From c1738fdadd1c969f13bbf09fe9c36fff56ccd709 Mon Sep 17 00:00:00 2001
From: Arnold Schwaighofer <aschwaighofer@apple.com>
Date: Sun, 5 May 2013 01:54:42 +0000
Subject: [PATCH] LoopVectorize: We don't need an identity element for min/max
 reductions

We can just use the initial element that feeds the reduction.

  max(max(x, y), z) == max(max(x,y), max(x,z))

radar://13723044

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181141 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/Transforms/Vectorize/LoopVectorize.cpp    | 49 +++++++------------
 .../LoopVectorize/minmax_reduction.ll         |  5 +-
 2 files changed, 22 insertions(+), 32 deletions(-)

diff --git a/lib/Transforms/Vectorize/LoopVectorize.cpp b/lib/Transforms/Vectorize/LoopVectorize.cpp
index 2250712a8e0..1c7397bfa66 100644
--- a/lib/Transforms/Vectorize/LoopVectorize.cpp
+++ b/lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -498,8 +498,7 @@ public:
 
   /// This function returns the identity element (or neutral element) for
   /// the operation K.
-  static Constant *getReductionIdentity(ReductionKind K, Type *Tp,
-                                        MinMaxReductionKind MinMaxK);
+  static Constant *getReductionIdentity(ReductionKind K, Type *Tp);
 private:
   /// Check if a single basic block loop is vectorizable.
   /// At this point we know that this is a loop with a constant trip count
@@ -1501,8 +1500,7 @@ InnerLoopVectorizer::createEmptyLoop(LoopVectorizationLegality *Legal) {
 /// This function returns the identity element (or neutral element) for
 /// the operation K.
 Constant*
-LoopVectorizationLegality::getReductionIdentity(ReductionKind K, Type *Tp,
-                                                MinMaxReductionKind MinMaxK) {
+LoopVectorizationLegality::getReductionIdentity(ReductionKind K, Type *Tp) {
   switch (K) {
   case RK_IntegerXor:
   case RK_IntegerAdd:
@@ -1521,24 +1519,6 @@ LoopVectorizationLegality::getReductionIdentity(ReductionKind K, Type *Tp,
   case  RK_FloatAdd:
     // Adding zero to a number does not change it.
     return ConstantFP::get(Tp, 0.0L);
-  case  RK_IntegerMinMax:
-    switch(MinMaxK) {
-    default: llvm_unreachable("Unknown min/max predicate");
-    case MRK_UIntMin:
-      return ConstantInt::getAllOnesValue(Tp);
-    case MRK_UIntMax:
-      return ConstantInt::get(Tp, 0);
-    case MRK_SIntMin: {
-      unsigned BitWidth = Tp->getPrimitiveSizeInBits();
-      return ConstantInt::get(Tp->getContext(),
-                              APInt::getSignedMaxValue(BitWidth));
-    }
-    case LoopVectorizationLegality::MRK_SIntMax: {
-      unsigned BitWidth = Tp->getPrimitiveSizeInBits();
-      return ConstantInt::get(Tp->getContext(),
-                              APInt::getSignedMinValue(BitWidth));
-    }
-    }
   default:
     llvm_unreachable("Unknown reduction kind");
   }
@@ -1761,16 +1741,23 @@ InnerLoopVectorizer::vectorizeLoop(LoopVectorizationLegality *Legal) {
 
     // Find the reduction identity variable. Zero for addition, or, xor,
     // one for multiplication, -1 for And.
-    Constant *Iden =
-      LoopVectorizationLegality::getReductionIdentity(RdxDesc.Kind,
-                                                      VecTy->getScalarType(),
-                                                      RdxDesc.MinMaxKind);
-    Constant *Identity = ConstantVector::getSplat(VF, Iden);
+    Value *Identity;
+    Value *VectorStart;
+    if (RdxDesc.Kind == LoopVectorizationLegality::RK_IntegerMinMax)
+      // MinMax reduction have the start value as their identify.
+      VectorStart = Identity = Builder.CreateVectorSplat(VF, RdxDesc.StartValue,
+                                                         "minmax.ident");
+    else {
+      Constant *Iden =
+        LoopVectorizationLegality::getReductionIdentity(RdxDesc.Kind,
+                                                        VecTy->getScalarType());
+      Identity = ConstantVector::getSplat(VF, Iden);
 
-    // This vector is the Identity vector where the first element is the
-    // incoming scalar reduction.
-    Value *VectorStart = Builder.CreateInsertElement(Identity,
-                                                     RdxDesc.StartValue, Zero);
+      // This vector is the Identity vector where the first element is the
+      // incoming scalar reduction.
+      VectorStart = Builder.CreateInsertElement(Identity,
+                                                RdxDesc.StartValue, Zero);
+    }
 
     // Fix the vector-loop phi.
     // We created the induction variable so we know that the
diff --git a/test/Transforms/LoopVectorize/minmax_reduction.ll b/test/Transforms/LoopVectorize/minmax_reduction.ll
index 99dd09386f5..36a8758e2cc 100644
--- a/test/Transforms/LoopVectorize/minmax_reduction.ll
+++ b/test/Transforms/LoopVectorize/minmax_reduction.ll
@@ -6,8 +6,11 @@ target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f3
 
 ; Signed tests.
 
-; Turn this into a max reduction.
+; Turn this into a max reduction. Make sure we use a splat to initialize the
+; vector for the reduction.
 ; CHECK: @max_red
+; CHECK: %[[VAR:.*]] = insertelement <2 x i32> undef, i32 %max, i32 0
+; CHECK: {{.*}} = shufflevector <2 x i32> %[[VAR]], <2 x i32> undef, <2 x i32> zeroinitializer
 ; CHECK: icmp sgt <2 x i32>
 ; CHECK: select <2 x i1>
 ; CHECK: middle.block