[WebAssembly] LSDA info generation

Summary:
This adds support for LSDA (exception table) generation for wasm EH.
Wasm EH mostly follows the structure of Itanium-style exception tables,
with one exception: a call site table entry in wasm EH corresponds to
not a call site but a landing pad.

In wasm EH, the VM is responsible for stack unwinding. After an
exception occurs and the stack is unwound, the control flow is
transferred to wasm 'catch' instruction by the VM, after which the
personality function is called from the compiler-generated code. (Refer
to WasmEHPrepare pass for more information on this part.)

This patch:
- Changes wasm.landingpad.index intrinsic to take a token argument, to
make this 1:1 match with a catchpad instruction
- Stores landingpad index info and catch type info MachineFunction in
before instruction selection
- Lowers wasm.lsda intrinsic to an MCSymbol pointing to the start of an
exception table
- Adds WasmException class with overridden methods for table generation
- Adds support for LSDA section in Wasm object writer

Reviewers: dschuff, sbc100, rnk

Subscribers: mgorny, jgravelle-google, sunfish, llvm-commits

Differential Revision: https://reviews.llvm.org/D52748

llvm-svn: 344575

1 files changed, 7 insertions, 3 deletions

diff --git a/llvm/lib/CodeGen/AsmPrinter/EHStreamer.cpp b/llvm/lib/CodeGen/AsmPrinter/EHStreamer.cpp
index be04b9a6e8c..7599121de2b 100644
--- a/llvm/lib/CodeGen/AsmPrinter/EHStreamer.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/EHStreamer.cpp
@@ -345,7 +345,9 @@ computeCallSiteTable(SmallVectorImpl<CallSiteEntry> &CallSites,
 ///     unwound and handling continues.
 ///  3. Type ID table contains references to all the C++ typeinfo for all
 ///     catches in the function.  This tables is reverse indexed base 1.
-void EHStreamer::emitExceptionTable() {
+///
+/// Returns the starting symbol of an exception table.
+MCSymbol *EHStreamer::emitExceptionTable() {
   const MachineFunction *MF = Asm->MF;
   const std::vector<const GlobalValue *> &TypeInfos = MF->getTypeInfos();
   const std::vector<unsigned> &FilterIds = MF->getFilterIds();
@@ -375,6 +377,7 @@ void EHStreamer::emitExceptionTable() {
   computeCallSiteTable(CallSites, LandingPads, FirstActions);
 
   bool IsSJLJ = Asm->MAI->getExceptionHandlingType() == ExceptionHandling::SjLj;
+  bool IsWasm = Asm->MAI->getExceptionHandlingType() == ExceptionHandling::Wasm;
   unsigned CallSiteEncoding =
       IsSJLJ ? dwarf::DW_EH_PE_udata4 : dwarf::DW_EH_PE_uleb128;
   bool HaveTTData = !TypeInfos.empty() || !FilterIds.empty();
@@ -457,8 +460,8 @@ void EHStreamer::emitExceptionTable() {
   Asm->EmitLabelDifferenceAsULEB128(CstEndLabel, CstBeginLabel);
   Asm->OutStreamer->EmitLabel(CstBeginLabel);
 
-  // SjLj Exception handling
-  if (IsSJLJ) {
+  // SjLj / Wasm Exception handling
+  if (IsSJLJ || IsWasm) {
     unsigned idx = 0;
     for (SmallVectorImpl<CallSiteEntry>::const_iterator
          I = CallSites.begin(), E = CallSites.end(); I != E; ++I, ++idx) {
@@ -604,6 +607,7 @@ void EHStreamer::emitExceptionTable() {
   }
 
   Asm->EmitAlignment(2);
+  return GCCETSym;
 }
 
 void EHStreamer::emitTypeInfos(unsigned TTypeEncoding, MCSymbol *TTBaseLabel) {