the main goal, presumably, even without the hoisting it's still beneficial to do)