[release/9.0-preview7] JIT: Fix placement of GT_START_NOGC for tailcalls in face of bulk copy with write barrier calls
#105572
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…opy with write barrier calls When the JIT generates code for a tailcall it must generate code to write the arguments into the incoming parameter area. Since the GC ness of the arguments of the tailcall may not match the GC ness of the parameters, we have to disable GC before we start writing these. This is done by finding the earliest `GT_PUTARG_STK` node and placing the start of the NOGC region right before it. In addition, there is logic to take care of potential overlap between the arguments and parameters. For example, if the call has an operand that uses one of the parameters, then we must take care that we do not override that parameter with the tailcall argument before the use of it. To do so, we sometimes may need to introduce copies from the parameter locals to locals on the stack frame. This used to work fine, however, with #101761 we started transforming block copies into managed calls in certain scenarios. It was possible for the JIT to decide to introduce a copy to a local and for this transformation to then kick in. This would cause us to end up with the managed helper call after starting the nogc region. In checked builds this would hit an assert during GC scan; in release builds, it would end up with corrupted data. The fix here is to make sure we insert the `GT_START_NOGC` after all the potential temporary copies we may introduce as part of the tailcat stll logic. There was an additional assumption that the first `PUTARG_STK` operand was the earliest one in execution order. That is not guaranteed, so this change stops relying on that as well by introducing a new `LIR::FirstNode` and using that to determine the earliest `PUTARG_STK` node. Fix #102370 Fix #104123 Fix #105441
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EgorBo
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Jul 26, 2024
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CC @jeffschwMSFT for approval. |
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cc @carlossanlop. |
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This was referenced Jul 30, 2024
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Backport of #105551 to release/9.0-preview7
/cc @hoyosjs @jakobbotsch
Customer Impact
Customer code is subject to crashes, memory corruption, and process control reaching unexplainable points.
For instance, Roslyn has been observed crashing in several unexplainable ways. Some of the observed cases were tracked down to a GC hole caused by a safe point contained in a no-gc region of parameter setup in fast tail calls. The GC would kick when the method was calling
BulkMoveWithWriteBarrierto copy a large struct into the frame and this would result in a bad object pointer getting copied. This fix removed the safe point from the region.#102370
#104123
#105441
dotnet/roslyn-analyzers#7349
dotnet/dnceng#3305
Testing
Regression test added and manual verification of partner scenarios.