[PATCH] D37093: [coroutines] Promote cleanup.dest.slot-like allocas to registers to avoid storing them in the coroutine frame

[Gor Nishanov via Phabricator via llvm-commits]

GorNishanov added a comment.

> That's fine, but does not in itself explain why the variable cannot be extracted from the frame, or otherwise made available to the cleanup code. I'm not worried about the size of the stack frame at -O0 being larger than necessary (it will, compared to optimized code, be larger regardless). Moreover, the lowering algorithm must be sound at the IR level (not just in a state where it happens to work for some patterns that Clang happens to create).

For cleanup, clang creates a pattern that current coroutine frame building algorithm cannot handle. To handle it, we will probably need to convert allocas to SSA and see def/use chains, but, once converted to SSA, the problematic pattern disappears.
Another approach is to prove that for a particular alloca store-load pairs never cross a suspend point, if that is the case, we can duplicate the alloca (so that every coroutine part has its own copy) since the value is never carried over from one suspend/resume to another.  However, this algorithm seems complicated and have to be hand built, whereas, we can reuse well-tested alloca promotion algorithm to eliminate the problematic pattern.

Probably moving the promotion from CoroSplit to CoroEarly which rans immediately after front-end will help to make sure that later optimization passes will not interfere with cleanup.dest alloca promotion.


https://reviews.llvm.org/D37093