[AMDGPU] Filter candidates of LiveRegOptimizer for profitable cases

It is known that for vector whose element fits in i16 will be split
and scalarized in SelectionDag's type legalizer
(see SIISelLowering::getPreferredVectorAction).

LRO attempts to undo the scalarizing of vectors across basic block
boundary and shoehorn Values in VGPRs. LRO is beneficial for operations
that natively work on illegal vector types to prevent flip-flopping
between SGPR and VGPR. If we know that operations on vector will be
split and scalarized, then we don't want to shoehorn them back to VGPR.

Operations that we know to work natively on illegal vector types
usually come in the form of intrinsics (MFMA, DOT8), buffer store,
shuffle, phi nodes to name a few.

Author: choikwa

llvm/lib/Target/AMDGPU/AMDGPULateCodeGenPrepare.cpp

@@ -14,6 +14,7 @@
 
 #include "AMDGPU.h"
 #include "AMDGPUTargetMachine.h"
+#include "AMDGPUTargetTransformInfo.h"
 #include "llvm/Analysis/AssumptionCache.h"
 #include "llvm/Analysis/UniformityAnalysis.h"
 #include "llvm/Analysis/ValueTracking.h"
@@ -45,6 +46,7 @@ class AMDGPULateCodeGenPrepare
   Function &F;
   const DataLayout &DL;
   const GCNSubtarget &ST;
+  const TargetTransformInfo &TTI;
 
   AssumptionCache *const AC;
   UniformityInfo &UA;
@@ -53,8 +55,9 @@ class AMDGPULateCodeGenPrepare
 
 public:
   AMDGPULateCodeGenPrepare(Function &F, const GCNSubtarget &ST,
-                           AssumptionCache *AC, UniformityInfo &UA)
-      : F(F), DL(F.getDataLayout()), ST(ST), AC(AC), UA(UA) {}
+                           const TargetTransformInfo &TTI, AssumptionCache *AC,
+                           UniformityInfo &UA)
+      : F(F), DL(F.getDataLayout()), ST(ST), TTI(TTI), AC(AC), UA(UA) {}
   bool run();
   bool visitInstruction(Instruction &) { return false; }
 
@@ -75,6 +78,8 @@ class LiveRegOptimizer {
   Module &Mod;
   const DataLayout &DL;
   const GCNSubtarget &ST;
+  const TargetTransformInfo &TTI;
+
   /// The scalar type to convert to
   Type *const ConvertToScalar;
   /// The set of visited Instructions
@@ -125,8 +130,41 @@ class LiveRegOptimizer {
     return LK.first != TargetLoweringBase::TypeLegal;
   }
 
-  LiveRegOptimizer(Module &Mod, const GCNSubtarget &ST)
-      : Mod(Mod), DL(Mod.getDataLayout()), ST(ST),
+  // Filtering based on operation or its cost.
+  // If an operation incurs high enough cost or natively work on
+  // vector of illegal type, ie. v2i8, then it makes sense to try
+  // to avoid scalarizing across BB.
+  bool shouldReplaceBasedOnOp(Instruction *II) {
+    // Ignore pseudos
+    if (II->isDebugOrPseudoInst())
+      return false;
+
+    // Instruction Cost
+    const auto Cost = TTI.getInstructionCost(
+        II, TargetTransformInfo::TargetCostKind::TCK_SizeAndLatency);
+    LLVM_DEBUG(dbgs() << "shouldReplaceBasedOnOp: " << *II << " Cost=" << Cost
+                      << '\n';);
+    if (Cost >= 8)
+      return true;
+
+    // Intrinsics - assume they natively handle illegal type
+    if (dyn_cast<IntrinsicInst>(II))
+      return true;
+
+    // Stores
+    if (dyn_cast<StoreInst>(II))
+      return true;
+
+    // Shuffles
+    if (dyn_cast<ShuffleVectorInst>(II))
+      return true;
+
+    return false;
+  }
+
+  LiveRegOptimizer(Module &Mod, const GCNSubtarget &ST,
+                   const TargetTransformInfo &TTI)
+      : Mod(Mod), DL(Mod.getDataLayout()), ST(ST), TTI(TTI),
         ConvertToScalar(Type::getInt32Ty(Mod.getContext())) {}
 };
 
@@ -140,7 +178,7 @@ bool AMDGPULateCodeGenPrepare::run() {
   // vectors to equivalent vectors of legal type (which are converted back
   // before uses in subsequent blocks), to pack the bits into fewer physical
   // registers (used in CopyToReg/CopyFromReg pairs).
-  LiveRegOptimizer LRO(*F.getParent(), ST);
+  LiveRegOptimizer LRO(*F.getParent(), ST, TTI);
 
   bool Changed = false;
 
@@ -259,6 +297,9 @@ bool LiveRegOptimizer::optimizeLiveType(
     if (!shouldReplace(II->getType()))
       continue;
 
+    if (!shouldReplaceBasedOnOp(II))
+      continue;
+
     if (PHINode *Phi = dyn_cast<PHINode>(II)) {
       PhiNodes.insert(Phi);
       // Collect all the incoming values of problematic PHI nodes.
@@ -478,11 +519,12 @@ bool AMDGPULateCodeGenPrepare::visitLoadInst(LoadInst &LI) {
 PreservedAnalyses
 AMDGPULateCodeGenPreparePass::run(Function &F, FunctionAnalysisManager &FAM) {
   const GCNSubtarget &ST = TM.getSubtarget<GCNSubtarget>(F);
+  const TargetTransformInfo &TTI = TM.getTargetTransformInfo(F);
 
   AssumptionCache &AC = FAM.getResult<AssumptionAnalysis>(F);
   UniformityInfo &UI = FAM.getResult<UniformityInfoAnalysis>(F);
 
-  bool Changed = AMDGPULateCodeGenPrepare(F, ST, &AC, UI).run();
+  bool Changed = AMDGPULateCodeGenPrepare(F, ST, TTI, &AC, UI).run();
 
   if (!Changed)
     return PreservedAnalyses::all();
@@ -518,13 +560,14 @@ bool AMDGPULateCodeGenPrepareLegacy::runOnFunction(Function &F) {
   const TargetPassConfig &TPC = getAnalysis<TargetPassConfig>();
   const TargetMachine &TM = TPC.getTM<TargetMachine>();
   const GCNSubtarget &ST = TM.getSubtarget<GCNSubtarget>(F);
+  const TargetTransformInfo &TTI = TM.getTargetTransformInfo(F);
 
   AssumptionCache &AC =
       getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
   UniformityInfo &UI =
       getAnalysis<UniformityInfoWrapperPass>().getUniformityInfo();
 
-  return AMDGPULateCodeGenPrepare(F, ST, &AC, UI).run();
+  return AMDGPULateCodeGenPrepare(F, ST, TTI, &AC, UI).run();
 }
 
 INITIALIZE_PASS_BEGIN(AMDGPULateCodeGenPrepareLegacy, DEBUG_TYPE,