[PATCH] D108729: [WebAssembly] Use entry block only for initializations in EmSjLj

[Heejin Ahn via Phabricator via llvm-commits]

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Emscripten SjLj transformation is done in four steps. This will be
mostly the same for the soon-to-be-added Wasm SjLj; the step 1, 3, and 4
will be shared and there will be separate way of doing step 2.

1. Initialize `setjmpTable` and `setjmpTableSize` in the entry BB
2. Handle `setjmp` callsites
3. Handle `longjmp` callsites
4. Cleanup and update SSA

We initialize `setjmpTable` and `setjmpTableSize` in the entry BB. But
if the entry BB contains a `setjmp` call, some `setjmp` handling
transformation will also happen in the entry BB, such as calling
`saveSetjmp`.

This is fine for Emscripten SjLj but not for Wasm SjLj, because in Wasm
SjLj we will add a dispatch BB that contains a `switch` right after the
entry BB, from which we jump to one of post-`setjmp` BBs. And this
dispatch BB should precede all `setjmp` calls.

Emscripten SjLj (current):

  entry:
    %setjmpTable = ...
    %setjmpTableSize = ...
    ...
    call @saveSetjmp(...)

Wasm SjLj (follow-up):

  entry:
    %setjmpTable = ...
    %setjmpTableSize = ...
  
  setjmp.dispatch:
    ...
    ; Jump to the right post-setjmp BB, if we are returning from a
    ; longjmp. If this is the first setjmp call, go to %entry.split.
    switch i32 %no, label %entry.split [
      i32 1, label %post.setjmp1
      i32 2, label %post.setjmp2
      ...
      i32 N, label %post.setjmpN
    ]
  
  entry.split:
    ...
    call @saveSetjmp(...)

So in Wasm SjLj we split the entry BB to make the entry block only for
`setjmpTable` and `setjmpTableSize` initialization and insert a
`setjmp.dispatch` BB. (This part is not in this CL. This will be a
follow-up.) But note that Emscripten SjLj and Wasm SjLj share all
steps except for the step 2. If we only split the entry BB only for Wasm
SjLj, there will be one more `if`-`else` and the code will be more
complicated.

So this CL splits the entry BB in Emscripten SjLj and put only
initialization stuff there as follows:
Emscripten SjLj (this CL):

  entry:
    %setjmpTable = ...
    %setjmpTableSize = ...
    br %entry.split
  
  entry.split:
    ...
    call @saveSetjmp(...)

This is just done to share code with Wasm SjLj. It adds an unnecessary
branch but this will be removed in later optimization passes anyway.

This is in effect NFC, meaning the program behavior will not change, but
existing ll tests files have changed because the entry block was split.
The reason I upload this in a separate CL is to make the Wasm SjLj diff
tidier, because this changes many existing Emscripten SjLj tests, which
can be confusing for the follow-up Wasm SjLj CL.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D108729

Files:
  llvm/lib/Target/WebAssembly/WebAssemblyLowerEmscriptenEHSjLj.cpp
  llvm/test/CodeGen/WebAssembly/lower-em-ehsjlj.ll
  llvm/test/CodeGen/WebAssembly/lower-em-sjlj-alias.ll
  llvm/test/CodeGen/WebAssembly/lower-em-sjlj-debuginfo.ll
  llvm/test/CodeGen/WebAssembly/lower-em-sjlj.ll

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