Mark more phis as uniform with undefs

Mark more phis as uniform with undefs

Author: dlei6g

IGC/Compiler/CISACodeGen/WIAnalysis.cpp

@@ -930,6 +930,49 @@ bool WIAnalysisRunner::isWaveBroadcastIndex(const llvm::Instruction* inst)
     return false;
 }
 
+// Checks all incoming values of PHI for a unique value, including PHI nodes that
+// loop back on itself. We ignore undef values as not counting towards a unique incoming value.
+static bool hasUniqueIncoming(llvm::PHINode* PN, Value*& UniqueVal, std::set<PHINode*>& phiLoop)
+{
+    UniqueVal = nullptr;
+    phiLoop.insert(PN);
+
+    for (Value* V : PN->incoming_values())
+    {
+        if (isa<UndefValue>(V))
+            continue;
+
+        if (auto* phi = dyn_cast<PHINode>(V))
+        {
+            // Ignore previously seen PHIs
+            if (phiLoop.count(phi) > 0)
+                continue;
+
+            Value* temp;
+            if (hasUniqueIncoming(phi, temp, phiLoop))
+            {
+                V = temp;
+            }
+        }
+
+        if (!UniqueVal)
+            UniqueVal = V;
+        else if (UniqueVal != V)
+        {
+            UniqueVal = nullptr;
+            return false;
+        }
+    }
+    return true;
+}
+
+bool WIAnalysisRunner::hasUniqueNonUndef(llvm::PHINode* PN)
+{
+    Value* temp;
+    std::set<PHINode*> phiLoop;
+    return hasUniqueIncoming(PN, temp, phiLoop);
+}
+
 void WIAnalysisRunner::update_cf_dep(const IGCLLVM::TerminatorInst* inst)
 {
     IGC_ASSERT(hasDependency(inst));
@@ -1098,14 +1141,23 @@ void WIAnalysisRunner::updatePHIDepAtJoin(BasicBlock* blk, BranchInfo* brInfo)
             continue;
         }
         Value* trickySrc = nullptr;
+        bool isUnique = hasUniqueNonUndef(phi);
         for (unsigned predIdx = 0; predIdx < phi->getNumOperands(); ++predIdx)
         {
             Value* srcVal = phi->getOperand(predIdx);
             Instruction* defi = dyn_cast<Instruction>(srcVal);
             if (defi && brInfo->influence_region.count(defi->getParent()))
             {
-                updateDepMap(phi, brDep);
-                break;
+                // If we have a phi join that looks like this:
+                // %v = phi i32 [ %x, %if ], [ undef, %else ]
+                //
+                // Then if %x is uniform, %v isn't really non-uniform because
+                // there will be no phi-mov associated with the undef.
+                if (!isUnique)
+                {
+                    updateDepMap(phi, brDep);
+                    break;
+                }
             }
             else
             {
@@ -1120,7 +1172,7 @@ void WIAnalysisRunner::updatePHIDepAtJoin(BasicBlock* blk, BranchInfo* brInfo)
                     {
                         trickySrc = srcVal;
                     }
-                    else if (trickySrc != srcVal)
+                    else if (trickySrc != srcVal && !isUnique)
                     {
                         updateDepMap(phi, brDep);
                         break;

IGC/Compiler/CISACodeGen/WIAnalysis.hpp

@@ -245,6 +245,10 @@ namespace IGC
             bool& IsLyUniform,
             bool& IsLzUniform);
 
+        /// @brief return true if the phi has at most one incoming value that is
+        ///        not an undef value.
+        static bool hasUniqueNonUndef(llvm::PHINode* PN);
+
     private:
 
         /// The WIAnalysis follows pointer arithmetic