[GVN] Support rnflow pattern matching and transform

Author: madhur13490

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 +407,8 @@ class GVNPass : public PassInfoMixin<GVNPass> {
   void addDeadBlock(BasicBlock *BB);
   void assignValNumForDeadCode();
   void assignBlockRPONumber(Function &F);
+  
+  bool optimizeMinMaxFindingSelectPattern(SelectInst *Select);
 };
 
 /// 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 (optimizeMinMaxFindingSelectPattern(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 +3334,124 @@ void GVNPass::assignValNumForDeadCode() {
   }
 }
 
+bool GVNPass::optimizeMinMaxFindingSelectPattern(SelectInst *Select) {
+  LLVM_DEBUG(
+      dbgs()
+      << "GVN: Analyzing select instruction for minimum finding pattern\n");
+  LLVM_DEBUG(dbgs() << "GVN: Select: " << *Select << "\n");
+  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 ULT 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;
+  }
+
+  LLVM_DEBUG(dbgs() << "GVN: Found minimum finding pattern in Block: "
+                    << Select->getParent()->getName() << "\n");
+
+  // Transform the pattern.
+  // Hoist the chain of operations for the second load to preheader.
+  // Get predecessor of the block containing the select instruction.
+  BasicBlock *BB = Select->getParent();
+
+  // Get preheader of the loop.
+  Loop *L = LI->getLoopFor(BB);
+  if (!L) {
+    LLVM_DEBUG(dbgs() << "GVN: Could not find loop\n");
+    return false;
+  }
+  BasicBlock *Preheader = L->getLoopPreheader();
+  if (!Preheader) {
+    LLVM_DEBUG(dbgs() << "GVN: Could not find loop preheader\n");
+    return false;
+  }
+
+  // Hoist the chain of operations for the second load to preheader.
+  // %90 = sext i32 %.05.i to i64
+  // %91 = getelementptr float, ptr %0, i64 %90 ; %0 + (sext i32 %85 to i64)*4
+  // %92 = getelementptr i8, ptr %91, i64 -4 ; %0 + (sext i32 %85 to i64)*4 - 4
+  // %93 = load float, ptr %92, align 4
+
+  Value *BasePtr = nullptr, *IndexVal = nullptr, *OffsetVal = nullptr;
+  IRBuilder<> Builder(Preheader->getTerminator());
+  if (match(RHS,
+            m_Load(m_GEP(m_GEP(m_Value(BasePtr), m_SExt(m_Value(IndexVal))),
+                         m_Value(OffsetVal))))) {
+    LLVM_DEBUG(dbgs() << "GVN: Found pattern: " << *RHS << "\n");
+    LLVM_DEBUG(dbgs() << "GVN: Found pattern: " << "\n");
+
+    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.
+    PHINode *KnownMinPhi =
+        PHINode::Create(Builder.getFloatTy(), 2, "known_min", BB->begin());
+
+    // Build the GEP chain in the preheader.
+    // 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(Builder.getFloatTy(), 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(Builder.getFloatTy(), HoistedGEP2, "hoisted_load");
+
+    // Replace all uses of load with new load.
+    RHS->replaceAllUsesWith(NewLoad);
+    dyn_cast<LoadInst>(RHS)->eraseFromParent();
+
+    // Replace second operand of comparison with KnownMinPhi.
+    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 PHI node.
+    KnownMinPhi->addIncoming(NewLoad, Preheader);
+    KnownMinPhi->addIncoming(CurrentMinSelect, BB);
+    LLVM_DEBUG(dbgs() << "Transformed the code\n");
+    return true;
+  } else {
+    LLVM_DEBUG(dbgs() << "GVN: Could not find pattern: " << *RHS << "\n");
+    LLVM_DEBUG(dbgs() << "GVN: Could not find pattern: " << "\n");
+    return false;
+  }
+  return false;
+}
+
+
 class llvm::gvn::GVNLegacyPass : public FunctionPass {
 public:
   static char ID; // Pass identification, replacement for typeid.

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

@@ -0,0 +1,59 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 6
+; Minimal test case containing only the .lr.ph.i basic block
+; RUN: opt -passes=gvn -S < %s | FileCheck %s
+
+define void @test_lr_ph_i(ptr %0) {
+; CHECK-LABEL: define void @test_lr_ph_i(
+; 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 %[[DOTLR_PH_I:.*]]
+; CHECK:       [[_LR_PH_I:.*:]]
+; CHECK-NEXT:    [[KNOWN_MIN:%.*]] = phi float [ [[HOISTED_LOAD]], %[[ENTRY]] ], [ [[CURRENT_MIN:%.*]], %[[DOTLR_PH_I]] ]
+; CHECK-NEXT:    [[INDVARS_IV_I:%.*]] = phi i64 [ 1, %[[ENTRY]] ], [ [[INDVARS_IV_NEXT_I:%.*]], %[[DOTLR_PH_I]] ]
+; CHECK-NEXT:    [[TMP1:%.*]] = phi i64 [ 0, %[[ENTRY]] ], [ [[TMP10:%.*]], %[[DOTLR_PH_I]] ]
+; CHECK-NEXT:    [[DOT05_I:%.*]] = phi i32 [ 1, %[[ENTRY]] ], [ [[DOT1_I:%.*]], %[[DOTLR_PH_I]] ]
+; CHECK-NEXT:    [[INDVARS_IV_NEXT_I]] = add nsw i64 [[INDVARS_IV_I]], -1
+; CHECK-NEXT:    [[TMP2:%.*]] = getelementptr float, ptr [[TMP0]], i64 [[INDVARS_IV_I]]
+; CHECK-NEXT:    [[TMP3:%.*]] = getelementptr i8, ptr [[TMP2]], i64 -8
+; CHECK-NEXT:    [[TMP4:%.*]] = load float, ptr [[TMP3]], align 4
+; CHECK-NEXT:    [[TMP5:%.*]] = sext i32 [[DOT05_I]] to i64
+; CHECK-NEXT:    [[TMP6:%.*]] = getelementptr float, ptr [[TMP0]], i64 [[TMP5]]
+; CHECK-NEXT:    [[TMP7:%.*]] = getelementptr i8, ptr [[TMP6]], i64 -4
+; CHECK-NEXT:    [[TMP8:%.*]] = fcmp contract olt float [[TMP4]], [[KNOWN_MIN]]
+; CHECK-NEXT:    [[TMP9:%.*]] = trunc nsw i64 [[INDVARS_IV_NEXT_I]] to i32
+; CHECK-NEXT:    [[DOT1_I]] = select i1 [[TMP8]], i32 [[TMP9]], i32 [[DOT05_I]]
+; CHECK-NEXT:    [[TMP10]] = add nsw i64 [[TMP1]], -1
+; CHECK-NEXT:    [[TMP11:%.*]] = icmp samesign ugt i64 [[TMP1]], 1
+; CHECK-NEXT:    [[CURRENT_MIN]] = select i1 [[TMP8]], float [[TMP4]], float [[KNOWN_MIN]]
+; CHECK-NEXT:    br i1 [[TMP11]], label %[[DOTLR_PH_I]], label %[[EXIT:.*]]
+; CHECK:       [[EXIT]]:
+; CHECK-NEXT:    ret void
+;
+entry:
+  br label %.lr.ph.i
+
+.lr.ph.i:                                         ; preds = %.lr.ph.i, %entry
+  %indvars.iv.i = phi i64 [ 1, %entry ], [ %indvars.iv.next.i, %.lr.ph.i ]
+  %86 = phi i64 [ 0, %entry ], [ %96, %.lr.ph.i ]
+  %.05.i = phi i32 [ 1, %entry ], [ %.1.i, %.lr.ph.i ]
+  %indvars.iv.next.i = add nsw i64 %indvars.iv.i, -1
+  %87 = getelementptr float, ptr %0, i64 %indvars.iv.i
+  %88 = getelementptr i8, ptr %87, i64 -8 ; first load : %0 + 4 * 1 - 8
+  %89 = load float, ptr %88, align 4
+  %90 = sext i32 %.05.i to i64
+  %91 = getelementptr float, ptr %0, i64 %90 ; %0 + 4 * 1
+  %92 = getelementptr i8, ptr %91, i64 -4 ; second load : %0 + 4 * 1 - 4
+  %93 = load float, ptr %92, align 4
+  %94 = fcmp contract olt float %89, %93
+  %95 = trunc nsw i64 %indvars.iv.next.i to i32
+  %.1.i = select i1 %94, i32 %95, i32 %.05.i
+  %96 = add nsw i64 %86, -1
+  %97 = icmp samesign ugt i64 %86, 1
+  br i1 %97, label %.lr.ph.i, label %exit
+
+exit:
+  ret void
+}