[WebAssembly] Add support for exception handling instructions

[aheejin]

Summary

This adds backend support for throw, rethrow, try, and try_end instructions.
This needs the corresponding clang builtin support:
https://reviews.llvm.org/D34783
This follows the Wasm exception handling proposal in
https://github.com/WebAssembly/exception-handling/blob/master/proposals/Exceptions.md

include/llvm/IR/IntrinsicsWebAssembly.td

 
 let TargetPrefix = "wasm" in {  // All intrinsics start with "llvm.wasm.".
 
 // Note that current_memory is not IntrNoMem because it must be sequenced with
 // respect to grow_memory calls.
 def int_wasm_current_memory : Intrinsic<[llvm_anyint_ty], [], [IntrReadMem]>;
 def int_wasm_grow_memory : Intrinsic<[llvm_anyint_ty], [LLVMMatchType<0>], []>;
 
+// Exception handling intrinsics
+def int_wasm_throw: Intrinsic<[], [llvm_i32_ty, llvm_ptr_ty], [Throws]>;
+def int_wasm_rethrow: Intrinsic<[], [], [Throws]>;
+
 }

lib/Target/WebAssembly/WebAssemblyInstrControl.td

 def BR_TABLE_I64 : I<(outs), (ins I64:$index, variable_ops),
                      [(WebAssemblybr_table I64:$index)],
                      "br_table \t$index"> {
   let TSFlags{0} = 1;
   let TSFlags{1} = 1;
 }
 } // isTerminator = 1, hasCtrlDep = 1, isBarrier = 1
 
-// Placemarkers to indicate the start or end of a block or loop scope. These
-// use/clobber VALUE_STACK to prevent them from being moved into the middle of
-// an expression tree.
+// Placemarkers to indicate the start or end of a block, loop, or try scope.
+// These use/clobber VALUE_STACK to prevent them from being moved into the
+// middle of an expression tree.
 let Uses = [VALUE_STACK], Defs = [VALUE_STACK] in {
 def BLOCK     : I<(outs), (ins Signature:$sig), [], "block   \t$sig", 0x02>;
 def LOOP      : I<(outs), (ins Signature:$sig), [], "loop    \t$sig", 0x03>;
+def TRY       : I<(outs), (ins Signature:$sig), [], "try     \t$sig", 0x06>;
 
-// END_BLOCK, END_LOOP, and END_FUNCTION are represented with the same opcode
-// in wasm.
+// END_BLOCK, END_LOOP, END_TRY, and END_FUNCTION are represented with the same
+// opcode in wasm.
 def END_BLOCK : I<(outs), (ins), [], "end_block", 0x0b>;
 def END_LOOP  : I<(outs), (ins), [], "end_loop", 0x0b>;
+def END_TRY   : I<(outs), (ins), [], "end_try", 0x0b>;
 let isTerminator = 1, isBarrier = 1 in
 def END_FUNCTION : I<(outs), (ins), [], "end_function", 0x0b>;
 } // Uses = [VALUE_STACK], Defs = [VALUE_STACK]
 
 multiclass RETURN<WebAssemblyRegClass vt> {
   def RETURN_#vt : I<(outs), (ins vt:$val), [(WebAssemblyreturn vt:$val)],
                      "return  \t$val", 0x0f>;
   // Equivalent to RETURN_#vt, for use at the end of a function when wasm
let isReturn = 1 in {
 
   // This is to RETURN_VOID what FALLTHROUGH_RETURN_#vt is to RETURN_#vt.
   let isCodeGenOnly = 1 in
   def FALLTHROUGH_RETURN_VOID : I<(outs), (ins), []>;
 } // isReturn = 1
 
 def UNREACHABLE : I<(outs), (ins), [(trap)], "unreachable", 0x00>;
 
+def THROW_I32 : I<(outs), (ins i32imm:$tag, I32:$obj),
+                  [(int_wasm_throw imm:$tag, I32:$obj)], "throw   \t$tag, $obj",
+                  0x08>;
+def THROW_I64 : I<(outs), (ins i32imm:$tag, I64:$obj),
+                  [(int_wasm_throw imm:$tag, I64:$obj)], "throw   \t$tag, $obj",
+                  0x08>;
+def RETHROW : I<(outs), (ins i32imm:$rel_depth), [], "rethrow \t$rel_depth",
+                0x09>;
+
 } // isTerminator = 1, hasCtrlDep = 1, isBarrier = 1
 
 } // Defs = [ARGUMENTS]
+
+// rethrow takes a relative depth as an argument, for which currently only 0 is
+// possible for C++. Once other languages need depths other than 0, depths will
+// be computed in CFGStackify.
+def : Pat<(int_wasm_rethrow), (RETHROW 0)>;

test/CodeGen/WebAssembly/exception.ll

+; RUN: llc < %s -asm-verbose=false -disable-wasm-fallthrough-return-opt -disable-wasm-explicit-locals | FileCheck %s
+
+target datalayout = "e-m:e-p:32:32-i64:64-n32:64-S128"
+target triple = "wasm32-unknown-unknown-wasm"
+
+declare void @llvm.wasm.throw(i32, i8*)
+declare void @llvm.wasm.rethrow()
+
+; CHECK-LABEL: throw:
+; CHECK-NEXT: i32.const $push0=, 0
+; CHECK-NEXT: throw 0, $pop0
+define void @throw() {
+  call void @llvm.wasm.throw(i32 0, i8* null)
+  ret void
+}
+
+; CHECK-LABEL: rethrow:
+; CHECK-NEXT: rethrow 0
+define void @rethrow() {
+  call void @llvm.wasm.rethrow()
+  ret void
+}