[PATCH] D137497: [ArgumentPromotion] Allow the frontend to specify the maximum number of elements to promote on a per-function basis via metadata.

[Patrick Walton via Phabricator via llvm-commits]

pcwalton added a comment.

Sure. Check this one out. It's `backtrace::symbolize::gimli::macho::Object::parse` in `backtrace-rs`.

No argument promotion: https://gist.github.com/pcwalton/38ff053dd1973ca383b85183e13de0fc

With argument promotion with elements = 99 (converting hidden to internal so arg promotion works): https://gist.github.com/pcwalton/6195ccd7b1b49cde8920f3e65c048b84

Notice that:

- Many of the structures have more elements than 2.

- There are no memcpys in `parse()` with argument promotion. Without it there are 384 bytes in 13 stack to stack memcpys. Teaching memcpyopt to eliminate these would require a lot of expensive flow sensitive analysis if it's even feasible at all.

Here is the codegen without the argument promotion: https://gist.github.com/pcwalton/6c589ee875d5f6a639d2139990db66d2

And with argument promotion with max elements at 99: https://gist.github.com/pcwalton/f359acd836a61ceb8669c6ce823d141e

Notice that, with the argument promotion, we reduce the size of the stack in that function from 408 bytes to 328 bytes, a 24% savings. The main remaining memcpy-like codegen segment is basically just copying into the retptr which we may be able to eliminate in other ways.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D137497/new/

https://reviews.llvm.org/D137497