JIT: Fix RBO introducing data races #89710
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RBO cannot safely duplicate a GTF_GLOB_REF tree without additional checks because this may introduce data races. To be able to duplicate such a tree we need to verify that the local defined it is truly dead after the transformation. Fix dotnet#78713
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Tagging subscribers to this area: @JulieLeeMSFT, @jakobbotsch Issue DetailsRBO cannot safely duplicate a GTF_GLOB_REF tree without additional checks because this may introduce data races. To be able to duplicate such a tree we need to verify that the local defined it is truly dead after the transformation. Fix #78713
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cc @dotnet/jit-contrib PTAL @AndyAyersMS Diffs. As you can see from the diffs CSE generally comes along and removes the data race that RBO introduces in these cases, so the problem currently is more theoretical than not. But overall the diffs of the fix are quite small and removes this cross-phase dependency, so I think we can take the change. Let me know if you disagree, I don't have strong feelings about this. |
Sadly not that easy, probably. For example, one case looks like: ------------ BB01 [000..00C) -> BB05 (cond), preds={} succs={BB02,BB05}
***** BB01
STMT00000 ( 0x000[E-] ... 0x00A )
N007 ( 9, 6) [000005] DA-XG------ ▌ STORE_LCL_VAR int V04 tmp1 d:2 $244
N006 ( 9, 6) [000004] ---XG------ └──▌ NE int <l:$283, c:$282>
N004 ( 4, 4) [000002] ---XG------ ├──▌ IND ref <l:$242, c:$243>
N003 ( 2, 2) [000096] -------N--- │ └──▌ ADD byref $180
N001 ( 1, 1) [000000] ----------- │ ├──▌ LCL_VAR ref V00 this u:1 $80
N002 ( 1, 1) [000095] ----------- │ └──▌ CNS_INT long 40 Fseq[<unknown field>] $140
N005 ( 1, 1) [000003] ----------- └──▌ CNS_INT ref null $VN.Null
***** BB01
STMT00002 ( ??? ... 0x00A )
N002 ( 5, 4) [000011] DA--------- ▌ STORE_LCL_VAR int V05 tmp2 d:2 $VN.Void
N001 ( 1, 1) [000006] ----------- └──▌ LCL_VAR int V04 tmp1 u:2 <l:$280, c:$281>
***** BB01
STMT00001 ( ??? ... ??? )
N004 ( 5, 5) [000010] ----------- ▌ JTRUE void $VN.Void
N003 ( 3, 3) [000009] J------N--- └──▌ NE int <l:$280, c:$281>
N001 ( 1, 1) [000007] ----------- ├──▌ LCL_VAR int V05 tmp2 u:2 (last use) <l:$280, c:$281>
N002 ( 1, 1) [000008] ----------- └──▌ CNS_INT int 0 $40
What could be doable is to do a "mini" CSE here inside of RBO by allowing the duplication here after realizing that it's fine if we then manually CSE |
RBO cannot safely duplicate a GTF_GLOB_REF tree without additional checks because this may introduce data races. To be able to duplicate such a tree we need to verify that the local defined is truly dead after the transformation.
Fix #78713