[clang] [lld] [llvm] [X86] Use generic CPU tuning when tune-cpu is empty (PR #83631)

[Phoebe Wang via cfe-commits]

phoebewang wrote:

This is a turbulent change to both upstream and downstream tests without any profit as far as I can tell.

I did a similar change for 64-bit a few years ago: https://reviews.llvm.org/D129647

In comparison, this patch is not to solve a specific problem. It should not show any value because we don't care about 32-bit performance. Not to mention, we need to keep the test not changed as many as possible. The way we used in D129647 is to add an explicit "tune-cpu". We cannot blindly update tests in case distort the original intention.

https://github.com/llvm/llvm-project/pull/83631