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I am 100% in agreement with the C standard committee on this one. Treating all use of uninitialized memory as undefined behavior is a reasonable limitation that gives compiler writers a great deal of flexibility to improve performance. If the only loss is the ability to use uninitialized memory as a source of entropy for random number generators, then that is an incredibly low price to pay for the increased performance.

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Then why isn't the code being disallowed? Why is it being silently removed in the middle of the night through an optimization back door?

It is disallowed by the standard.

It's not disallowed by compilers, because they're deficient. In this case I'd guess compiler front-end doesn't track which values in an array are initialized, and when optimizing back-end sees UB it's too late to emit an error.

They are improving performance in what way? My compiler: read input; emit nop, improves performance too.

If your program ALSO compiles VALID programs correctly, and somehow those are faster because it replaces INVALID programs with a nop, that sounds great!

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