[release/9.0-staging] JIT: Read back all replacements before statements with implicit EH control flow

[github-actions]

Backport of #109107 to release/9.0

/cc @jakobbotsch

Customer Impact

• Customer reported
• Found internally

#108969

Silent bad code generation in some cases involving structs, type tests, and exception handling. Programs may unexpectedly crash or compute incorrectly.

As an example, given

    private struct S
    {
        public int A, B, C, D;
    }
   ...
        S v = default;
        try
        {
            v = Bar();
            Use((int?)o);
        }
        catch (Exception)
        {
        }

        Use(v.A);
        Use(v.A);

The JIT will promote the v.A field to a local, and so must ensure that all calls to Use have the correct value, even if an exception is thrown. If the exception site is within a type test (here induced by the (int?)o cast) or similar construct within a try, then value of the promoted local may not be initialized correctly.

There is a workaround (using the undocumented env var DOTNET_JitEnablePhysicalPromotion=0) but it will not be easy for customers to determine the underlying problem and realize there is a workaround available.

Regression

• Yes
• No

Regression from .NET 8.

Testing

Verified on the example in #108969 and new test case added to capture this particular pattern.

Risk

Low. The fix is fairly surgical. A small number of methods impacted in our SPMI suite.

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[jakobbotsch]

This one should target 9.0-staging, but looks like that branch doesn't exist yet.

[jeffschwMSFT]

lgtm. please get a code review. we will take for consideration in 9.0.x

[carlossanlop]

@jakobbotsch - Please retarget this PR to the release/9.0-staging branch.

You can retarget to that branch by clicking on the Edit button on the top right (next to the PR title) and choosing the release/9.0-staging branch from the dropdown. Important: Please make sure you don't bring any unrelated changes from the release/9.0 branch when retargeting to release/9.0-staging.

[jakobbotsch]

@carlossanlop I retargeted this, but you are still set as changes requested. Can it be merged?

[jeffschwMSFT]

once ready, this can be merged

[jakobbotsch]

Ping @carlossanlop

[kirsan31]

So we aiming to 9.0.2 now?

[jakobbotsch]

So we aiming to 9.0.2 now?

Yes, it's looking like this fix will be published with the January servicing release at this point.

[jeffschwMSFT]

9.0.2 is our February servicing release

[jakobbotsch]

My apologies @kirsan31, it looks like I dropped the ball here on getting this PR merged in time for the first servicing release (which I believe to be the January one).

[jakobbotsch]

/ba-g Failures are known