[EH] Fix missing outer block for catchless try

[aheejin]

When translating a try expression, we may need an 'outer' block that wraps the newly generated try_table so we can jump out of the expression when an exception does not occur. (The condition we use is when the try has any catches or if the try is a target of any inner try-delegates:

binaryen/src/passes/TranslateEH.cpp

Line 677 in 219e668

if (it != delegateTargetToBrTarget.end() || curr->isCatch()) {

)

In case the try has either of catch or delegate, when we have the 'outer' block, we add the newly created try_table in the 'outer' block and replace the whole expression with the block:

binaryen/src/passes/TranslateEH.cpp

Line 670 in 219e668

replaceCurrent(outerBlock);

binaryen/src/passes/TranslateEH.cpp

Lines 332 to 340 in 219e668

	// If we need an outer block for other reasons (if this is a target of a
	// delegate), we insert the new try_table into it. If not we just replace
	// the current try with the new try_table.
	if (outerBlock) {
	outerBlock->list.push_back(tryTable);
	replaceCurrent(outerBlock);
	} else {
	replaceCurrent(tryTable);
	}

But in case of a catchless try, we forgot to do that:

binaryen/src/passes/TranslateEH.cpp

Line 388 in 219e668

replaceCurrent(tryTable);

So this PR fixes it.

[kripken]

Why was this outer block needed?

[aheejin]

Before we go into actually processing catches and delegates, we create an outer block in this condition:

binaryen/src/passes/TranslateEH.cpp

Line 677 in 219e668

if (it != delegateTargetToBrTarget.end() || curr->isCatch()) {

Because

1. In case of a delegate, it will be translated into a throw_ref to the delegate target. But when an exception does not occur, we have to bail out of that throw_ref. And this outer block is that bail-out destination. It is explained here:
   

   binaryen/src/passes/TranslateEH.cpp

   Line 251 in e50a2ba

   // (br $outer)

   
   

   binaryen/src/passes/TranslateEH.cpp

   Line 267 in e50a2ba

   // (br $outer ;; Now has the try_table as a child.

2. In case of catches, we proceed into catch handler parts unless we bail out in case an exception does not occur. It is explained here:
   

   binaryen/src/passes/TranslateEH.cpp

   Line 415 in e50a2ba

   // (br $outer)

   
   

   binaryen/src/passes/TranslateEH.cpp

   Line 436 in e50a2ba

   // (br $outer

But this test does not need a br because the body's type is unreachable due to a return. We omit that br here:

binaryen/src/passes/TranslateEH.cpp

Lines 298 to 300 in e50a2ba

	auto* brToOuter = curr->body->type == Type::unreachable
	? nullptr
	: builder.makeBreak(outerBlock->name);

It is hard to check this condition before creating an outer block. We can run BranchSeeker to find out if the block is actually used as a destination for a branch afterwards to remove this unnecessary block. Do you want me to do that? It is not hard, but I didn't end up doing that because we might be adding too much optimization in this pass which should ideally be done by other passes later. Currently we run this pass at the end and don't run any other optimization passes after this, I already baked some amount of optimization into it though.

[aheejin]

Tried removing it without using BranchSeeker: 8c8fcc5

[kripken]

Great!