[SPIR-V] Add partial order tests, assert reducible (#117887)

Add testing for the visitor and added a note explaining irreducible CFG
are not supported.
Related to #116692

---------

Signed-off-by: Nathan Gauër <[email protected]>

Author: Keenuts

llvm/lib/Target/SPIRV/SPIRVUtils.cpp

@@ -519,8 +519,11 @@ bool PartialOrderingVisitor::CanBeVisited(BasicBlock *BB) const {
 }
 
 size_t PartialOrderingVisitor::GetNodeRank(BasicBlock *BB) const {
-  size_t result = 0;
+  auto It = BlockToOrder.find(BB);
+  if (It != BlockToOrder.end())
+    return It->second.Rank;
 
+  size_t result = 0;
   for (BasicBlock *P : predecessors(BB)) {
     // Ignore back-edges.
     if (DT.dominates(BB, P))
@@ -552,15 +555,20 @@ size_t PartialOrderingVisitor::visit(BasicBlock *BB, size_t Unused) {
   ToVisit.push(BB);
   Queued.insert(BB);
 
+  size_t QueueIndex = 0;
   while (ToVisit.size() != 0) {
     BasicBlock *BB = ToVisit.front();
     ToVisit.pop();
 
     if (!CanBeVisited(BB)) {
       ToVisit.push(BB);
+      assert(QueueIndex < ToVisit.size() &&
+             "No valid candidate in the queue. Is the graph reducible?");
+      QueueIndex++;
       continue;
     }
 
+    QueueIndex = 0;
     size_t Rank = GetNodeRank(BB);
     OrderInfo Info = {Rank, BlockToOrder.size()};
     BlockToOrder.emplace(BB, Info);

llvm/lib/Target/SPIRV/SPIRVUtils.h

@@ -41,6 +41,8 @@ class SPIRVSubtarget;
 // ignores back-edges. The cycle is visited from the entry in the same
 // topological-like ordering.
 //
+// Note: this visitor REQUIRES a reducible graph.
+//
 // This means once we visit a node, we know all the possible ancestors have been
 // visited.
 //
@@ -84,10 +86,11 @@ class PartialOrderingVisitor {
   // Visits |BB| with the current rank being |Rank|.
   size_t visit(BasicBlock *BB, size_t Rank);
 
-  size_t GetNodeRank(BasicBlock *BB) const;
   bool CanBeVisited(BasicBlock *BB) const;
 
 public:
+  size_t GetNodeRank(BasicBlock *BB) const;
+
   // Build the visitor to operate on the function F.
   PartialOrderingVisitor(Function &F);
 

llvm/test/CodeGen/SPIRV/structurizer/cf.switch.ifstmt.simple2.ll

@@ -19,21 +19,21 @@
 ;       b += 2;
 ;       break;
 ;     case 3:
-;       {
-;         b += 3;
-;         break;
-;       }
+;     {
+;       b += 3;
+;       break;
+;     }
 ;     case t:
 ;       b += t;
 ;     case 4:
 ;     case 5:
 ;       b += 5;
 ;       break;
 ;     case 6: {
-;               case 7:
-;                 break;}
+;     case 7:
+;       break;}
 ;     default:
-;             break;
+;       break;
 ;   }
 ;
 ;   return a + b + c;

llvm/unittests/Target/SPIRV/CMakeLists.txt

@@ -16,5 +16,6 @@ set(LLVM_LINK_COMPONENTS
 add_llvm_target_unittest(SPIRVTests
   SPIRVConvergenceRegionAnalysisTests.cpp
   SPIRVSortBlocksTests.cpp
+  SPIRVPartialOrderingVisitorTests.cpp
   SPIRVAPITest.cpp
 )