Heejin Ahn 5e290d0f9b [WebAssembly] Fix incorrect grouping and sorting of exceptions ... This CL is not big but contains changes that span multiple analyses and passes. This description is very long because it tries to explain basics on what each pass/analysis does and why we need this change on top of that. Please feel free to skip parts that are not necessary for your understanding. --- `WasmEHFuncInfo` contains the mapping of <EH pad, the EH pad's next unwind destination>. The value (unwind dest) here is where an exception should end up when it is not caught by the key (EH pad). We record this info in WasmEHPrepare to fix catch mismatches, because the CFG itself does not have this info. A CFG only contains BBs and predecessor-successor relationship between them, but in `WasmEHFuncInfo` the unwind destination BB is not necessarily a successor or the key EH pad BB. Their relationship can be intuitively explained by this C++ code snippet: ``` try { try { foo(); } catch (int) { // EH pad ... } } catch (...) { // unwind destination } ``` So when `foo()` throws, it goes to `catch (int)` first. But if it is not caught by it, it ends up in the next unwind destination `catch (...)`. This unwind destination is what you see in `catchswitch`'s `unwind label %bb` part. --- `WebAssemblyExceptionInfo` groups exceptions so that they can be sorted continuously together in CFGSort, as we do for loops. What this analysis does is very simple: it creates a single `WebAssemblyException` per EH pad, and all BBs that are dominated by that EH pad are included in this exception. We also identify subexception relationship in this way: if EHPad A domiantes EHPad B, EHPad B's exception is a subexception of EHPad A's exception. This simple rule turns out to be incorrect in some cases. In `WasmEHFuncInfo`, if EHPad A's unwind destination is EHPad B, it means semantically EHPad B should not be included in EHPad A's exception, because it does not make sense to rethrow/delegate to an inner scope. This is what happened in CFGStackify as a result of this: ``` try try catch ... <- %dest_bb is among here! end delegate %dest_bb ``` So this patch adds a phase in `WebAssemblyExceptionInfo::recalculate` to make sure excptions' unwind destinations are not subexceptions of their unwind sources in `WasmEHFuncInfo`. But this alone does not prevent `dest_bb` in the example above from being sorted within the inner `catch`'s exception, even if its exception is not a subexception of that `catch`'s exception anymore, because of how CFGSort works, which will be explained below. --- CFGSort places BBs within the same `SortRegion` (loop or exception) continuously together so they can be demarcated with `loop`-`end_loop` or `catch`-`end_try` in CFGStackify. `SortRegion` is a wrapper for one of `MachineLoop` or `WebAssemblyException`. `SortRegionInfo` already does some complicated things because there discrepancies between those two data structures. `WebAssemblyException` is what we control, and it is defined as an EH pad as its header and BBs dominated by the header as its BBs (with a newly added exception of unwind destinations explained in the previous paragraph). But `MachineLoop` is an LLVM data structure and uses the standard loop detection algorithm. So by the algorithm, BBs that are 1. dominated by the loop header and 2. have a path back to its header. Because of the second condition, many BBs that are dominated by the loop header are not included in the loop. So BBs that contain `return` or branches to outside of the loop are not technically included in `MachineLoop`, but they can be sorted together with the loop with no problem. Maybe to relax the condition, in CFGSort, when we are in a `SortRegion` we allow sorting of not only BBs that belong to the current innermost region but also BBs that are by the current region header. (This was written this way from the first version written by Dan, when only loops existed.) But now, we have cases in exceptions when EHPad B is the unwind destination for EHPad A, even if EHPad B is dominated by EHPad A it should not be included in EHPad A's exception, and should not be sorted within EHPad A. One way to make things work, at least correctly, is change `dominates` condition to `contains` condition for `SortRegion` when sorting BBs, but this will change compilation results for existing non-EH code and I can't be sure it will not degrade performance or code size. I think it will degrade performance because it will force many BBs dominated by a loop, which don't have the path back to the header, to be placed after the loop and it will likely to create more branches and blocks. So this does a little hacky check when adding BBs to `Preferred` list: (`Preferred` list is a ready list. CFGSort maintains ready list in two priority queues: `Preferred` and `Ready`. I'm not very sure why, but it was written that way from the beginning. BBs are first added to `Preferred` list and then some of them are pushed to `Ready` list, so here we only need to guard condition for `Preferred` list.) When adding a BB to `Preferred` list, we check if that BB is an unwind destination of another BB. To do this, this adds the reverse mapping, `UnwindDestToSrc`, and getter methods to `WasmEHFuncInfo`. And if the BB is an unwind destination, it checks if the current stack of regions (`Entries`) contains its source BB by traversing the stack backwards. If we find its unwind source in there, we add the BB to its `Deferred` list, to make sure that unwind destination BB is added to `Preferred` list only after that region with the unwind source BB is sorted and popped from the stack. --- This does not contain a new test that crashes because of this bug, but this fix changes the result for one of existing test case. This test case didn't crash because it fortunately didn't contain `delegate` to the incorrectly placed unwind destination BB. Fixes https://github.com/emscripten-core/emscripten/issues/13514. Reviewed By: dschuff, tlively Differential Revision: https://reviews.llvm.org/D97247		2021-02-23 14:54:55 -08:00
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CMakeLists.txt
WebAssemblyExceptionInfoTest.cpp	[WebAssembly] Fix incorrect grouping and sorting of exceptions	2021-02-23 14:54:55 -08:00