Fix il corruption #2966
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Fix il corruption #2966
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adamsitnik
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Aug 12, 2022
vitek-karas
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Aug 12, 2022
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The code is currently not written to handle replacing instruction sequences of various length for conditions - in this case we would have to remove the ldc.i4.1 and brtrue.s to replace it with a simple br.s. Obviously doable, but would require much more code churn.
#2905 runs into a similar limitation. I haven't look in detail if we could use the same approach for both though. In any case I think this targeted fix is OK for 7.
Co-authored-by: Adam Sitnik <[email protected]>
agocke
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This prevents invalid IL in situations where removing the last instruction of a method would result in the last instruction being a conditional branch. There needs to be some IL in the not-taken branch for the IL to be valid, so this fixes the issue by injecting "ldnull; throw" at the end. Co-authored-by: vitek-karas <[email protected]> Co-authored-by: Adam Sitnik <[email protected]> Commit migrated from dotnet/linker@d559347
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Thanks for sharing the fix @vitek-karas. I cleaned it up a little to be more specific (existing tests which ended in removed ret instructions - for example in a removed else branch - were failing), and changed the test to check the instruction sequence.
This is intended to fix dotnet/runtime#73474. I had trouble creating a repro of the original issue encountered on runtime bits, but the new testcase exhibits what looks like the same issue.
To me it would feel like a cleaner fix to rewrite the always-taken conditional branches to be unconditional branches, but maybe there is some reason we don't do so today. The current fix has lower risk since it is so specifically targeted.