tree 3e43a86c6439e18376e5484c486108a14a3ef510
parent fe96839f4601e141b8e577eeb9515e39252accf4
author Dan Zheng <danielzheng@google.com> 1571331569 -0700
committer GitHub <noreply@github.com> 1571331569 -0700
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[AutoDiff] SILGen derivative function thunks must not be transparent. (#27752)

SILGen generates thunks for derivative functions registered via
`@differentiable` and `@differentiating` attributes.

The thunks have a canonical, consistent naming (currently based on
the original function name and parameter indices) that TBDGen can also
generate via `SILDeclRef::mangle`.

These SILGen derivative function thunks must not be transparent; otherwise,
the functions will be inlined during MandatoryInlining and will not be
exposed publicly.

This bug was discovered as a DeadFunctionElimination crash:
`function_ref` instructions referencing transparent derivative thunks
had their referenced functions set to null by MandatoryInling and became
invalid.