[WebAssembly] Support 'unreachable' expression

[dschuff]

Summary

Lower LLVM's 'unreachable' terminator to ISD::TRAP, and lower ISD::TRAP to
wasm's 'unreachable' expression.

WebAssembly type-checks expressions, but a noreturn function with a
return type that doesn't match the context will cause a check
failure. So we lower LLVM 'unreachable' to ISD::TRAP and then lower that
to WebAssembly's 'unreachable' expression, which typechecks in any
context and causes a trap if executed.

[sunfish]

lib/Target/WebAssembly/WebAssemblyInstrControl.td

65

Trap-like instructions are not considered "Return" instructions in the LLVM CodeGen sense, so we should rearrange the settings here so that UNREACHABLE is outside the isReturn = 1 block.

[dschuff]

• Make UNREACHABLE not isReturn

[sunfish]

lgtm, thanks!

lib/Target/WebAssembly/WebAssemblyISelLowering.cpp

WebAssemblyTargetLowering::WebAssemblyTargetLowering(
   //  - Floating-point extending loads.
   //  - Floating-point truncating stores.
   //  - i1 extending loads.
   setLoadExtAction(ISD::EXTLOAD, MVT::f32, MVT::f64, Expand);
   setTruncStoreAction(MVT::f64, MVT::f32, Expand);
   for (auto T : MVT::integer_valuetypes())
     for (auto Ext : {ISD::EXTLOAD, ISD::ZEXTLOAD, ISD::SEXTLOAD})
       setLoadExtAction(Ext, T, MVT::i1, Promote);
+
+  // Trap lowers to wasm unreachable
+  setOperationAction(ISD::TRAP, MVT::Other, Legal);
 }
 
 FastISel *WebAssemblyTargetLowering::createFastISel(
     FunctionLoweringInfo &FuncInfo, const TargetLibraryInfo *LibInfo) const {
   return WebAssembly::createFastISel(FuncInfo, LibInfo);
 }
 
 bool WebAssemblyTargetLowering::isOffsetFoldingLegal(

lib/Target/WebAssembly/WebAssemblyInstrControl.td

 // Placemarkers to indicate the start of a block or loop scope.
 def BLOCK     : I<(outs), (ins bb_op:$dst), [], "block $dst">;
 def LOOP      : I<(outs), (ins bb_op:$dst), [], "loop $dst">;
 
 multiclass RETURN<WebAssemblyRegClass vt> {
   def RETURN_#vt : I<(outs), (ins vt:$val), [(WebAssemblyreturn vt:$val)],
                      "return $val">;
 }
-let isReturn = 1, isTerminator = 1, hasCtrlDep = 1, isBarrier = 1 in {
+
+let isTerminator = 1, hasCtrlDep = 1, isBarrier = 1 in {
+let isReturn = 1 in {
   defm : RETURN<I32>;
   defm : RETURN<I64>;
   defm : RETURN<F32>;
   defm : RETURN<F64>;
   def RETURN_VOID : I<(outs), (ins), [(WebAssemblyreturn)], "return">;
-} // isReturn = 1, isTerminator = 1, hasCtrlDep = 1, isBarrier = 1
+} // isReturn = 1
+  def UNREACHABLE : I<(outs), (ins), [(trap)], "unreachable">;
+} // isTerminator = 1, hasCtrlDep = 1, isBarrier = 1

lib/Target/WebAssembly/WebAssemblyTargetMachine.cpp

WebAssemblyTargetMachine::WebAssemblyTargetMachine(
     const Target &T, const Triple &TT, StringRef CPU, StringRef FS,
     const TargetOptions &Options, Reloc::Model RM, CodeModel::Model CM,
     CodeGenOpt::Level OL)
     : LLVMTargetMachine(T, TT.isArch64Bit()
                                ? "e-p:64:64-i64:64-n32:64-S128"
                                : "e-p:32:32-i64:64-n32:64-S128",
                         TT, CPU, FS, Options, RM, CM, OL),
       TLOF(make_unique<WebAssemblyTargetObjectFile>()) {
+  // WebAssembly type-checks expressions, but a noreturn function with a return
+  // type that doesn't match the context will cause a check failure. So we lower
+  // LLVM 'unreachable' to ISD::TRAP and then lower that to WebAssembly's
+  // 'unreachable' expression which is meant for that case.
+  this->Options.TrapUnreachable = true;
+
   initAsmInfo();
 
   // We need a reducible CFG, so disable some optimizations which tend to
   // introduce irreducibility.
   setRequiresStructuredCFG(true);
 }
 
 WebAssemblyTargetMachine::~WebAssemblyTargetMachine() {}

test/CodeGen/WebAssembly/unreachable.ll

+; RUN: llc < %s -asm-verbose=false | FileCheck %s
+; RUN: llc < %s -asm-verbose=false -fast-isel | FileCheck %s
+
+; Test that LLVM unreachable instruction and trap intrinsic are lowered to
+; wasm unreachable
+
+target datalayout = "e-p:32:32-i64:64-n32:64-S128"
+target triple = "wasm32-unknown-unknown"
+
+declare void @llvm.trap()
+declare void @llvm.debugtrap()
+declare void @abort()
+
+; CHECK-LABEL: f1:
+; CHECK: call $abort
+; CHECK: unreachable
+define i32 @f1() {
+  call void @abort()
+  unreachable
+}
+
+; CHECK-LABEL: f2:
+; CHECK: unreachable
+define void @f2() {
+  call void @llvm.trap()
+  ret void
+}
+
+; CHECK-LABEL: f3:
+; CHECK: unreachable
+define void @f3() {
+  call void @llvm.debugtrap()
+  ret void
+}