JIT: Add support for async variant delegate GDV

[jakobbotsch]

Switch to the async variant if user implemented when doing delegate GDV. This avoids calling through the task-returning thunk, and further allows inlining the async variant (under the current restrictions).

class Program
{
    public static async Task Main()
    {
        Func<int, Task<int>> del = async x => x * x;

        for (int i = 0; i < 10; i++)
        {
            for (int j = 0; j < 100; j++)
            {
                await Sum(100, del);
            }

            await Task.Delay(100);
        }

        Stopwatch timer = Stopwatch.StartNew();
        await Sum(100_000_000, del);
        Console.WriteLine("Took {0} ms", timer.ElapsedMilliseconds);
    }

    [MethodImpl(MethodImplOptions.NoInlining)]
    private static async Task<int> Sum(int n, Func<int, Task<int>> func)
    {
        int sum = 0;
        for (int i = 0; i < n; i++)
            sum += await func(i);
        return sum;
    }
}

Before: Took 1334 ms
After: Took 122 ms

There are some unnecessary context saves/restores in the inner loop, but at least we do GDV, hoisting and inlining now.

Fix #128260

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[Copilot]

Pull request overview

This PR extends CoreCLR JIT indirect-call transformations so that guarded devirtualization (GDV) of delegate Invoke within async methods can opportunistically switch a guessed target to its user-implemented async variant (when available), enabling additional hoisting/inlining and avoiding the thunk path. It also introduces a small CallArgs helper to standardize insertion of the async-continuation well-known argument.

Changes:

• Enhance GuardedDevirtualizationTransformer to detect AsyncHelpers.Await consumption of a GDV return value, split the await region, and (for applicable guesses) switch to the async “other variant” method handle.
• Add CallArgs::InsertAsyncContinuation and use it from the importer path for ldvirtftn async calls (and from the new GDV async-variant path).
• Refactor/rename some transformer state to m_ members and adjust block weight handling when an await remainder block is introduced.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 6 comments.

File	Description
src/coreclr/jit/indirectcalltransformer.cpp	Adds async-variant selection for delegate GDV in async methods, plus await splitting and related refactoring.
src/coreclr/jit/importercalls.cpp	Switches async-continuation insertion for ldvirtftn async calls to a shared CallArgs helper.
src/coreclr/jit/gentree.h	Declares the new CallArgs::InsertAsyncContinuation helper.
src/coreclr/jit/gentree.cpp	Implements CallArgs::InsertAsyncContinuation insertion policy for the async continuation arg.

Comments suppressed due to low confidence (1)

src/coreclr/jit/indirectcalltransformer.cpp:873

• CreateOrGetLocalForRetExpr writes the unused-return result to *isUnused, but the subsequent conditional checks m_returnValueUnused instead of *isUnused. This breaks callers that pass a different output flag (e.g. m_awaitReturnValueUnused in SplitAwait) and can cause the ret-expr to be linked/handled incorrectly (temp allocated vs NOP substitution) depending on stale member state.

            unsigned resultLcl = BAD_VAR_NUM;
            if (noReturnValue)
            {
                JITDUMP("Linking GT_RET_EXPR [%06u] for VOID return to NOP\n",
                        compiler->dspTreeID(inlineInfo->retExpr));
                inlineInfo->retExpr->gtSubstExpr = compiler->gtNewNothingNode();
            }
            else if (m_returnValueUnused)
            {
                JITDUMP("Linking GT_RET_EXPR [%06u] for UNUSED return to NOP\n",
                        compiler->dspTreeID(inlineInfo->retExpr));
                inlineInfo->retExpr->gtSubstExpr = compiler->gtNewNothingNode();
            }

[Copilot]

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 4 comments.