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[release/5.0] Fix for Issue 44895 - Incorrect codegen with multiple returns of a field of a Nullable or struct type - #45989
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* Fix for Issue 44895 - Fix: Don't allow an unwrapped promoted field of TYP_REF to be returned when we are expecting a TYP_STRUCT Backout change in gtGetStructHandleIfPresent for GT_RETURN as it isn't needed for this fix Deoptimize all GT_RETURN's with mismatched types for promoted struct fields. * Allow both GT_ADDR and GT_ASG as a parent node * Add second test case Repro2_44895.cs * Change assert about Incompatible types to be a noway_assert in gtNewTempAssign * Only use the smaller repro case for Runtime_44895.cs * Added noway_assert in release build for an assignment of a TYP_REF to a TYP_STRUCT * rerun jit-format
AndyAyersMS
approved these changes
Dec 12, 2020
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Backport of #45284 to release/5.0
Fixes: #44895
Customer Impact
Reported by customer running .net 5.0. When optimizing we would incorrectly optimize a method that returns a nullable type.
The method needed to have more than 4 returns in the method body. This is silent bad codegen when optimizing.
The user would see an invalid return value from the function call.
Note that this issue also applies to other struct types beyond the nullable types. It is just more commonly seen with a nullable type.
The return statement must be field of a struct that was struct promoted and the return value must be a struct with a single ref field.
Regression?
Yes, this is a regression from .NET Core 3.1
Testing
Manual, new unit test, CLR outerloop, asm diffs.
Risk
Low
Allow only GT_ADDR and GT_ASG as a parent node
Add test case Runtime_44895.cs
Change assert about Incompatible types to be a noway_assert in gtNewTempAssign
Added noway_assert in release build for an assignment of a TYP_REF to a TYP_STRUCT