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

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-ir

Author: Joshua Cao (caojoshua)

<details>
<summary>Changes</summary>

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).

---
Full diff: https://github.com/llvm/llvm-project/pull/93387.diff


3 Files Affected:

- (modified) llvm/include/llvm/IR/Function.h (+2-1) 
- (modified) llvm/lib/CodeGen/MachineOutliner.cpp (+1-2) 
- (modified) llvm/lib/IR/Attributes.cpp (+3-7) 


``````````diff
diff --git a/llvm/include/llvm/IR/Function.h b/llvm/include/llvm/IR/Function.h
index cb514cde95b51..be745a0083cb7 100644
--- a/llvm/include/llvm/IR/Function.h
+++ b/llvm/include/llvm/IR/Function.h
@@ -654,7 +654,8 @@ class LLVM_EXTERNAL_VISIBILITY Function : public GlobalObject,
     return getUWTableKind() != UWTableKind::None;
   }
   void setUWTableKind(UWTableKind K) {
-    addFnAttr(Attribute::getWithUWTableKind(getContext(), K));
+    if (K != UWTableKind::None)
+      addFnAttr(Attribute::getWithUWTableKind(getContext(), K));
   }
   /// True if this function needs an unwind table.
   bool needsUnwindTableEntry() const {
diff --git a/llvm/lib/CodeGen/MachineOutliner.cpp b/llvm/lib/CodeGen/MachineOutliner.cpp
index dc2f5ef15206e..f174dd857def0 100644
--- a/llvm/lib/CodeGen/MachineOutliner.cpp
+++ b/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);
diff --git a/llvm/lib/IR/Attributes.cpp b/llvm/lib/IR/Attributes.cpp
index c8d6bdd423878..cc3a2ceb6de84 100644
--- a/llvm/lib/IR/Attributes.cpp
+++ b/llvm/lib/IR/Attributes.cpp
@@ -526,13 +526,9 @@ 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)) {

``````````

</details>


https://github.com/llvm/llvm-project/pull/93387