[WebAssembly] Support instruction selection for catching exceptions

Summary:
This lowers exception catching-related instructions:
1. Lowers `wasm.catch` intrinsic to `catch` instruction
2. Removes `catchpad` and `cleanuppad` instructions; they are not
necessary after isel phase. (`MachineBasicBlock::isEHFuncletEntry()` or
`MachineBasicBlock::isEHPad()` can be used instead.)
3. Lowers `catchret` and `cleanupret` instructions to pseudo `catchret`
and `cleanupret` instructions in isel, which will be replaced with other
instructions in `WebAssemblyExceptionPrepare` pass.
4. Adds 'WebAssemblyExceptionPrepare` pass, which is for running various
transformation for EH. Currently this pass only replaces `catchret` and
`cleanupret` instructions into appropriate wasm instructions to make
this patch successfully run until the end.

Currently this does not handle lowering of intrinsics related to LSDA
info generation (`wasm.landingpad.index` and `wasm.lsda`), because they
cannot be tested without implementing `EHStreamer`'s wasm-specific
handlers. They are marked as TODO, which is needed to make isel pass.
Also this does not generate `try` and `end_try` markers yet, which will
be handled in later patches.

This patch is based on the first wasm EH proposal.
(https://github.com/WebAssembly/exception-handling/blob/master/proposals/Exceptions.md)

Reviewers: dschuff, majnemer

Subscribers: jfb, sbc100, jgravelle-google, sunfish, llvm-commits

Differential Revision: https://reviews.llvm.org/D44090

llvm-svn: 333705

1 files changed, 0 insertions, 3 deletions

diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyCFGStackify.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyCFGStackify.cpp
index 1245ac03727..a53fe28d264 100644
--- a/llvm/lib/Target/WebAssembly/WebAssemblyCFGStackify.cpp
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyCFGStackify.cpp
@@ -145,9 +145,6 @@ static void PlaceBlockMarker(
            std::prev(InsertPos)->getOpcode() != WebAssembly::END_LOOP)
       --InsertPos;
   }
-  // The header block in which a 'block' mark will be inserted should have a
-  // terminator because it is branching to a non-layout successor.
-  assert(InsertPos != Header->end());
 
   // Add the BLOCK.
   MachineInstr *Begin =