[all-commits] [llvm/llvm-project] b8fc71: [WebAssembly] Share rethrowing BBs in LowerEmscrip...

[Heejin Ahn via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: b8fc71b7aeeda0c86a4920da201cc19132ad6d03
      https://github.com/llvm/llvm-project/commit/b8fc71b7aeeda0c86a4920da201cc19132ad6d03
  Author: Heejin Ahn <aheejin at gmail.com>
  Date:   2021-08-30 (Mon, 30 Aug 2021)

  Changed paths:
    M llvm/lib/Target/WebAssembly/WebAssemblyLowerEmscriptenEHSjLj.cpp
    M llvm/test/CodeGen/WebAssembly/lower-em-ehsjlj.ll
    M llvm/test/CodeGen/WebAssembly/lower-em-sjlj-debuginfo.ll
    M llvm/test/CodeGen/WebAssembly/lower-em-sjlj.ll

  Log Message:
  -----------
  [WebAssembly] Share rethrowing BBs in LowerEmscriptenEHSjLj

There are three kinds of "rethrowing" BBs in this pass:
1. In Emscripten SjLj, after a possibly longjmping function call, we
   check if the thrown longjmp corresponds to one of setjmps within the
   current function. If not, we rethrow the longjmp by calling
   `emscripten_longjmp`.
2. In Emscripten EH, after a possibly throwing function call, we check
   if the thrown exception corresponds to the current `catch` clauses.
   If not, we rethrow the exception by calling `__resumeException`.
3. When both Emscripten EH and SjLj are used, when we check for an
   exception after a possibly throwing function call, it is possible
   that we get not an exception but a longjmp. In this case, we
   shouldn't swallow it; we should rethrow the longjmp by calling
   `emscripten_longjmp`.
4. When both Emscripten EH and SjLj are used, when we check for a
   longjmp after a possibly longjmping function call, it is possible
   that we get not a longjmp but an exception. In this case, we
   shouldn't swallot it; we should rethrow the exception by calling
   `__resumeException`.

Case 1 is in Emscripten SjLj, 2 is in Emscripten EH, and 3 and 4 are
relevant when both Emscripten EH and SjLj are used. 3 and 4 were first
implemented in D106525.

We create BBs for 1, 3, and 4 in this pass. We create those BBs for
every throwing/longjmping function call, along with other BBs that
contain condition checks. What this CL does is to create a single BB
within a function for each of 1, 3, and 4 cases. These BBs are exiting
BBs in the function and thus don't have successors, so easy to be shared
between calls.

The names of BBs created are:
Case 1: `call.em.longjmp`
Case 3: `rethrow.exn`
Case 4: `rethrow.longjmp`

For the case 2 we don't currently create BBs; we only replace the
existing `resume` instruction with `call @__resumeException`. And Clang
already creates only a single `resume` BB per function and reuses it,
so we don't need to optimize this case.

Not sure what are good benchmarks for EH/SjLj, but this decreases the
size of the object file for `grfmt_jpeg.bc` (presumably from opencv) we
got from one of our users by 8.9%. Even after running `wasm-opt -O4` on
them, there is still 4.8% improvement.

Reviewed By: dschuff

Differential Revision: https://reviews.llvm.org/D108945