[WebAssembly] Fix landingpad-only case in Emscripten EH

[aheejin]

Summary

Previously we didn't set Changed to true when there are only landing
pads but not invokes. This fixes it and we set Changed to true
whenever we have landing pads. (There can't be invokes without landing
pads, so that case is covered too)

The test case for this has to be a separate file because this pass is a
ModulePass and Changed is computed based on the whole module.

[merge_guards_bot]

Unit tests: pass. 61259 tests passed, 0 failed and 736 were skipped.

clang-tidy: pass.

clang-format: pass.

Build artifacts: diff.json, clang-tidy.txt, clang-format.patch, CMakeCache.txt, console-log.txt, test-results.xml

[tlively]

LGTM! I checked locally and this fixes https://github.com/rust-lang/rust/issues/66308. Thanks for solving this so quickly :)

llvm/lib/Target/WebAssembly/WebAssemblyLowerEmscriptenEHSjLj.cpp

bool WebAssemblyLowerEmscriptenEHSjLj::runEHOnFunction(Function &F) {
   SmallPtrSet<LandingPadInst *, 32> LandingPads;
   bool AllowExceptions =
       areAllExceptionsAllowed() || EHWhitelistSet.count(F.getName());
 
   for (BasicBlock &BB : F) {
     auto *II = dyn_cast<InvokeInst>(BB.getTerminator());
     if (!II)
       continue;
-    Changed = true;
     LandingPads.insert(II->getLandingPadInst());
     IRB.SetInsertPoint(II);
 
     bool NeedInvoke = AllowExceptions && canThrow(II->getCalledValue());
     if (NeedInvoke) {
       // Wrap invoke with invoke wrapper and generate preamble/postamble
       Value *Threw = wrapInvoke(II);
       ToErase.push_back(II);
bool WebAssemblyLowerEmscriptenEHSjLj::runEHOnFunction(Function &F) {
   }
 
   // Look for orphan landingpads, can occur in blocks with no predecessors
   for (BasicBlock &BB : F) {
     Instruction *I = BB.getFirstNonPHI();
     if (auto *LPI = dyn_cast<LandingPadInst>(I))
       LandingPads.insert(LPI);
   }
+  Changed = !LandingPads.empty();
 
   // Handle all the landingpad for this function together, as multiple invokes
   // may share a single lp
   for (LandingPadInst *LPI : LandingPads) {
     IRB.SetInsertPoint(LPI);
     SmallVector<Value *, 16> FMCArgs;
     for (unsigned I = 0, E = LPI->getNumClauses(); I < E; ++I) {
       Constant *Clause = LPI->getClause(I);

llvm/test/CodeGen/WebAssembly/lower-em-exceptions-lpad-only.ll

+; RUN: opt < %s -wasm-lower-em-ehsjlj -S | FileCheck %s
+
+target datalayout = "e-m:e-p:32:32-i64:64-n32:64-S128"
+target triple = "wasm32-unknown-unknown"
+
+@_ZTIi = external constant i8*
+
+; Checks if a module that only contains a landingpad (and resume) but not an
+; invoke works correctly and does not crash.
+; CHECK-LABEL: @landingpad_only
+define void @landingpad_only() personality i8* bitcast (i32 (...)* @__gxx_personality_v0 to i8*) {
+entry:
+  br label %cont
+
+lpad:                                             ; preds = %entry
+  %0 = landingpad { i8*, i32 }
+          catch i8* bitcast (i8** @_ZTIi to i8*)
+          catch i8* null
+  resume { i8*, i32 } %0
+
+cont:
+  ret void
+}
+
+declare i32 @__gxx_personality_v0(...)