[WebAssembly] Add Wasm exception handling prepare pass

Summary:
This adds a pass that transforms a program to be prepared for Wasm
exception handling. This is using Windows EH instructions and based on
the previous Wasm EH proposal.
(https://github.com/WebAssembly/exception-handling/blob/master/proposals/Exceptions.md)

Reviewers: dschuff, majnemer

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

Differential Revision: https://reviews.llvm.org/D43746

llvm-svn: 333696

1 files changed, 6 insertions, 1 deletions

diff --git a/llvm/lib/CodeGen/TargetPassConfig.cpp b/llvm/lib/CodeGen/TargetPassConfig.cpp
index 46152aa09e8..8c5bdde888e 100644
--- a/llvm/lib/CodeGen/TargetPassConfig.cpp
+++ b/llvm/lib/CodeGen/TargetPassConfig.cpp
@@ -656,7 +656,12 @@ void TargetPassConfig::addPassesToHandleExceptions() {
     addPass(createDwarfEHPass());
     break;
   case ExceptionHandling::Wasm:
-    // TODO to prevent warning
+    // Wasm EH uses Windows EH instructions, but it does not need to demote PHIs
+    // on catchpads and cleanuppads because it does not outline them into
+    // funclets. Catchswitch blocks are not lowered in SelectionDAG, so we
+    // should remove PHIs there.
+    addPass(createWinEHPass(/*DemoteCatchSwitchPHIOnly=*/false));
+    addPass(createWasmEHPass());
     break;
   case ExceptionHandling::None:
     addPass(createLowerInvokePass());