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The Swift calling convention can return values in r8 so switch the GS cookie check to use r9 for its temporary register.

Fix #120557

The Swift calling convention can return values in r8 so switch the GS
cookie check to use r9 for its temporary register.
Copilot AI review requested due to automatic review settings October 15, 2025 11:59
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Pull Request Overview

This PR updates the JIT's GS cookie check on x64 platforms to use register r9 instead of r8 as a temporary register. This change is necessary because the Swift calling convention can return values in r8, and using it for the GS cookie check would overwrite those return values.

Key Changes

  • Changed the temporary register for GS cookie checks from r8 to r9 on AMD64 platforms
  • Updated the corresponding comment to reflect support for Swift calling convention

@github-actions github-actions bot added the area-CodeGen-coreclr CLR JIT compiler in src/coreclr/src/jit and related components such as SuperPMI label Oct 15, 2025
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/azp run runtime

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@jakobbotsch jakobbotsch merged commit cd65a7e into dotnet:main Oct 23, 2025
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@jakobbotsch jakobbotsch deleted the fix-swift-reg branch October 23, 2025 15:22
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Test failure: Interop/Swift/SwiftCallbackAbiStress/SwiftCallbackAbiStress/SwiftCallbackAbiStress.cmd

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