LLVMContext: add getSyncScopeName() to lookup individual scope name (llvm#109484)

This PR adds a `getSyncScopeString(Id)` API to `LLVMContext` that
returns the `StringRef` for that ID, if any.

Author: gonzalobg

llvm/include/llvm/IR/LLVMContext.h

@@ -130,6 +130,10 @@ class LLVMContext {
   /// scope names are ordered by increasing synchronization scope IDs.
   void getSyncScopeNames(SmallVectorImpl<StringRef> &SSNs) const;
 
+  /// getSyncScopeName - Returns the name of a SyncScope::ID
+  /// registered with LLVMContext, if any.
+  std::optional<StringRef> getSyncScopeName(SyncScope::ID Id) const;
+
   /// Define the GC for a function
   void setGC(const Function &Fn, std::string GCName);
 

llvm/lib/IR/LLVMContext.cpp

@@ -330,6 +330,10 @@ void LLVMContext::getSyncScopeNames(SmallVectorImpl<StringRef> &SSNs) const {
   pImpl->getSyncScopeNames(SSNs);
 }
 
+std::optional<StringRef> LLVMContext::getSyncScopeName(SyncScope::ID Id) const {
+  return pImpl->getSyncScopeName(Id);
+}
+
 void LLVMContext::setGC(const Function &Fn, std::string GCName) {
   pImpl->GCNames[&Fn] = std::move(GCName);
 }

llvm/lib/IR/LLVMContextImpl.cpp

@@ -244,6 +244,16 @@ void LLVMContextImpl::getSyncScopeNames(
     SSNs[SSE.second] = SSE.first();
 }
 
+std::optional<StringRef>
+LLVMContextImpl::getSyncScopeName(SyncScope::ID Id) const {
+  for (const auto &SSE : SSC) {
+    if (SSE.second != Id)
+      continue;
+    return SSE.first();
+  }
+  return std::nullopt;
+}
+
 /// Gets the OptPassGate for this LLVMContextImpl, which defaults to the
 /// singleton OptBisect if not explicitly set.
 OptPassGate &LLVMContextImpl::getOptPassGate() const {

llvm/lib/IR/LLVMContextImpl.h

@@ -1665,6 +1665,10 @@ class LLVMContextImpl {
   /// scope names are ordered by increasing synchronization scope IDs.
   void getSyncScopeNames(SmallVectorImpl<StringRef> &SSNs) const;
 
+  /// getSyncScopeName - Returns the name of a SyncScope::ID
+  /// registered with LLVMContext, if any.
+  std::optional<StringRef> getSyncScopeName(SyncScope::ID Id) const;
+
   /// Maintain the GC name for each function.
   ///
   /// This saves allocating an additional word in Function for programs which

llvm/lib/Target/AMDGPU/SIISelLowering.cpp

@@ -16144,11 +16144,8 @@ static bool atomicIgnoresDenormalModeOrFPModeIsFTZ(const AtomicRMWInst *RMW) {
 
 static OptimizationRemark emitAtomicRMWLegalRemark(const AtomicRMWInst *RMW) {
   LLVMContext &Ctx = RMW->getContext();
-  SmallVector<StringRef> SSNs;
-  Ctx.getSyncScopeNames(SSNs);
-  StringRef MemScope = SSNs[RMW->getSyncScopeID()].empty()
-                           ? "system"
-                           : SSNs[RMW->getSyncScopeID()];
+  StringRef SS = Ctx.getSyncScopeName(RMW->getSyncScopeID()).value_or("");
+  StringRef MemScope = SS.empty() ? StringRef("system") : SS;
 
   return OptimizationRemark(DEBUG_TYPE, "Passed", RMW)
          << "Hardware instruction generated for atomic "