[NVPTX] Add more clear error message for using invalid syncscope (#165737)

Using invalid syncscopes on certain NVVM intrinsics causes an obscure
error to appear: (error 9: NVVM_ERROR_COMPILATION), libNVVM extra log:
Could not find scope ID=5.

This is not a very helpful error. A much more useful error would be
something like 'NVPTX does not support syncscope "agent"'

This would immediately make it clear that the issue is not NVPTX
specific, but actually from code being fed to NVPTX. This would save
users time in debugging issues related to this.

Author: smada3

llvm/lib/Target/NVPTX/NVPTXISelDAGToDAG.cpp

@@ -1836,7 +1836,7 @@ bool NVPTXDAGToDAGISel::tryFence(SDNode *N) {
   return true;
 }
 
-NVPTXScopes::NVPTXScopes(LLVMContext &C) {
+NVPTXScopes::NVPTXScopes(LLVMContext &C) : Context(&C) {
   Scopes[C.getOrInsertSyncScopeID("singlethread")] = NVPTX::Scope::Thread;
   Scopes[C.getOrInsertSyncScopeID("")] = NVPTX::Scope::System;
   Scopes[C.getOrInsertSyncScopeID("block")] = NVPTX::Scope::Block;
@@ -1851,11 +1851,21 @@ NVPTX::Scope NVPTXScopes::operator[](SyncScope::ID ID) const {
 
   auto S = Scopes.find(ID);
   if (S == Scopes.end()) {
-    // TODO:
-    // - Add API to LLVMContext to get the name of a single scope.
-    // - Use that API here to print an error containing the name
-    //   of this Unknown ID.
-    report_fatal_error(formatv("Could not find scope ID={}.", int(ID)));
+    auto scopeName = Context->getSyncScopeName(ID);
+    assert(scopeName.has_value() && "Scope name must exist.");
+
+    // Build list of supported syncscopes programmatically
+    SmallVector<StringRef> supportedScopes;
+    for (const auto &Entry : Scopes) {
+      if (auto name = Context->getSyncScopeName(Entry.first))
+        supportedScopes.push_back(name->empty() ? "<empty string>" : *name);
+    }
+
+    reportFatalUsageError(
+        formatv("NVPTX backend does not support syncscope \"{0}\" (ID={1}).\n"
+                "Supported syncscopes are: {2}.",
+                scopeName.value(), int(ID),
+                make_range(supportedScopes.begin(), supportedScopes.end())));
   }
   return S->second;
 }

llvm/lib/Target/NVPTX/NVPTXISelDAGToDAG.h

@@ -35,6 +35,7 @@ struct NVPTXScopes {
 
 private:
   SmallMapVector<SyncScope::ID, NVPTX::Scope, 8> Scopes{};
+  LLVMContext *Context = nullptr;
 };
 
 class LLVM_LIBRARY_VISIBILITY NVPTXDAGToDAGISel : public SelectionDAGISel {

llvm/test/CodeGen/NVPTX/cmpxchg-unsupported-syncscope.err.ll

@@ -0,0 +1,11 @@
+; RUN: not llc -mcpu=sm_100a -mtriple=nvptx64 -mattr=+ptx86 %s 2>&1 | FileCheck %s
+
+; Test that we get a clear error message when using an unsupported syncscope.
+
+; CHECK: NVPTX backend does not support syncscope "agent"
+; CHECK: Supported syncscopes are: singlethread, <empty string>, block, cluster, device
+define i32 @cmpxchg_unsupported_syncscope_agent(ptr %addr, i32 %cmp, i32 %new) {
+  %result = cmpxchg ptr %addr, i32 %cmp, i32 %new syncscope("agent") monotonic monotonic
+  %value = extractvalue { i32, i1 } %result, 0
+  ret i32 %value
+}