llvm: migrate to new parameter-bearing uwtable attr

In https://reviews.llvm.org/D114543 the uwtable attribute gained a flag
so that we can ask for sync uwtables instead of async, as the former are
much cheaper. The default is async, so that's what I've done here, but I
left a TODO that we might be able to do better.

While in here I went ahead and dropped support for removing uwtable
attributes in rustc: we never did it, so I didn't write the extra C++
bridge code to make it work. Maybe I should have done the same thing
with the `sync|async` parameter but we'll see.

compiler/rustc_codegen_llvm/src/allocator.rs

@@ -64,7 +64,7 @@ pub(crate) unsafe fn codegen(
             llvm::LLVMRustSetVisibility(llfn, llvm::Visibility::Hidden);
         }
         if tcx.sess.must_emit_unwind_tables() {
-            attributes::emit_uwtable(llfn, true);
+            attributes::emit_uwtable(llfn);
         }
 
         let callee = kind.fn_name(method.name);
@@ -111,7 +111,7 @@ pub(crate) unsafe fn codegen(
         llvm::LLVMRustSetVisibility(llfn, llvm::Visibility::Hidden);
     }
     if tcx.sess.must_emit_unwind_tables() {
-        attributes::emit_uwtable(llfn, true);
+        attributes::emit_uwtable(llfn);
     }
 
     let kind = if has_alloc_error_handler { AllocatorKind::Global } else { AllocatorKind::Default };

compiler/rustc_codegen_llvm/src/attributes.rs

@@ -59,8 +59,11 @@ pub fn sanitize<'ll>(cx: &CodegenCx<'ll, '_>, no_sanitize: SanitizerSet, llfn: &
 
 /// Tell LLVM to emit or not emit the information necessary to unwind the stack for the function.
 #[inline]
-pub fn emit_uwtable(val: &Value, emit: bool) {
-    Attribute::UWTable.toggle_llfn(Function, val, emit);
+pub fn emit_uwtable(val: &Value) {
+    // NOTE: We should determine if we even need async unwind tables, as they
+    // take have more overhead and if we can use sync unwind tables we
+    // probably should.
+    llvm::EmitUWTableAttr(val, true);
 }
 
 /// Tell LLVM if this function should be 'naked', i.e., skip the epilogue and prologue.
@@ -275,7 +278,7 @@ pub fn from_fn_attrs<'ll, 'tcx>(
     // You can also find more info on why Windows always requires uwtables here:
     //      https://bugzilla.mozilla.org/show_bug.cgi?id=1302078
     if cx.sess().must_emit_unwind_tables() {
-        attributes::emit_uwtable(llfn, true);
+        attributes::emit_uwtable(llfn);
     }
 
     if cx.sess().opts.debugging_opts.profile_sample_use.is_some() {

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -1182,6 +1182,7 @@ extern "C" {
     pub fn LLVMRustAddByValAttr(Fn: &Value, index: c_uint, ty: &Type);
     pub fn LLVMRustAddStructRetAttr(Fn: &Value, index: c_uint, ty: &Type);
     pub fn LLVMRustAddFunctionAttribute(Fn: &Value, index: c_uint, attr: Attribute);
+    pub fn LLVMRustEmitUWTableAttr(Fn: &Value, async_: bool);
     pub fn LLVMRustAddFunctionAttrStringValue(
         Fn: &Value,
         index: c_uint,

compiler/rustc_codegen_llvm/src/llvm/mod.rs

@@ -31,6 +31,10 @@ impl LLVMRustResult {
     }
 }
 
+pub fn EmitUWTableAttr(llfn: &Value, async_: bool) {
+    unsafe { LLVMRustEmitUWTableAttr(llfn, async_) }
+}
+
 pub fn AddFunctionAttrStringValue(llfn: &Value, idx: AttributePlace, attr: &CStr, value: &CStr) {
     unsafe {
         LLVMRustAddFunctionAttrStringValue(llfn, idx.as_uint(), attr.as_ptr(), value.as_ptr())

compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp

@@ -334,6 +334,17 @@ extern "C" void LLVMRustAddStructRetAttr(LLVMValueRef Fn, unsigned Index,
   AddAttribute(F, Index, Attr);
 }
 
+extern "C" void LLVMRustEmitUWTableAttr(LLVMValueRef Fn, bool Async) {
+  Function *F = unwrap<Function>(Fn);
+#if LLVM_VERSION_LT(15, 0)
+  Attribute Attr = Attribute::get(F->getContext(), Attribute::UWTable);
+#else
+  Attribute Attr = Attribute::getWithUWTableKind(
+      F->getContext(), Async ? UWTableKind::Async : UWTableKind::Sync);
+#endif
+  AddAttribute(F, AttributeList::AttrIndex::FunctionIndex, Attr);
+}
+
 extern "C" void LLVMRustAddFunctionAttrStringValue(LLVMValueRef Fn,
                                                    unsigned Index,
                                                    const char *Name,