Avoid unnecessary return val copying in tailcall-via-help

[jakobbotsch]

My initial implementation of this did not handle the fact that retbuf
can point to GC heap during reflection invoke. It was fixed in #39815,
but the way it was fixed was by copying it into a local. This changes
the fix so that we simply report the return value pointer as a byref
throughout the mechanism, which simplifies the JIT's handling and is a
perf improvement as well.

Benchmark:

[Benchmark]
public long Bench()
{
    return Tail(100_000_000, new int[5]).Length;
}

private static Span<int> Tail(int count, Span<int> s)
{
    if (count == 0)
        return s;

    IL.Push(count - 1);
    IL.Emit.Ldarg(nameof(s));
    IL.Emit.Tail();
    IL.Emit.Call(new MethodRef(typeof(Benchmark), nameof(Tail)));
    IL.Emit.Ret();
    throw IL.Unreachable();
}

Result:

Method	Job	Toolchain	Mean	Error	StdDev	Ratio	RatioSD
Bench	Job-BWZGON	\base\corerun.exe	3.949 s	0.0788 s	0.0809 s	1.00	0.00
Bench	Job-SJJEVX	\diff\corerun.exe	3.785 s	0.0154 s	0.0120 s	0.96	0.02

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Issue Details

---

My initial implementation of this did not handle the fact that retbuf
can point to GC heap during reflection invoke. It was fixed in #39815,
but the way it was fixed was by copying it into a local. This changes
the fix so that we simply report the return value pointer as a byref
throughout the mechanism, which simplifies the JIT's handling and is a
perf improvement as well.

Benchmark:

[Benchmark]
public long Bench()
{
    return Tail(100_000_000, new int[5]).Length;
}

private static Span<int> Tail(int count, Span<int> s)
{
    if (count == 0)
        return s;

    IL.Push(count - 1);
    IL.Emit.Ldarg(nameof(s));
    IL.Emit.Tail();
    IL.Emit.Call(new MethodRef(typeof(Benchmark), nameof(Tail)));
    IL.Emit.Ret();
    throw IL.Unreachable();
}

Result:

Method	Job	Toolchain	Mean	Error	StdDev	Ratio	RatioSD
Bench	Job-BWZGON	\base\corerun.exe	3.949 s	0.0788 s	0.0809 s	1.00	0.00
Bench	Job-SJJEVX	\diff\corerun.exe	3.785 s	0.0154 s	0.0120 s	0.96	0.02

Author:	jakobbotsch
Assignees:	jakobbotsch
Labels:	area-CodeGen-coreclr
Milestone:	-