[release/10.0] JIT: Disallow widening tailcalls in async functions #119175
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The JIT allows tailcalls that depend on widening happening in the callee: e.g. an int32 returning function can tailcall an int16 returning function because the managed ABI specifies that the callee will widen anyway. However, this is not legal for async methods. For async methods the int32 and int16 returning functions come with two different resumption stubs that are responsible for propagating the returned result into the next continuation in asynchronous cases. Allowing an int32 -> int16 tailcall means only a 16-bit value gets propagated into a caller that expects 32 bits to be written into its continuation. Fix #119060
jakobbotsch
reviewed
Aug 28, 2025
jeffschwMSFT
approved these changes
Aug 28, 2025
EgorBo
approved these changes
Aug 28, 2025
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Backport of #119145 to release/10.0
/cc @jakobbotsch
Customer Impact
In some cases a runtime async method may tailcall another runtime async method even though such a tailcall is incompatible. In particular a function returning an an int16 can tailcall a function returning an int8. If the runtime async function finishes asynchronously, then only an 8 bit result value will be propagated to a caller expecting a 16 bit result value.
Regression
Testing
Unit test added.
Risk
Low, only has an impact when runtime async is enabled.