JIT: Fix register move ordering bug in genConsumeBlockOp for CpObjUnroll

[Copilot]

Under JitStressRegs, LSRA can assign the source address of a block copy to the register that is the fixed requirement for the destination (e.g. source → RDI, dest → RAX, with CpObjUnroll needing dst=RDI, src=RSI on Linux x64 SysV ABI). The original code always moved the destination first, clobbering the source:

mov rdi, rax   ; dst → dstReg  ← overwrites source already in rdi
mov rsi, rdi   ; src → srcReg  ← copies destination, not source → corrupt struct copy

This manifested as MetricsTests.PassingVariableTagsParametersTest corrupting the first KeyValuePair element when passing 4+ tags via params ReadOnlySpan<KeyValuePair<string, object?>> (which triggers inline-array-at-call-site codegen, producing a CpObjUnroll with GC-ref struct fields).

Changes

• src/coreclr/jit/codegenlinear.cpp — genConsumeBlockOp
  • After consuming all operands, detect when the source address's current register equals the required destination register (srcCurReg == dstReg). In that case, emit the source move first, then the destination — avoiding the clobber.
  • Only applies to CopyBlk with a GT_IND source; local sources use a LEA (no conflict possible).
  • Adds assert(dstAddr->GetRegNum() != srcReg) to surface any circular-dependency (register-swap) case, which LSRA is not expected to produce.
  • Corrects the function comment, which incorrectly claimed LSRA guarantees conflict-freedom when copying to fixed-register requirements.

Security

No security impact. Pure codegen correctness fix affecting register-constrained struct copy sequences.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Test failure: System.Diagnostics.Metrics.Tests.MetricsTests.PassingVariableTagsParametersTest</issue_title>
<issue_description>Failed in: runtime-coreclr libraries-jitstress2-jitstressregs 20260307.1

Failed tests:

net11.0-linux-Release-x64-jitstress2_jitstressregs2-AzureLinux.3.Amd64.Open
- System.Diagnostics.Metrics.Tests.MetricsTests.PassingVariableTagsParametersTest
net11.0-linux-Release-x64-jitstress2_jitstressregs0x80-AzureLinux.3.Amd64.Open
- System.Diagnostics.Metrics.Tests.MetricsTests.PassingVariableTagsParametersTest

Error message:

Microsoft.DotNet.RemoteExecutor.RemoteExecutionException : Remote process failed with an unhandled exception.

Stack trace:

Child exception:
  Xunit.Sdk.EqualException: Assert.Equal() Failure: Collections differ
           ↓ (pos 0)
Expected: [["K1"] = "V1", ["K2"] = "V2", ["K3"] = "V3", ["K4"] = "V4", ["K5"] = "V5", ···]
Actual:   [[null] = null, ["K2"] = "V2", ["K3"] = "V3", ["K4"] = "V4", ["K5"] = "V5", ···]
           ↑ (pos 0)
   at System.Diagnostics.Metrics.Tests.MetricsTests.<>c__DisplayClass32_0`1.<InstrumentPassingVariableTagsParametersTest>b__1(Instrument inst, T measurement, ReadOnlySpan`1 tags, Object state) in /_/src/libraries/System.Diagnostics.DiagnosticSource/tests/MetricsTests.cs:line 1965
   at System.Diagnostics.Metrics.Instrument`1.RecordMeasurement(T measurement, ReadOnlySpan`1 tags) in /_/src/libraries/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/Metrics/Instrument.common.cs:line 99
   at System.Diagnostics.Metrics.Tests.MetricsTests.PublishCounterMeasurement[T](Counter`1 counter, T value, KeyValuePair`2[] tags) in /_/src/libraries/System.Diagnostics.DiagnosticSource/tests/MetricsTests.cs:line 1791
   at System.Diagnostics.Metrics.Tests.MetricsTests.<PassingVariableTagsParametersTest>b__10_3(Instrument`1 instrument, Int32 value, KeyValuePair`2[] tags) in /_/src/libraries/System.Diagnostics.DiagnosticSource/tests/MetricsTests.cs:line 751
   at System.Diagnostics.Metrics.Tests.MetricsTests.InstrumentPassingVariableTagsParametersTest[T](Instrument`1 instrument, Func`4 record) in /_/src/libraries/System.Diagnostics.DiagnosticSource/tests/MetricsTests.cs:line 2015
   at System.Diagnostics.Metrics.Tests.MetricsTests.<PassingVariableTagsParametersTest>b__10_0() in /_/src/libraries/System.Diagnostics.DiagnosticSource/tests/MetricsTests.cs:line 749
   at System.Reflection.MethodBaseInvoker.InterpretedInvoke_Method(Object obj, IntPtr* args) in /_/src/coreclr/System.Private.CoreLib/src/System/Reflection/MethodBaseInvoker.CoreCLR.cs:line 36
   at System.Reflection.MethodBaseInvoker.InvokeWithNoArgs(Object obj, BindingFlags invokeAttr) in /_/src/libraries/System.Private.CoreLib/src/System/Reflection/MethodBaseInvoker.cs:line 57

Child process:
  System.Diagnostics.DiagnosticSource.Tests, Version=11.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51 System.Diagnostics.Metrics.Tests.MetricsTests Void <PassingVariableTagsParametersTest>b__10_0()
```</issue_description>

<agent_instructions>Please look through the suggested fix in the latest comment and create a PR with the fix. </agent_instructions>

## Comments on the Issue (you are @copilot in this section)

<comments>
<comment_new><author>@mangod9</author><body>
Here is some copilot analysis for this failure:

```text 
Crash Context

  ┌────────────┬───────────────────────────────────────────────────────────────────────────────────────────────────┐
  │ Field      │ Value                                                                                             │
  ├────────────┼───────────────────────────────────────────────────────────────────────────────────────────────────┤
  │ Build      │ runtime-coreclr libraries-jitstress2-jitstressregs 20260307.1                                     │
  │            │ (https://dev.azure.com/dnceng-public/cbb18261-c48f-4abb-8651-8cdcb5474649/_buil                   │
  │            │ d/results?buildId=1324993)                                                                        │
  ├────────────┼───────────────────────────────────────────────────────────────────────────────────────────────────┤
  │ Helix Job  │ 3f3cc108-6550-42e6-80a5-5e70ed9c556a                                                              │
  │ (regs2)    │                                                                                                   │
  ├────────────┼───────────────────────────────────────────────────────────────────────────────────────────────────┤
  │ Helix Job  │ acfbbeb5-9555-4c5b-b5ea-892f475ef22b                                                              │
  │ (regs0x80) │                                                                                                   │
  ├────────────┼───────────────────────────────────...

</details>



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- Fixes dotnet/runtime#125312

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[mangod9]

Closing, not sure why copilot is creating multiple PRs