JIT: Add a "default value analysis" for async hoisting

[jakobbotsch]

This analysis computes the set of locals that still have their default value and uses it to skip hoisting these locals into continuations around async suspension points.

Fix #124512

Example from #124512:

using System;
using System.Runtime.CompilerServices;
using System.Threading.Tasks;

public class Program
{
    public static async Task Main()
    {
        await FooAsync();
    }

    private static async Task FooAsync()
    {
        await Task.Yield();
        Inline();
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    private static void Inline()
    {
        S s = new S();
        Console.WriteLine(s.A);
    }

    private struct S
    {
        public long A, B, C, D, E;
        public object F;

        [MethodImpl(MethodImplOptions.NoInlining)]
        public S()
        {
            A = B = C = D = E = 10;
            F = null;
        }
    }
}

...
@@ -137,15 +130,10 @@ G_M000_IG19:
        mov      rdx, 0xD1FFAB1E
        call     [CORINFO_HELP_ALLOC_CONTINUATION]
        mov      rbx, rax
-       lea      rcx, [reloc @RWD00]
-       mov      qword ptr [rbx+0x10], rcx
-       xor      ecx, ecx
-       mov      qword ptr [rbx+0x18], rcx
-       lea      rdi, bword ptr [rbx+0x28]
-       lea      rsi, bword ptr [rbp-0x58]
-       call     CORINFO_HELP_ASSIGN_BYREF
-       mov      ecx, 5
-       rep movsq 
+       lea      rax, [reloc @RWD00]
+       mov      qword ptr [rbx+0x10], rax
+       xor      eax, eax
+       mov      qword ptr [rbx+0x18], rax
        call     [System.Runtime.CompilerServices.AsyncHelpers:CaptureExecutionContext():System.Threading.ExecutionContext]
        lea      rcx, bword ptr [rbx+0x20]
        mov      rdx, rax
@@ -153,55 +141,43 @@ G_M000_IG19:
        cmp      gword ptr [rbp+0x10], 0
        setne    cl
        movzx    rcx, cl
-       mov      rdx, gword ptr [rbp-0x20]
-       mov      r8, gword ptr [rbp-0x28]
+       mov      rdx, gword ptr [rbp-0x18]
+       mov      r8, gword ptr [rbp-0x20]
        call     [System.Runtime.CompilerServices.AsyncHelpers:RestoreContextsOnSuspension(bool,System.Threading.ExecutionContext,System.Threading.SynchronizationContext)]
        mov      rcx, rbx
  
 G_M000_IG20:
-       vzeroupper 
-       add      rsp, 104
+       add      rsp, 96
        pop      rbx
        pop      rsi
-       pop      rdi
        pop      rbp
        ret      
  
 G_M000_IG21:
-       mov      rdx, gword ptr [rbp+0x10]
-       mov      rcx, gword ptr [rdx+0x20]
+       mov      rcx, gword ptr [rbp+0x10]
+       mov      rcx, gword ptr [rcx+0x20]
        call     [System.Runtime.CompilerServices.AsyncHelpers:RestoreExecutionContext(System.Threading.ExecutionContext)]
- 
-G_M000_IG22:
-       mov      rdx, gword ptr [rbp+0x10]
-       vmovdqu  ymm0, ymmword ptr [rdx+0x28]
-       vmovdqu  ymmword ptr [rbp-0x58], ymm0
-       vmovdqu  xmm0, xmmword ptr [rdx+0x48]
-       vmovdqu  xmmword ptr [rbp-0x38], xmm0
- 
-G_M000_IG23:
        jmp      G_M000_IG05
  
-G_M000_IG24:
+G_M000_IG22:
        sub      rsp, 40
  
-G_M000_IG25:
+G_M000_IG23:
        cmp      gword ptr [rbp+0x10], 0
        setne    cl
        movzx    rcx, cl
-       mov      rdx, gword ptr [rbp-0x20]
-       mov      r8, gword ptr [rbp-0x28]
+       mov      rdx, gword ptr [rbp-0x18]
+       mov      r8, gword ptr [rbp-0x20]
        call     [System.Runtime.CompilerServices.AsyncHelpers:RestoreContexts(bool,System.Threading.ExecutionContext,System.Threading.SynchronizationContext)]
        nop      
  
-G_M000_IG26:
-       vzeroupper 
+G_M000_IG24:
        add      rsp, 40
        ret      
  
 RWD00  	dq	(dynamicClass):IL_STUB_AsyncResume(System.Object,byref):System.Object
 	dq	G_M000_IG19
 
-; Total bytes of code 544
+; Total bytes of code 471

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[Copilot]

Pull request overview

This PR adds a new "default value analysis" for async hoisting to address issue #124512, where the JIT incorrectly hoists default-valued locals at async suspension points. The issue occurs when the JIT suppresses explicit zeroing of locals (expecting them to be zeroed by the prolog), but then hoists these uninitialized locals into async continuations when their address is taken.

Changes:

• Introduces a two-phase DefaultValueAnalysis class that tracks which locals retain their default (zero) values through program execution
• Integrates this analysis with the existing AsyncLiveness class to skip hoisting locals that are live but still have default values at suspension points
• Handles promoted struct fields correctly by checking if all fields have default values

[Copilot]

Copilot encountered an error and was unable to review this pull request. You can try again by re-requesting a review.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

Comments suppressed due to low confidence (1)

src/coreclr/jit/async.cpp:597

• There's an extra line separator here (line 597). Following the pattern seen elsewhere in the file, function header comments should start with a single line separator, not a double one.

//------------------------------------------------------------------------

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

Comments suppressed due to low confidence (1)

src/coreclr/jit/async.cpp:583

• There are duplicate comment separator lines (------------------------------------------------------------------------). The second separator on line 583 should be removed as it's redundant.

//------------------------------------------------------------------------
//------------------------------------------------------------------------

[jakobbotsch]

cc @dotnet/jit-contrib PTAL @AndyAyersMS

Diffs. Throughput wise pretty much a wash.

The analysis here was written almost completely autonomously by Copilot with Opus 4.6. I made a few changes:

• I added handling for promoted locals
• The bit vector handling was pretty wasteful, so I optimized it a bit
• I added correct handling for locals not initialized in prolog

Still, I was quite impressed by the result.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

[jakobbotsch]

/azp run runtime-nativeaot-outerloop

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[AndyAyersMS]

Just a comment nit you can fix later.

[jakobbotsch]

/ba-g Failure looks similar to #124370 and not related to async