[AMDGPU] System scope atomics are emulated over PCIe in gfx1250 (llvm#152369)

HW will emulate unsupported PCIe atomics via CAS loop, we do not need to
expand these anymore.

Author: rampitec

llvm/lib/Target/AMDGPU/AMDGPU.td

@@ -1013,6 +1013,14 @@ def FeatureAgentScopeFineGrainedRemoteMemoryAtomics
   "device memory."
 >;
 
+def FeatureEmulatedSystemScopeAtomics
+  : SubtargetFeature<"emulated-system-scope-atomics",
+  "HasEmulatedSystemScopeAtomics",
+  "true",
+  "System scope atomics unsupported by the PCI-e are emulated in HW via CAS "
+  "loop and functional."
+>;
+
 def FeatureDefaultComponentZero : SubtargetFeature<"default-component-zero",
   "HasDefaultComponentZero",
   "true",
@@ -2062,6 +2070,7 @@ def FeatureISAVersion12_50 : FeatureSet<
    FeatureAtomicFMinFMaxF64FlatInsts,
    FeatureFlatBufferGlobalAtomicFaddF64Inst,
    FeatureMemoryAtomicFAddF32DenormalSupport,
+   FeatureEmulatedSystemScopeAtomics,
    FeatureGloballyAddressableScratch,
    FeatureKernargPreload,
    FeatureVmemPrefInsts,

llvm/lib/Target/AMDGPU/GCNSubtarget.h

@@ -187,6 +187,7 @@ class GCNSubtarget final : public AMDGPUGenSubtargetInfo,
   bool HasFlatBufferGlobalAtomicFaddF64Inst = false;
   bool HasDefaultComponentZero = false;
   bool HasAgentScopeFineGrainedRemoteMemoryAtomics = false;
+  bool HasEmulatedSystemScopeAtomics = false;
   bool HasDefaultComponentBroadcast = false;
   bool HasXF32Insts = false;
   /// The maximum number of instructions that may be placed within an S_CLAUSE,
@@ -950,6 +951,12 @@ class GCNSubtarget final : public AMDGPUGenSubtargetInfo,
     return HasAgentScopeFineGrainedRemoteMemoryAtomics;
   }
 
+  /// \return true is HW emulates system scope atomics unsupported by the PCI-e
+  /// via CAS loop.
+  bool hasEmulatedSystemScopeAtomics() const {
+    return HasEmulatedSystemScopeAtomics;
+  }
+
   bool hasDefaultComponentZero() const { return HasDefaultComponentZero; }
 
   bool hasDefaultComponentBroadcast() const {

llvm/lib/Target/AMDGPU/SIISelLowering.cpp

@@ -17695,6 +17695,8 @@ static bool globalMemoryFPAtomicIsLegal(const GCNSubtarget &Subtarget,
     if (Subtarget.supportsAgentScopeFineGrainedRemoteMemoryAtomics() &&
         RMW->hasMetadata("amdgpu.no.remote.memory"))
       return true;
+    if (Subtarget.hasEmulatedSystemScopeAtomics())
+      return true;
   } else if (Subtarget.supportsAgentScopeFineGrainedRemoteMemoryAtomics())
     return true;
 
@@ -17942,8 +17944,7 @@ SITargetLowering::shouldExpandAtomicRMWInIR(AtomicRMWInst *RMW) const {
   case AtomicRMWInst::UMax: {
     if (AMDGPU::isFlatGlobalAddrSpace(AS) ||
         AS == AMDGPUAS::BUFFER_FAT_POINTER) {
-      // Always expand system scope min/max atomics.
-      if (HasSystemScope)
+      if (HasSystemScope && !Subtarget->hasEmulatedSystemScopeAtomics())
         return AtomicExpansionKind::CmpXChg;
     }