[ARM64] R2R methods require RCPC2 without using RCPC2 instructions

Description

Crossgen2 adds RCPC2 dependencies to methods that contain no RCPC2 instructions, causing unnecessary R2R rejection on CPUs without RCPC2.

Context: dotnet/android#12722

Reproduction

Create Probe/Probe.csproj:

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFramework>net11.0</TargetFramework>
    <PublishReadyToRun>true</PublishReadyToRun>
  </PropertyGroup>
</Project>

Add Probe/Probe.cs:

using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;

[StructLayout(LayoutKind.Explicit)]
public sealed class Fields
{
    [FieldOffset(0)]
    public volatile int Small;

    [FieldOffset(320)]
    public volatile int Large;
}

public sealed class ReferenceField
{
    public volatile object Value;
}

public static class Probe
{
    [MethodImpl(MethodImplOptions.NoInlining)]
    public static int ReadSmall(Fields value) => value.Small;

    [MethodImpl(MethodImplOptions.NoInlining)]
    public static int ReadLarge(Fields value) => value.Large;

    [MethodImpl(MethodImplOptions.NoInlining)]
    public static void WriteSmall(Fields value, int data) => value.Small = data;

    [MethodImpl(MethodImplOptions.NoInlining)]
    public static void WriteLarge(Fields value, int data) => value.Large = data;

    [MethodImpl(MethodImplOptions.NoInlining)]
    public static void WriteReference(ReferenceField value, object data) => value.Value = data;
}

Publish:

dotnet publish Probe/Probe.csproj -c Release -r linux-arm64

Disassembly and dependency graphs for stock and RCPC1-only builds

Run from the directory containing Probe/. Clean between variants to avoid reusing R2R output:

set -euo pipefail
for policy in stock rcpc-only; do
  output="$PWD/evidence/$policy"
  mkdir -p "$output"
  extra="--mapcsv;--parallelism:1;--dgmllog:$output/dependencies.dgml;--fulllog"
  extra+=";--codegenopt:JitDisasm=*;--codegenopt:JitDisasmTesting=1"
  extra+=";--codegenopt:JitDisasmWithCodeBytes=1;--codegenopt:JitStdOutFile=$output/jit-disasm.txt"
  if [ "$policy" = rcpc-only ]; then
    extra+=";--instruction-set:rcpc,-optimistic"
  fi

  escaped=${extra//;/%3B}
  escaped=${escaped//,/%2C}
  dotnet clean Probe/Probe.csproj -c Release -r linux-arm64
  dotnet publish Probe/Probe.csproj -c Release -r linux-arm64 \
    "-p:PublishReadyToRunCrossgen2ExtraArgs=$escaped" \
    -o "$output/publish"
  cp Probe/obj/Release/net11.0/linux-arm64/R2R/Probe.map.csv "$output/"
done

Actual behavior

The stock build declares RCPC2 for all five methods, but only two use it:

Method	Generated code	Uses RCPC2?
`ReadSmall`	`ldapur w0, [x0, #8]`	Yes
`WriteSmall`	`stlur w1, [x0, #8]`	Yes
`ReadLarge`	`add x0, x0, #328; ldapr w0, [x0]`	No
`WriteLarge`	`add x0, x0, #328; stlr w1, [x0]`	No
`WriteReference`	Call to `CORINFO_HELP_ASSIGN_REF`	No

ReadLarge:

stp     x29, x30, [sp, #-0x10]!
mov     x29, sp
add     x0, x0, #328
ldapr   w0, [x0]
ldp     x29, x30, [sp], #0x10
ret

Despite using only RCPC1, it has this dependency:

MethodWithGCInfo(Probe_Probe__ReadLarge)
  -> PrecodeHelperImport->SignaturePointer_ReadyToRunInstructionSets_Rcpc+Rcpc2,
     reason: classMustBeLoadedBeforeCodeIsRun

The emitted ISA signature also requires RCPC2. With --instruction-set:rcpc,-optimistic, ReadLarge, WriteLarge and WriteReference have byte-identical machine code without that dependency.

Expected behavior

ReadLarge, WriteLarge and WriteReference should not require RCPC2.

Likely cause

Lowering::TryCreateAddrMode calls compOpportunisticallyDependsOn(InstructionSet_Rcpc2) before checking whether the addressing mode is usable.

That call records the dependency, which survives when a later offset/index/containment check fails.

Here, offset 328 is outside the signed 9-bit range. The reference-store case also records RCPC2 even though codegen takes the GC write-barrier path.

Impact and environment

In a MAUI sample-content composite, 117 of 564 RCPC2-guarded bodies contain no RCPC2 instruction. The other 447 genuinely use RCPC2. Startup impact of a fix has not been measured.

Reproduced on a macOS ARM64 host, publishing a net11.0 library for linux-arm64, with:

.NET SDK 11.0.100-rc.1.26431.118
crossgen2/runtime 11.0.0-rc.1.26431.118

The same check is present in runtime main at c307f70 (source inspection only).

Investigation and discussion.

Drafted with the Copilot App.

@JulieLeeMSFT

Tagging subscribers to this area: @JulieLeeMSFT, @jakobbotsch
See info in area-owners.md if you want to be subscribed.

Tagging subscribers to this area: @dotnet/crossgen-contrib
See info in area-owners.md if you want to be subscribed.

@JulieLeeMSFT

Tagging subscribers to this area: @JulieLeeMSFT, @jakobbotsch
See info in area-owners.md if you want to be subscribed.

This is JIT bug in this code:

runtime/src/coreclr/jit/codegenarmarch.cpp

Lines 1656 to 1693 in b44cd90

    
           // With RCPC2 (arm64 v8.4+) we can work with simple addressing modes like [reg + simm9] 
        
           const bool shouldUseRcpc2 = !addrIsInReg && indir->Addr()->OperIs(GT_LEA) && !indir->HasIndex() && 
        
                                       (indir->Scale() == 1) && 
        
                                       emitter::emitIns_valid_imm_for_unscaled_ldst_offset(indir->Offset()) && 
        
                                       m_compiler->compOpportunisticallyDependsOn(InstructionSet_Rcpc2); 
        
           if (shouldUseRcpc2) 
        
           { 
        
               assert(!addrIsInReg); 
        
               switch (currentIns) 
        
               { 
        
                       // Loads 
        
                   case INS_ldrb: 
        
                       return INS_ldapurb; 
        
                   case INS_ldrh: 
        
                       return INS_ldapurh; 
        
                   case INS_ldr: 
        
                       return INS_ldapur; 
        
                       // Stores 
        
                   case INS_strb: 
        
                       return INS_stlurb; 
        
                   case INS_strh: 
        
                       return INS_stlurh; 
        
                   case INS_str: 
        
                       return INS_stlur; 
        
                   default: 
        
                       *needsBarrier = true; 
        
                       return currentIns; 
        
               } 
        
           }

@JulieLeeMSFT

Tagging subscribers to this area: @JulieLeeMSFT, @jakobbotsch
See info in area-owners.md if you want to be subscribed.

Fix JIT over reporting RCPC2 (#135134)

Navigation Menu

Repository navigation

[ARM64] R2R methods require RCPC2 without using RCPC2 instructions #133671

Description

Reproduction

Actual behavior

Expected behavior

Likely cause

Impact and environment

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

	// With RCPC2 (arm64 v8.4+) we can work with simple addressing modes like [reg + simm9]
	const bool shouldUseRcpc2 = !addrIsInReg && indir->Addr()->OperIs(GT_LEA) && !indir->HasIndex() &&
	(indir->Scale() == 1) &&
	emitter::emitIns_valid_imm_for_unscaled_ldst_offset(indir->Offset()) &&
	m_compiler->compOpportunisticallyDependsOn(InstructionSet_Rcpc2);

	if (shouldUseRcpc2)
	{
	assert(!addrIsInReg);
	switch (currentIns)
	{
	// Loads

	case INS_ldrb:
	return INS_ldapurb;

	case INS_ldrh:
	return INS_ldapurh;

	case INS_ldr:
	return INS_ldapur;

	// Stores

	case INS_strb:
	return INS_stlurb;

	case INS_strh:
	return INS_stlurh;

	case INS_str:
	return INS_stlur;

	default:
	*needsBarrier = true;
	return currentIns;
	}
	}

[ARM64] R2R methods require RCPC2 without using RCPC2 instructions #133671

Description

Description

Reproduction

Actual behavior

Expected behavior

Likely cause

Impact and environment

Activity

dotnet-policy-service commented on Sep 11, 2026

EgorBo commented on Sep 11, 2026

JulieLeeMSFT commented on Sep 11, 2026

dotnet-policy-service commented on Sep 11, 2026

dotnet-policy-service commented on Sep 11, 2026

jkotas commented on Sep 11, 2026

dotnet-policy-service commented on Sep 26, 2026

Metadata

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Issue actions