[PATCH] D77950: [WebAssembly] Fix try placement in fixing unwind mismatches

[Heejin Ahn via Phabricator via llvm-commits]

aheejin created this revision.
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aheejin updated this revision to Diff 256784.
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rebase


In CFGStackify, `fixUnwindMismatches` function fixes unwind destination
mismatches created by `try` marker placement. For example,

  try
    ...
    call @qux  ;; This should throw to the caller!
  catch
    ...
  end

When `call @qux` is supposed to throw to the caller, it is possible that
it is wrapped inside a `catch` so in case it throws it ends up unwinding
there incorrectly. (Also it is possible `call @qux` is supposed to
unwind to another `catch` within the same function.)

To fix this, we wrap this inner `call @qux` with a nested
`try`-`catch`-`end` sequence, and within the nested `catch` body, branch
to the right destination:

  block $l0
    try
      ...
      try                 ;; new nested try
        call @qux
      catch               ;; new nested catch
        local.set n       ;; store exnref to a local
        br $l0
      end
    catch
      ...
    end
  end
  local.get n             ;; retrieve exnref back
  rethrow                 ;; rethrow to the caller

The previous algorithm placed the nested `try` right before the `call`.
But it is possible that there are stackified instructions before the
call from which the call takes arguments.

  try
    ...
    i32.const 5
    call @qux  ;; This should throw to the caller!
  catch
    ...
  end

In this case we have to place `try` before those stackified
instructions.

  block $l0
    try
      ...
      try                 ;; this should go *before* 'i32.const 5'
        i32.const 5
        call @qux
      catch
        local.set n
        br $l0
      end
    catch
      ...
    end
  end
  local.get n
  rethrow

We correctly handle this in the first normal `try` placement phase
(`placeTryMarker` function), but failed to handle this in this
`fixUnwindMismatches`.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D77950

Files:
  llvm/lib/Target/WebAssembly/WebAssemblyCFGStackify.cpp
  llvm/test/CodeGen/WebAssembly/cfg-stackify-eh.ll

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