[GVN] Support rnflow pattern matching and transform

llvm/include/llvm/Transforms/Scalar/GVN.h

@@ -22,6 +22,7 @@
 #include "llvm/IR/Dominators.h"
 #include "llvm/IR/InstrTypes.h"
 #include "llvm/IR/PassManager.h"
+#include "llvm/Analysis/LoopInfo.h"
 #include "llvm/IR/ValueHandle.h"
 #include "llvm/Support/Allocator.h"
 #include "llvm/Support/Compiler.h"
@@ -45,6 +46,7 @@ class FunctionPass;
 class GetElementPtrInst;
 class ImplicitControlFlowTracking;
 class LoadInst;
+class SelectInst;
 class LoopInfo;
 class MemDepResult;
 class MemoryAccess;
@@ -405,6 +407,14 @@ class GVNPass : public PassInfoMixin<GVNPass> {
   void addDeadBlock(BasicBlock *BB);
   void assignValNumForDeadCode();
   void assignBlockRPONumber(Function &F);
+
+  bool recognizeMinFindingSelectPattern(SelectInst *Select);
+  bool transformMinFindingSelectPattern(Loop *L, Type *LoadType,
+                                        BasicBlock *Preheader, BasicBlock *BB,
+                                        Value *LHS, Value *RHS,
+                                        CmpInst *Comparison, SelectInst *Select,
+                                        Value *BasePtr, Value *IndexVal,
+                                        Value *OffsetVal);
 };
 
 /// Create a legacy GVN pass.

llvm/lib/Transforms/Scalar/GVN.cpp

@@ -2743,6 +2743,10 @@ bool GVNPass::processInstruction(Instruction *I) {
     }
     return Changed;
   }
+  if (SelectInst *Select = dyn_cast<SelectInst>(I)) {
+    if (recognizeMinFindingSelectPattern(Select))
+      return true;
+  }
 
   // Instructions with void type don't return a value, so there's
   // no point in trying to find redundancies in them.
@@ -3330,6 +3334,128 @@ void GVNPass::assignValNumForDeadCode() {
   }
 }
 
+bool GVNPass::transformMinFindingSelectPattern(
+    Loop *L, Type *LoadType, BasicBlock *Preheader, BasicBlock *BB, Value *LHS,
+    Value *RHS, CmpInst *Comparison, SelectInst *Select, Value *BasePtr,
+    Value *IndexVal, Value *OffsetVal) {
+  // Hoist the chain of operations for the second load to preheader.
+  // %min.idx.ext = sext i32 %min.idx to i64
+  // %ptr.float.min = getelementptr float, ptr %0, i64 %min.idx.ext
+  // %ptr.second.load = getelementptr i8, ptr %ptr.float.min, i64 -4
+  // %val.current.min = load float, ptr %ptr.second.load, align 4
+  IRBuilder<> Builder(Preheader->getTerminator());
+
+  PHINode *Phi = dyn_cast<PHINode>(IndexVal);
+  if (!Phi) {
+    LLVM_DEBUG(dbgs() << "GVN: IndexVal is not a PHI node\n");
+    return false;
+  }
+
+  Value *InitialMinIndex = Phi->getIncomingValueForBlock(Preheader);
+
+  // Insert PHI node at the top of this block.
+  // This PHI node will be used to memoize the current minimum value so far.
+  PHINode *KnownMinPhi = PHINode::Create(LoadType, 2, "known_min", BB->begin());
+
+  // Hoist the load and build the necessary operations.
+  // 1. hoist_0 = sext i32 to i64
+  Value *HoistedSExt =
+      Builder.CreateSExt(InitialMinIndex, Builder.getInt64Ty(), "hoist_sext");
+
+  // 2. hoist_gep1 = getelementptr float, ptr BasePtr, i64 HoistedSExt
+  Value *HoistedGEP1 =
+      Builder.CreateGEP(LoadType, BasePtr, HoistedSExt, "hoist_gep1");
+
+  // 3. hoist_gep2 = getelementptr i8, ptr HoistedGEP1, i64 OffsetVal
+  Value *HoistedGEP2 = Builder.CreateGEP(Builder.getInt8Ty(), HoistedGEP1,
+                                         OffsetVal, "hoist_gep2");
+
+  // 4. hoisted_load = load float, ptr HoistedGEP2
+  LoadInst *NewLoad = Builder.CreateLoad(LoadType, HoistedGEP2, "hoisted_load");
+
+  // Let the new load now take the place of the old load.
+  RHS->replaceAllUsesWith(NewLoad);
+  dyn_cast<LoadInst>(RHS)->eraseFromParent();
+
+  // Comparison should now compare the current value and the newly inserted
+  // PHI node.
+  Comparison->setOperand(1, KnownMinPhi);
+
+  // Create new select instruction for selecting the minimum value.
+  IRBuilder<> SelectBuilder(BB->getTerminator());
+  SelectInst *CurrentMinSelect = dyn_cast<SelectInst>(
+      SelectBuilder.CreateSelect(Comparison, LHS, KnownMinPhi, "current_min"));
+
+  // Populate the newly created PHI node
+  // with (hoisted) NewLoad from the preheader and CurrentMinSelect.
+  KnownMinPhi->addIncoming(NewLoad, Preheader);
+  KnownMinPhi->addIncoming(CurrentMinSelect, BB);
+  LLVM_DEBUG(dbgs() << "Transformed the code\n");
+  return true;
+}
+
+bool GVNPass::recognizeMinFindingSelectPattern(SelectInst *Select) {
+  Value *BasePtr = nullptr, *IndexVal = nullptr, *OffsetVal = nullptr;
+  LLVM_DEBUG(
+      dbgs()
+      << "GVN: Analyzing select instruction for minimum finding pattern.\n");
+  LLVM_DEBUG(dbgs() << "GVN: Select: " << *Select << "\n");
+  BasicBlock *BB = Select->getParent();
+
+  // If the block is not in a loop, bail out.
+  Loop *L = LI->getLoopFor(BB);
+  if (!L) {
+    LLVM_DEBUG(dbgs() << "GVN: Could not find loop.\n");
+    return false;
+  }
+
+  // If preheader of the loop is not found, bail out.
+  BasicBlock *Preheader = L->getLoopPreheader();
+  if (!Preheader) {
+    LLVM_DEBUG(dbgs() << "GVN: Could not find loop preheader.\n");
+    return false;
+  }
+  Value *Condition = Select->getCondition();
+  CmpInst *Comparison = dyn_cast<CmpInst>(Condition);
+  if (!Comparison) {
+    LLVM_DEBUG(dbgs() << "GVN: Condition is not a comparison.\n");
+    return false;
+  }
+
+  // Check if this is less-than comparison.
+  CmpInst::Predicate Pred = Comparison->getPredicate();
+  if (Pred != CmpInst::ICMP_SLT && Pred != CmpInst::ICMP_ULT &&
+      Pred != CmpInst::FCMP_OLT && Pred != CmpInst::FCMP_ULT) {
+    LLVM_DEBUG(dbgs() << "GVN: Not a less-than comparison, predicate: " << Pred
+                      << "\n");
+    return false;
+  }
+
+  // Check that both operands are loads.
+  Value *LHS = Comparison->getOperand(0);
+  Value *RHS = Comparison->getOperand(1);
+  if (!isa<LoadInst>(LHS) || !isa<LoadInst>(RHS)) {
+    LLVM_DEBUG(dbgs() << "GVN: Not both operands are loads.\n");
+    return false;
+  }
+
+  if (!match(RHS,
+             m_Load(m_GEP(m_GEP(m_Value(BasePtr), m_SExt(m_Value(IndexVal))),
+                          m_Value(OffsetVal))))) {
+    LLVM_DEBUG(dbgs() << "GVN: Not a required load pattern.\n");
+    return false;
+  }
+  LLVM_DEBUG(dbgs() << "GVN: Found minimum finding pattern in Block: "
+                    << Select->getParent()->getName() << ".\n");
+
+  // Get type of load.
+  Type *LoadType = dyn_cast<LoadInst>(LHS)->getType();
+  LLVM_DEBUG(dbgs() << "GVN: Transforming minimum finding pattern.\n");
+  return transformMinFindingSelectPattern(L, LoadType, Preheader, BB, LHS, RHS,
+                                          Comparison, Select, BasePtr, IndexVal,
+                                          OffsetVal);
+}
+
 class llvm::gvn::GVNLegacyPass : public FunctionPass {
 public:
   static char ID; // Pass identification, replacement for typeid.

llvm/test/Transforms/GVN/PRE/gvn-min-pre.ll

@@ -0,0 +1,58 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 6
+; RUN: opt -passes=gvn -S < %s | FileCheck %s
+
+define void @test_gvn_min_pattern(ptr %0) {
+; CHECK-LABEL: define void @test_gvn_min_pattern(
+; CHECK-SAME: ptr [[TMP0:%.*]]) {
+; CHECK-NEXT:  [[ENTRY:.*]]:
+; CHECK-NEXT:    [[HOIST_GEP1:%.*]] = getelementptr float, ptr [[TMP0]], i64 1
+; CHECK-NEXT:    [[HOIST_GEP2:%.*]] = getelementptr i8, ptr [[HOIST_GEP1]], i64 -4
+; CHECK-NEXT:    [[HOISTED_LOAD:%.*]] = load float, ptr [[HOIST_GEP2]], align 4
+; CHECK-NEXT:    br label %[[LOOP:.*]]
+; CHECK:       [[LOOP]]:
+; CHECK-NEXT:    [[KNOWN_MIN:%.*]] = phi float [ [[HOISTED_LOAD]], %[[ENTRY]] ], [ [[CURRENT_MIN:%.*]], %[[LOOP]] ]
+; CHECK-NEXT:    [[INDVARS_IV_I:%.*]] = phi i64 [ 1, %[[ENTRY]] ], [ [[INDVARS_IV_NEXT_I:%.*]], %[[LOOP]] ]
+; CHECK-NEXT:    [[LOOP_COUNTER:%.*]] = phi i64 [ 0, %[[ENTRY]] ], [ [[LOOP_COUNTER_NEXT:%.*]], %[[LOOP]] ]
+; CHECK-NEXT:    [[MIN_IDX:%.*]] = phi i32 [ 1, %[[ENTRY]] ], [ [[MIN_IDX_NEXT:%.*]], %[[LOOP]] ]
+; CHECK-NEXT:    [[INDVARS_IV_NEXT_I]] = add nsw i64 [[INDVARS_IV_I]], -1
+; CHECK-NEXT:    [[PTR_FLOAT_IV:%.*]] = getelementptr float, ptr [[TMP0]], i64 [[INDVARS_IV_I]]
+; CHECK-NEXT:    [[PTR_FIRST_LOAD:%.*]] = getelementptr i8, ptr [[PTR_FLOAT_IV]], i64 -8
+; CHECK-NEXT:    [[VAL_FIRST:%.*]] = load float, ptr [[PTR_FIRST_LOAD]], align 4
+; CHECK-NEXT:    [[MIN_IDX_EXT:%.*]] = sext i32 [[MIN_IDX]] to i64
+; CHECK-NEXT:    [[PTR_FLOAT_MIN:%.*]] = getelementptr float, ptr [[TMP0]], i64 [[MIN_IDX_EXT]]
+; CHECK-NEXT:    [[PTR_SECOND_LOAD:%.*]] = getelementptr i8, ptr [[PTR_FLOAT_MIN]], i64 -4
+; CHECK-NEXT:    [[CMP:%.*]] = fcmp contract olt float [[VAL_FIRST]], [[KNOWN_MIN]]
+; CHECK-NEXT:    [[NEXT_IDX_TRUNC:%.*]] = trunc nsw i64 [[INDVARS_IV_NEXT_I]] to i32
+; CHECK-NEXT:    [[MIN_IDX_NEXT]] = select i1 [[CMP]], i32 [[NEXT_IDX_TRUNC]], i32 [[MIN_IDX]]
+; CHECK-NEXT:    [[LOOP_COUNTER_NEXT]] = add nsw i64 [[LOOP_COUNTER]], -1
+; CHECK-NEXT:    [[LOOP_CONTINUE:%.*]] = icmp samesign ugt i64 [[LOOP_COUNTER]], 1
+; CHECK-NEXT:    [[CURRENT_MIN]] = select i1 [[CMP]], float [[VAL_FIRST]], float [[KNOWN_MIN]]
+; CHECK-NEXT:    br i1 [[LOOP_CONTINUE]], label %[[LOOP]], label %[[EXIT:.*]]
+; CHECK:       [[EXIT]]:
+; CHECK-NEXT:    ret void
+;
+entry:
+  br label %loop
+
+loop:                                         ; preds = %loop, %entry
+  %indvars.iv.i = phi i64 [ 1, %entry ], [ %indvars.iv.next.i, %loop ]
+  %loop.counter = phi i64 [ 0, %entry ], [ %loop.counter.next, %loop ]
+  %min.idx = phi i32 [ 1, %entry ], [ %min.idx.next, %loop ]
+  %indvars.iv.next.i = add nsw i64 %indvars.iv.i, -1
+  %ptr.float.iv = getelementptr float, ptr %0, i64 %indvars.iv.i
+  %ptr.first.load = getelementptr i8, ptr %ptr.float.iv, i64 -8
+  %val.first = load float, ptr %ptr.first.load, align 4
+  %min.idx.ext = sext i32 %min.idx to i64
+  %ptr.float.min = getelementptr float, ptr %0, i64 %min.idx.ext
+  %ptr.second.load = getelementptr i8, ptr %ptr.float.min, i64 -4
+  %val.current.min = load float, ptr %ptr.second.load, align 4
+  %cmp = fcmp contract olt float %val.first, %val.current.min
+  %next.idx.trunc = trunc nsw i64 %indvars.iv.next.i to i32
+  %min.idx.next = select i1 %cmp, i32 %next.idx.trunc, i32 %min.idx
+  %loop.counter.next = add nsw i64 %loop.counter, -1
+  %loop.continue = icmp samesign ugt i64 %loop.counter, 1
+  br i1 %loop.continue, label %loop, label %exit
+
+exit:
+  ret void
+}