The place where I see this really hurts goes when Clang/LLVM gets too fancy, in situations like this:

  - Function A calls function B, which returns some struct S (for instance on the stack).
  - B writes S by individual (small) stores.
  - A wants to copy S from some place to another (e.g. store it in some other struct).
  - LLVM coalesces the individual loads/stores needed to copy S, into one or a series of large operations (e.g. 128-bit SSE2 loads+stores).
  - These large loads are issued while the small stores from B are still pending, and necessarily overlap them.

This happens with partial autovectorization, too. Compiler fails to vectorize a first loop and then vectorizes the second, result is a store forwarding failure at the start of the second loop trying to read the output of the first loop, erasing the vectorization gains.