As a result of -ftrivial-auto-var-init, clang generates instructions to set alloca'd memory to a given pattern, right after the allocation site. In some cases, this (somehow costly) operation could be delayed, leading to conditional execution in some cases.
This is not an uncommon situation: it happens ~600 times on the cPython code base, and much more on the LLVM codebase. The benefit greatly varies on the execution path, but it should not regress on performance.