[IR] Do not set none for function uwtable (llvm#93387)

This avoids the pitfall where we set the uwtable to none:
```
func.setUWTableKind(llvm::UWTableKind::None)
```
`Attribute::getAsString()` would see an unknown attribute and fail an
assertion. In this patch, we assert that we do not see a None uwtable
kind.

This also skips the check of `UWTableKind::Async`. It is dominated by
the check of `UWTableKind::Default`, which has the same enum value
(nfc).

Author: caojoshua

llvm/include/llvm/IR/Function.h

@@ -654,7 +654,10 @@ class LLVM_EXTERNAL_VISIBILITY Function : public GlobalObject,
     return getUWTableKind() != UWTableKind::None;
   }
   void setUWTableKind(UWTableKind K) {
-    addFnAttr(Attribute::getWithUWTableKind(getContext(), K));
+    if (K == UWTableKind::None)
+      removeFnAttr(Attribute::UWTable);
+    else
+      addFnAttr(Attribute::getWithUWTableKind(getContext(), K));
   }
   /// True if this function needs an unwind table.
   bool needsUnwindTableEntry() const {

llvm/lib/CodeGen/MachineOutliner.cpp

@@ -717,8 +717,7 @@ MachineFunction *MachineOutliner::createOutlinedFunction(
       [](UWTableKind K, const outliner::Candidate &C) {
         return std::max(K, C.getMF()->getFunction().getUWTableKind());
       });
-  if (UW != UWTableKind::None)
-    F->setUWTableKind(UW);
+  F->setUWTableKind(UW);
 
   BasicBlock *EntryBB = BasicBlock::Create(C, "entry", F);
   IRBuilder<> Builder(EntryBB);

llvm/lib/IR/Attributes.cpp

@@ -526,13 +526,8 @@ std::string Attribute::getAsString(bool InAttrGrp) const {
 
   if (hasAttribute(Attribute::UWTable)) {
     UWTableKind Kind = getUWTableKind();
-    if (Kind != UWTableKind::None) {
-      return Kind == UWTableKind::Default
-                 ? "uwtable"
-                 : ("uwtable(" +
-                    Twine(Kind == UWTableKind::Sync ? "sync" : "async") + ")")
-                       .str();
-    }
+    assert(Kind != UWTableKind::None && "uwtable attribute should not be none");
+    return Kind == UWTableKind::Default ? "uwtable" : "uwtable(sync)";
   }
 
   if (hasAttribute(Attribute::AllocKind)) {

llvm/unittests/IR/FunctionTest.cpp

@@ -486,4 +486,27 @@ TEST(FunctionTest, EraseBBs) {
   It = F->erase(F->begin(), F->end());
   EXPECT_EQ(F->size(), 0u);
 }
+
+TEST(FunctionTest, UWTable) {
+  LLVMContext Ctx;
+  std::unique_ptr<Module> M = parseIR(Ctx, R"(
+    define void @foo() {
+     bb1:
+       ret void
+    }
+)");
+
+  Function &F = *M->getFunction("foo");
+
+  EXPECT_FALSE(F.hasUWTable());
+  EXPECT_TRUE(F.getUWTableKind() == UWTableKind::None);
+
+  F.setUWTableKind(UWTableKind::Async);
+  EXPECT_TRUE(F.hasUWTable());
+  EXPECT_TRUE(F.getUWTableKind() == UWTableKind::Async);
+
+  F.setUWTableKind(UWTableKind::None);
+  EXPECT_FALSE(F.hasUWTable());
+  EXPECT_TRUE(F.getUWTableKind() == UWTableKind::None);
+}
 } // end namespace