Fix return type misclasiffication in TransitionBlock.cs#129143
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The ComputeReturnValueTreatment has a typo that causes wrong return type classification for structs passed in registers on x64 Unix. It causes wrong classification when there are two eightbytes, the first eightbyte is classified as SystemVClassificationTypeSSE and the second one is not. It may lead to GC hole in that case.
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Pull request overview
This PR corrects the System V AMD64 (x64 Unix) struct-return classification logic in the ReadyToRun transition block so that the second eightbyte’s classification is respected when computing the encoded FP return information.
Changes:
- Fixes a duplicated check so the second eightbyte uses
eightByteClassifications1instead of re-checkingeightByteClassifications0.
Comment on lines
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| // Size of the struct is 16 bytes | ||
| fpReturnSize = 16; | ||
| // The lowest two bits of the size encode the order of the int and SSE fields | ||
| if (descriptor.eightByteClassifications0 == SystemVClassificationType.SystemVClassificationTypeSSE) | ||
| { | ||
| fpReturnSize += 1; | ||
| } | ||
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| if (descriptor.eightByteClassifications0 == SystemVClassificationType.SystemVClassificationTypeSSE) | ||
| if (descriptor.eightByteClassifications1 == SystemVClassificationType.SystemVClassificationTypeSSE) | ||
| { | ||
| fpReturnSize += 2; |
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I agree that a test would be nice, but I am not sure how hard it is to create a test that deterministically fails due to this bug.
jkotas
approved these changes
Jun 8, 2026
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The ComputeReturnValueTreatment has a typo that causes wrong return type classification for structs passed in registers on x64 Unix. It causes wrong classification when there are two eightbytes, the first eightbyte is classified as SystemVClassificationTypeSSE and the second one is not. It may lead to GC hole in that case.