[llvm] [AA] Move Target Specific AA before BasicAA (PR #125965)

[Nikita Popov via llvm-commits]

nikic wrote:

> And do you think it is okay to move the target-specific AA before basic AA for all backends?

Hard to say, given that most users of this are probably out of tree. I looked at a handful of users from https://github.com/search?q=createExternalAAWrapperPass&type=code and e.g. Swift should be fine to run before, while this SeaDsa code looks like it is probably more expensive than BasicAA: https://github.com/seahorn/sea-dsa/blob/8fc33bd29b878c8975e958da594e67ba02bb42f4/lib/seadsa/SeaDsaAliasAnalysis.cc#L126

Another thing to consider here is that changing the order can also impact correctness. For example, we intentionally run TBAA after BasicAA, even though TBAA is a lot cheaper, because we don't want to miscompile "obvious" strict aliasing violations, even though technically we are allowed to do so. So if you have two accesses `(int *)p` and `(float *)p`, LLVM will not aggressively treat them as NoAlias, because BasicAA determines that they are MustAlias first.

https://github.com/llvm/llvm-project/pull/125965