[jit-experimental] Regression_7 tests failing in

[kunalspathak]

set DOTNET_TieredCompilation=1
set DOTNET_TC_OnStackReplacement=1
set DOTNET_TC_QuickJitForLoops=1
set DOTNET_TC_OnStackReplacement_InitialCounter=1
set DOTNET_OSR_HitLimit=1

23:57:20.255 Passed test: JIT/Regression/CLR-x86-JIT/V1-M09.5-PDC/b26558/b26558/b26558.dll
23:57:20.280 Running test: JIT/Regression/CLR-x86-JIT/V1-M09.5-PDC/b26560/b26560/b26560.dll
App Exit Code: 259
Expected: 100
Actual: 259
END EXECUTION - FAILED
FAILED

Started failing in https://dev.azure.com/dnceng-public/public/_build/results?buildId=1068397&view=ms.vss-test-web.build-test-results-tab&runId=28906490&resultId=122403&paneView=dotnet-dnceng.dnceng-build-release-tasks.helix-test-information-tab

Failing on all platforms.

[dotnet-policy-service]

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[JulieLeeMSFT]

@amanasifkhalid, PTAL.

[amanasifkhalid]

The JIT enters an infinite loop while compiling the OSR variant of the test. The hang happens during flow opts:

• We have the following unconditional jump pattern: block1 -> block2 -> block3. block2 and block3 are empty.
• fgOptimizeBranchToEmptyUnconditional optimizes this pattern to block1 -> block3 -> block2, failing to realize there is an infinite loop between block2 and block3.
• Because the above transformation succeeded, the JIT runs flow opts on block1 again, transforming it back into block1 -> block2 -> block3.

We can see this in the JIT dump:

Optimizing a jump to an unconditional jump (BB21 -> BB13 -> BB20)

Optimizing a jump to an unconditional jump (BB21 -> BB20 -> BB13)

Optimizing a jump to an unconditional jump (BB21 -> BB13 -> BB20)

Optimizing a jump to an unconditional jump (BB21 -> BB20 -> BB13)
...