[flang] [libcxx] [clang] [llvm] [lld] [libcxxabi] [lldb] [libc] [msan] Unpoison indirect outputs for userspace using llvm.memset.* (PR #79924)

[Fangrui Song via cfe-commits]

MaskRay wrote:

> LGTM in general.
> 
> But the downside here is that the compiler won't be able to optimize away repeated stores if they are transformed to memset calls. Maybe introduce some threshold and only memset() sizes greater than, say, 64? (IIRC that's what `-ftrivial-auto-var-init` does)

TIL. Changed to keep using StoreInst when size <= 32, similar to `shouldUseBZeroPlusStoresToInitialize` `-ftrivial-auto-var-init`)

https://github.com/llvm/llvm-project/pull/79924