[libcxx-commits] [libcxx] [libc++] Add a utility for checking the output of commands (PR #65917)

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EricWF wrote:

I'm not OK importing our own version of an LLVM tool that we should just use if we need it.
Also, reading the tests you added with the change look like they won't last more than one version before they start failing.

Further, these tests are a second order measure of the thing we're actually trying to measure, which is "what is the performance of this code". 

Lets say that the code that used to be lowered to `__memmove` start being lowered to some instruction that's faster. The tests start failing.

Tests should only fail when the property under test is no longer true. 

For example, microbenchmarks seem like the more appropriate way to test performance.  Sure they're noisy, but they're a 1st order derivative of what we're trying to test. 



https://github.com/llvm/llvm-project/pull/65917