JIT: Fix register move ordering in genConsumeBlockOp when srcAddr is in dstReg

[Copilot]

MetricsTests.PassingVariableTagsParametersTest fails under JitStress=2 + JitStressRegs={2,0x80} on Linux x64 because the JIT emits incorrect code for GC-aware block copies (CpObjUnroll) when the LSRA allocates the source address to the same register required for the destination (rdi).

Description

Bug: genConsumeBlockOp unconditionally performs the destination move before the source move. When the source address is in dstReg, this silently overwrites it:

; LSRA under stress: srcAddr=rdi, dstAddr=rax
mov  rdi, rax     ; dst → dstReg — clobbers srcAddr already in rdi!
mov  rsi, rdi     ; src → srcReg — copies destination value, not source!

Fix (src/coreclr/jit/codegenlinear.cpp, genConsumeBlockOp):

• When the source address (from a GT_IND CopyBlk) is currently in dstReg, emit the source move to srcReg first, then move the destination to dstReg.
• Add assert(dstAddr->GetRegNum() != srcReg) to guard the unhandled cyclic-swap case (srcAddr in dstReg and dstAddr in srcReg simultaneously), which would need a temp register. The assert makes this fail loudly rather than silently corrupting data.
• Update the existing comment that incorrectly claimed the LSRA always guarantees interference-free copies to required registers in execution order — this bug disproves that claim.

The fix is generic: it applies to all architectures that use genConsumeBlockOp with fixed destination/source register requirements (REG_RDI/REG_RSI on x64, REG_WRITE_BARRIER_DST_BYREF/REG_WRITE_BARRIER_SRC_BYREF on ARM64/LoongArch64/RISCV64).

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 make a fix based on the analysis on the issue. </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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[jakobbotsch]

I don't think this is the right fix. Likely what I am working on in #125333 will fix this in a better way.

[mangod9]

Ok will close this PR then if you are already working on a fix. Thx!

[mangod9]

btw @jakobbotsch is the analysis on the issue incorrect or the proposed fix is not complete. Looks like as part of the analysis it validated the fix worked for a sample test case it had built.

[jakobbotsch]

btw @jakobbotsch is the analysis on the issue incorrect or the proposed fix is not complete. Looks like as part of the analysis it validated the fix worked for a sample test case it had built.

The fix is not sufficient -- there is nothing that prevents the case that the comment is mentioning:

           // Note: a true swap (srcAddr in dstReg AND dstAddr in srcReg) would require a temporary
           // register, but the register allocator should prevent this by ensuring at most one of the
           // two operands ends up in the other's required register.