[SCEV] Simplify SCEVExpr for PHI to SCEV for operand if operands are identical

llvm/include/llvm/Analysis/ScalarEvolution.h

@@ -1761,6 +1761,10 @@ class ScalarEvolution {
   /// V.
   const SCEV *getOperandsToCreate(Value *V, SmallVectorImpl<Value *> &Ops);
 
+  /// Returns SCEV for the first operand of a phi if all phi operands have
+  /// identical opcodes and operands.
+  const SCEV *createNodeForPHIWithIdenticalOperands(PHINode *PN);
+
   /// Provide the special handling we need to analyze PHI SCEVs.
   const SCEV *createNodeForPHI(PHINode *PN);
 

llvm/lib/Analysis/ScalarEvolution.cpp

@@ -6019,6 +6019,42 @@ const SCEV *ScalarEvolution::createNodeFromSelectLikePHI(PHINode *PN) {
   return nullptr;
 }
 
+/// Returns SCEV for the first operand of a phi if all phi operands have
+/// identical opcodes and operands
+/// eg.
+/// a: %add = %a + %b
+///    br %c
+/// b: %add1 = %a + %b
+///    br %c
+/// c: %phi = phi [%add, a], [%add1, b]
+/// scev(%phi) => scev(%add)
+const SCEV *
+ScalarEvolution::createNodeForPHIWithIdenticalOperands(PHINode *PN) {
+  BinaryOperator *CommonInst = nullptr;
+  // check if instructions are identical
+  for (Value *Incoming : PN->incoming_values()) {
+    auto *IncomingInst = dyn_cast<BinaryOperator>(Incoming);
+    if (!IncomingInst)
+      return nullptr;
+    if (CommonInst) {
+      if (!CommonInst->isIdenticalToWhenDefined(IncomingInst))
+        return nullptr; // Not identical, give up
+    } else {
+      // Remember binary operator
+      CommonInst = IncomingInst;
+    }
+  }
+  if (!CommonInst)
+    return nullptr;
+
+  // check if SCEV exprs for instructions are identical
+  const SCEV *CommonSCEV = getSCEV(CommonInst);
+  bool SCEVExprsIdentical = std::all_of(
+      PN->incoming_values().begin(), PN->incoming_values().end(),
+      [this, CommonSCEV](Value *V) { return CommonSCEV == getSCEV(V); });
+  return SCEVExprsIdentical ? CommonSCEV : nullptr;
+}
+
 const SCEV *ScalarEvolution::createNodeForPHI(PHINode *PN) {
   if (const SCEV *S = createAddRecFromPHI(PN))
     return S;
@@ -6030,6 +6066,9 @@ const SCEV *ScalarEvolution::createNodeForPHI(PHINode *PN) {
                /*UseInstrInfo=*/true, /*CanUseUndef=*/false}))
     return getSCEV(V);
 
+  if (const SCEV *S = createNodeForPHIWithIdenticalOperands(PN))
+    return S;
+
   if (const SCEV *S = createNodeFromSelectLikePHI(PN))
     return S;
 

llvm/test/Analysis/ScalarEvolution/trip-count-phi-increment.ll

@@ -0,0 +1,46 @@
+; NOTE: Assertions have been autogenerated by utils/update_analyze_test_checks.py UTC_ARGS: --version 5
+; RUN: opt < %s -disable-output "-passes=print<scalar-evolution>" 2>&1 | FileCheck %s
+define void @test(ptr %x, ptr %y) {
+; CHECK-LABEL: 'test'
+; CHECK-NEXT:  Classifying expressions for: @test
+; CHECK-NEXT:    %v1.0 = phi i32 [ 0, %entry ], [ %k.0, %if.end ]
+; CHECK-NEXT:    --> {0,+,1}<nuw><nsw><%for.cond> U: [0,7) S: [0,7) Exits: 6 LoopDispositions: { %for.cond: Computable }
+; CHECK-NEXT:    %add = add nsw i32 %v1.0, 1
+; CHECK-NEXT:    --> {1,+,1}<nuw><nsw><%for.cond> U: [1,8) S: [1,8) Exits: 7 LoopDispositions: { %for.cond: Computable }
+; CHECK-NEXT:    %add6 = add nsw i32 %v1.0, 1
+; CHECK-NEXT:    --> {1,+,1}<nuw><nsw><%for.cond> U: [1,8) S: [1,8) Exits: 7 LoopDispositions: { %for.cond: Computable }
+; CHECK-NEXT:    %k.0 = phi i32 [ %add, %if.then ], [ %add6, %if.else ]
+; CHECK-NEXT:    --> {1,+,1}<nuw><nsw><%for.cond> U: [1,8) S: [1,8) Exits: 7 LoopDispositions: { %for.cond: Computable }
+; CHECK-NEXT:  Determining loop execution counts for: @test
+; CHECK-NEXT:  Loop %for.cond: backedge-taken count is i32 6
+; CHECK-NEXT:  Loop %for.cond: constant max backedge-taken count is i32 6
+; CHECK-NEXT:  Loop %for.cond: symbolic max backedge-taken count is i32 6
+; CHECK-NEXT:  Loop %for.cond: Trip multiple is 7
+;
+entry:
+  br label %for.cond
+
+for.cond:                                                ; preds = %6, %0
+  %v1.0 = phi i32 [ 0, %entry ], [ %k.0, %if.end ]
+  %cmp = icmp slt i32 %v1.0, 6
+  br i1 %cmp, label %for.body, label %exit
+
+for.body:                                                ; preds = %1
+  %cmp3 = icmp slt i32 %v1.0, 2
+  br i1 %cmp3, label %if.then, label %if.else
+
+if.then:                                                ; preds = %2
+  %add = add nsw i32 %v1.0, 1
+  br label %if.end
+
+if.else:                                                ; preds = %2
+  %add6 = add nsw i32 %v1.0, 1
+  br label %if.end
+
+if.end:                                                ; preds = %4, %3
+  %k.0 = phi i32 [ %add, %if.then ], [ %add6, %if.else ]
+  br label %for.cond
+
+exit:                                                ; preds = %5
+  ret void
+}