[WebAssembly] Support clang -fwasm-exceptions for bitcode

This supports bitcode compilation using `clang -fwasm-exceptions`.

---

The current situation:

Currently the backend requires two options for Wasm EH:
`-wasm-enable-eh` and `-exception-model=wasm`. Wasm SjLj requires two
options as well: `-wasm-enable-sjlj` and `-exception-model=wasm`. When
using Wasm EH via Emscripten, you only need to pass `-fwasm-exceptions`,
and these options will be added within the clang driver. This
description will focus on the case of Wasm EH going forward, but Wasm
SjLj's case is similar.

When you pass `-fwasm-exceptions` to emcc and clang driver, the clang
driver adds these options to the command line that calls the clang
frontend (`clang -cc1`): `-mllvm -wasm-enable-eh` and
`-exception-model=wasm`. `-wasm-enable-eh` is prefixed with `-mllvm`, so
it is passed as is to the backend. But `-exception-model` is parsed and
processed within the clang frontend and stored in `LangOptions` class.
This info is later transferred to `TargetOptions` class, and then
eventually passed to `MCAsmInfo` class. All LLVM code queries this
`MCAsmInfo` to get the exception model.

---

Problem:

The problem is the whole `LangOptions` processing is bypassed when
compiling bitcode, so the information transfer of `LangOptions` ->
`TargetOptions` -> `MCAsmInfo` does not happen. They are all set to
`ExceptionHandling::None`, which is the default value.

---

What other targets do, and why we can't do the same:

Other targets support bitcode compilation by the clang driver, but they
can do that by using different triples. For example, X86 target supports
multiple triples, each of which has its own subclass of `MCAsmInfo`, so
it can hardcode the appropriate exception model within those subclasses'
constructors. But we don't have separate triples for each exception
mode: none, emscripten, and wasm.

---

What this CL does:

If we can figure out whether `-wasm-enable-eh` is passed to the backend,
we can programatically set the exception model from the backend, rather
than requiring it to be passed.

So we check `WasmEnableEH` and `WasmEnableSjLj` variables, which are
`cl::opt` for `-wasm-enable-eh` and `-wasm-enable-sjlj`, in
`WebAssemblyMCAsmInfo` constructor, and if either of them is set, we set
`MCAsmInfo.ExceptionType` to Wasm. `TargetOptions` cannot be updated
there, so we make sure they are the same later.

Fixes emscripten-core/emscripten#15712.

Reviewed By: dschuff

Differential Revision: https://reviews.llvm.org/D115893

clang/test/CodeGen/WebAssembly/wasm-eh.ll

@@ -0,0 +1,38 @@
+; REQUIRES: webassembly-registered-target
+; RUN: %clang %s -target wasm32-unknown-unknown -fwasm-exceptions -c -S -o - | FileCheck %s
+
+; This tests whether clang driver can take -fwasm-exceptions and compile bitcode
+; files using Wasm EH.
+
+; CHECK-LABEL: test
+; CHECK: try
+; CHECK:   call foo
+; CHECK: catch __cpp_exception
+; CHECK: end
+define void @test() personality i8* bitcast (i32 (...)* @__gxx_wasm_personality_v0 to i8*) {
+entry:
+  invoke void @foo()
+          to label %try.cont unwind label %catch.dispatch
+
+catch.dispatch:                                   ; preds = %entry
+  %0 = catchswitch within none [label %catch.start] unwind to caller
+
+catch.start:                                      ; preds = %catch.dispatch
+  %1 = catchpad within %0 [i8* null]
+  %2 = call i8* @llvm.wasm.get.exception(token %1)
+  %3 = call i32 @llvm.wasm.get.ehselector(token %1)
+  %4 = call i8* @__cxa_begin_catch(i8* %2) #2 [ "funclet"(token %1) ]
+  call void @__cxa_end_catch() [ "funclet"(token %1) ]
+  catchret from %1 to label %try.cont
+
+try.cont:                                         ; preds = %entry, %catch.start
+  ret void
+}
+
+declare void @foo()
+declare i32 @__gxx_wasm_personality_v0(...)
+declare i8* @llvm.wasm.get.exception(token)
+declare i32 @llvm.wasm.get.ehselector(token)
+declare i8* @__cxa_begin_catch(i8*)
+declare void @__cxa_end_catch()
+

llvm/lib/Target/WebAssembly/MCTargetDesc/WebAssemblyMCAsmInfo.cpp

@@ -13,6 +13,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "WebAssemblyMCAsmInfo.h"
+#include "Utils/WebAssemblyUtilities.h"
 #include "llvm/ADT/Triple.h"
 
 using namespace llvm;
@@ -44,5 +45,13 @@ WebAssemblyMCAsmInfo::WebAssemblyMCAsmInfo(const Triple &T,
 
   SupportsDebugInformation = true;
 
+  // When compilation is done on a cpp file by clang, the exception model info
+  // is stored in LangOptions, which is later used to set the info in
+  // TargetOptions and then MCAsmInfo in LLVMTargetMachine::initAsmInfo(). But
+  // this process does not happen when compiling bitcode directly with clang, so
+  // we make sure this info is set correctly.
+  if (WebAssembly::WasmEnableEH || WebAssembly::WasmEnableSjLj)
+    ExceptionsType = ExceptionHandling::Wasm;
+
   // TODO: UseIntegratedAssembler?
 }

llvm/lib/Target/WebAssembly/Utils/WebAssemblyUtilities.cpp

@@ -18,6 +18,31 @@
 #include "llvm/MC/MCContext.h"
 using namespace llvm;
 
+// Exception handling & setjmp-longjmp handling related options. These are
+// defined here to be shared between WebAssembly and its subdirectories.
+
+// Emscripten's asm.js-style exception handling
+cl::opt<bool> WebAssembly::WasmEnableEmEH(
+    "enable-emscripten-cxx-exceptions",
+    cl::desc("WebAssembly Emscripten-style exception handling"),
+    cl::init(false));
+// Emscripten's asm.js-style setjmp/longjmp handling
+cl::opt<bool> WebAssembly::WasmEnableEmSjLj(
+    "enable-emscripten-sjlj",
+    cl::desc("WebAssembly Emscripten-style setjmp/longjmp handling"),
+    cl::init(false));
+// Exception handling using wasm EH instructions
+cl::opt<bool>
+    WebAssembly::WasmEnableEH("wasm-enable-eh",
+                              cl::desc("WebAssembly exception handling"),
+                              cl::init(false));
+// setjmp/longjmp handling using wasm EH instrutions
+cl::opt<bool>
+    WebAssembly::WasmEnableSjLj("wasm-enable-sjlj",
+                                cl::desc("WebAssembly setjmp/longjmp handling"),
+                                cl::init(false));
+
+// Function names in libc++abi and libunwind
 const char *const WebAssembly::CxaBeginCatchFn = "__cxa_begin_catch";
 const char *const WebAssembly::CxaRethrowFn = "__cxa_rethrow";
 const char *const WebAssembly::StdTerminateFn = "_ZSt9terminatev";

llvm/lib/Target/WebAssembly/Utils/WebAssemblyUtilities.h

@@ -16,6 +16,7 @@
 #define LLVM_LIB_TARGET_WEBASSEMBLY_UTILS_WEBASSEMBLYUTILITIES_H
 
 #include "llvm/IR/DerivedTypes.h"
+#include "llvm/Support/CommandLine.h"
 
 namespace llvm {
 
@@ -70,6 +71,12 @@ inline bool isRefType(const Type *Ty) {
 bool isChild(const MachineInstr &MI, const WebAssemblyFunctionInfo &MFI);
 bool mayThrow(const MachineInstr &MI);
 
+// Exception handling / setjmp-longjmp handling command-line options
+extern cl::opt<bool> WasmEnableEmEH;   // asm.js-style EH
+extern cl::opt<bool> WasmEnableEmSjLj; // asm.js-style SjLJ
+extern cl::opt<bool> WasmEnableEH;     // EH using Wasm EH instructions
+extern cl::opt<bool> WasmEnableSjLj;   // SjLj using Wasm EH instructions
+
 // Exception-related function names
 extern const char *const ClangCallTerminateFn;
 extern const char *const CxaBeginCatchFn;

llvm/lib/Target/WebAssembly/WebAssemblyAsmPrinter.cpp

@@ -51,8 +51,6 @@ using namespace llvm;
 #define DEBUG_TYPE "asm-printer"
 
 extern cl::opt<bool> WasmKeepRegisters;
-extern cl::opt<bool> WasmEnableEmEH;
-extern cl::opt<bool> WasmEnableEmSjLj;
 
 //===----------------------------------------------------------------------===//
 // Helpers.
@@ -322,8 +320,9 @@ void WebAssemblyAsmPrinter::emitExternalDecls(const Module &M) {
       // will discard it later if it turns out not to be necessary.
       auto Signature = signatureFromMVTs(Results, Params);
       bool InvokeDetected = false;
-      auto *Sym = getMCSymbolForFunction(&F, WasmEnableEmEH || WasmEnableEmSjLj,
-                                         Signature.get(), InvokeDetected);
+      auto *Sym = getMCSymbolForFunction(
+          &F, WebAssembly::WasmEnableEmEH || WebAssembly::WasmEnableEmSjLj,
+          Signature.get(), InvokeDetected);
 
       // Multiple functions can be mapped to the same invoke symbol. For
       // example, two IR functions '__invoke_void_i8*' and '__invoke_void_i32'

llvm/lib/Target/WebAssembly/WebAssemblyLowerEmscriptenEHSjLj.cpp

@@ -267,6 +267,7 @@
 ///
 ///===----------------------------------------------------------------------===//
 
+#include "Utils/WebAssemblyUtilities.h"
 #include "WebAssembly.h"
 #include "WebAssemblyTargetMachine.h"
 #include "llvm/ADT/StringExtras.h"
@@ -285,13 +286,6 @@ using namespace llvm;
 
 #define DEBUG_TYPE "wasm-lower-em-ehsjlj"
 
-// Emscripten's asm.js-style exception handling
-extern cl::opt<bool> WasmEnableEmEH;
-// Emscripten's asm.js-style setjmp/longjmp handling
-extern cl::opt<bool> WasmEnableEmSjLj;
-// Wasm setjmp/longjmp handling using wasm EH instructions
-extern cl::opt<bool> WasmEnableSjLj;
-
 static cl::list<std::string>
     EHAllowlist("emscripten-cxx-exceptions-allowed",
                 cl::desc("The list of function names in which Emscripten-style "
@@ -370,8 +364,9 @@ class WebAssemblyLowerEmscriptenEHSjLj final : public ModulePass {
   static char ID;
 
   WebAssemblyLowerEmscriptenEHSjLj()
-      : ModulePass(ID), EnableEmEH(WasmEnableEmEH),
-        EnableEmSjLj(WasmEnableEmSjLj), EnableWasmSjLj(WasmEnableSjLj) {
+      : ModulePass(ID), EnableEmEH(WebAssembly::WasmEnableEmEH),
+        EnableEmSjLj(WebAssembly::WasmEnableEmSjLj),
+        EnableWasmSjLj(WebAssembly::WasmEnableSjLj) {
     assert(!(EnableEmSjLj && EnableWasmSjLj) &&
            "Two SjLj modes cannot be turned on at the same time");
     assert(!(EnableEmEH && EnableWasmSjLj) &&

llvm/lib/Target/WebAssembly/WebAssemblyMCInstLower.cpp

@@ -40,9 +40,6 @@ cl::opt<bool>
                                " instruction output for test purposes only."),
                       cl::init(false));
 
-extern cl::opt<bool> WasmEnableEmEH;
-extern cl::opt<bool> WasmEnableEmSjLj;
-
 static void removeRegisterOperands(const MachineInstr *MI, MCInst &OutMI);
 
 MCSymbol *
@@ -112,7 +109,8 @@ WebAssemblyMCInstLower::GetGlobalAddressSymbol(const MachineOperand &MO) const {
 
   bool InvokeDetected = false;
   auto *WasmSym = Printer.getMCSymbolForFunction(
-      F, WasmEnableEmEH || WasmEnableEmSjLj, Signature.get(), InvokeDetected);
+      F, WebAssembly::WasmEnableEmEH || WebAssembly::WasmEnableEmSjLj,
+      Signature.get(), InvokeDetected);
   WasmSym->setSignature(Signature.get());
   Printer.addSignature(std::move(Signature));
   WasmSym->setType(wasm::WASM_SYMBOL_TYPE_FUNCTION);

llvm/lib/Target/WebAssembly/WebAssemblyTargetMachine.cpp

@@ -14,6 +14,7 @@
 #include "WebAssemblyTargetMachine.h"
 #include "MCTargetDesc/WebAssemblyMCTargetDesc.h"
 #include "TargetInfo/WebAssemblyTargetInfo.h"
+#include "Utils/WebAssemblyUtilities.h"
 #include "WebAssembly.h"
 #include "WebAssemblyMachineFunctionInfo.h"
 #include "WebAssemblyTargetObjectFile.h"
@@ -24,6 +25,7 @@
 #include "llvm/CodeGen/RegAllocRegistry.h"
 #include "llvm/CodeGen/TargetPassConfig.h"
 #include "llvm/IR/Function.h"
+#include "llvm/MC/MCAsmInfo.h"
 #include "llvm/MC/TargetRegistry.h"
 #include "llvm/Target/TargetOptions.h"
 #include "llvm/Transforms/Scalar.h"
@@ -33,28 +35,6 @@ using namespace llvm;
 
 #define DEBUG_TYPE "wasm"
 
-// Emscripten's asm.js-style exception handling
-cl::opt<bool>
-    WasmEnableEmEH("enable-emscripten-cxx-exceptions",
-                   cl::desc("WebAssembly Emscripten-style exception handling"),
-                   cl::init(false));
-
-// Emscripten's asm.js-style setjmp/longjmp handling
-cl::opt<bool> WasmEnableEmSjLj(
-    "enable-emscripten-sjlj",
-    cl::desc("WebAssembly Emscripten-style setjmp/longjmp handling"),
-    cl::init(false));
-
-// Exception handling using wasm EH instructions
-cl::opt<bool> WasmEnableEH("wasm-enable-eh",
-                           cl::desc("WebAssembly exception handling"),
-                           cl::init(false));
-
-// setjmp/longjmp handling using wasm EH instrutions
-cl::opt<bool> WasmEnableSjLj("wasm-enable-sjlj",
-                             cl::desc("WebAssembly setjmp/longjmp handling"),
-                             cl::init(false));
-
 // A command-line option to keep implicit locals
 // for the purpose of testing with lit/llc ONLY.
 // This produces output which is not valid WebAssembly, and is not supported
@@ -368,7 +348,23 @@ FunctionPass *WebAssemblyPassConfig::createTargetRegisterAllocator(bool) {
   return nullptr; // No reg alloc
 }
 
-static void basicCheckForEHAndSjLj(const TargetMachine *TM) {
+using WebAssembly::WasmEnableEH;
+using WebAssembly::WasmEnableEmEH;
+using WebAssembly::WasmEnableEmSjLj;
+using WebAssembly::WasmEnableSjLj;
+
+static void basicCheckForEHAndSjLj(TargetMachine *TM) {
+  // Before checking, we make sure TargetOptions.ExceptionModel is the same as
+  // MCAsmInfo.ExceptionsType. Normally these have to be the same, because clang
+  // stores the exception model info in LangOptions, which is later transferred
+  // to TargetOptions and MCAsmInfo. But when clang compiles bitcode directly,
+  // clang's LangOptions is not used and thus the exception model info is not
+  // correctly transferred to TargetOptions and MCAsmInfo, so we make sure we
+  // have the correct exception model in in WebAssemblyMCAsmInfo constructor.
+  // But in this case TargetOptions is still not updated, so we make sure they
+  // are the same.
+  TM->Options.ExceptionModel = TM->getMCAsmInfo()->getExceptionHandlingType();
+
   // Basic Correctness checking related to -exception-model
   if (TM->Options.ExceptionModel != ExceptionHandling::None &&
       TM->Options.ExceptionModel != ExceptionHandling::Wasm)