From 7b4cd228aa920b62a4bc1c04e4055f069dc11285 Mon Sep 17 00:00:00 2001
From: Nick Lewycky <nicholas@mxc.ca>
Date: Thu, 27 Sep 2012 08:33:56 +0000
Subject: [PATCH] Prefer shuffles to selects. Backends love shuffles!

llvm-svn: 164763
---
 .../Transforms/InstCombine/InstCombineSelect.cpp   | 20 ++++++++++++-
 .../Transforms/InstCombine/vec_demanded_elts.ll    |  2 +-
 llvm/test/Transforms/InstCombine/vec_shuffle.ll    | 35 ++++++++++++++++++++++
 3 files changed, 55 insertions(+), 2 deletions(-)

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
index 291e800..70483ce 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -903,7 +903,7 @@ Instruction *InstCombiner::visitSelectInst(SelectInst &SI) {
     return &SI;
   }
 
-  if (VectorType* VecTy = dyn_cast<VectorType>(SI.getType())) {
+  if (VectorType *VecTy = dyn_cast<VectorType>(SI.getType())) {
     unsigned VWidth = VecTy->getNumElements();
     APInt UndefElts(VWidth, 0);
     APInt AllOnesEltMask(APInt::getAllOnesValue(VWidth));
@@ -912,6 +912,24 @@ Instruction *InstCombiner::visitSelectInst(SelectInst &SI) {
         return ReplaceInstUsesWith(SI, V);
       return &SI;
     }
+
+    if (ConstantVector *CV = dyn_cast<ConstantVector>(CondVal)) {
+      // Form a shufflevector instruction.
+      SmallVector<Constant *, 8> Mask(VWidth);
+      Type *Int32Ty = Type::getInt32Ty(CV->getContext());
+      for (unsigned i = 0; i != VWidth; ++i) {
+        Constant *Elem = cast<Constant>(CV->getOperand(i));
+        if (ConstantInt *E = dyn_cast<ConstantInt>(Elem))
+          Mask[i] = ConstantInt::get(Int32Ty, i + (E->isZero() ? VWidth : 0));
+        else if (isa<UndefValue>(Elem))
+          Mask[i] = UndefValue::get(Int32Ty);
+        else
+          return 0;
+      }
+      Constant *MaskVal = ConstantVector::get(Mask);
+      Value *V = Builder->CreateShuffleVector(TrueVal, FalseVal, MaskVal);
+      return ReplaceInstUsesWith(SI, V);
+    }
   }
 
   return 0;
diff --git a/llvm/test/Transforms/InstCombine/vec_demanded_elts.ll b/llvm/test/Transforms/InstCombine/vec_demanded_elts.ll
index 0019a57..2d90750 100644
--- a/llvm/test/Transforms/InstCombine/vec_demanded_elts.ll
+++ b/llvm/test/Transforms/InstCombine/vec_demanded_elts.ll
@@ -196,7 +196,7 @@ define <4 x float> @test_select(float %f, float %g) {
 ; CHECK-NOT: insertelement
 ; CHECK: %a3 = insertelement <4 x float> %a0, float 3.000000e+00, i32 3
 ; CHECK-NOT: insertelement
-; CHECK: %ret = select <4 x i1> <i1 true, i1 false, i1 false, i1 true>, <4 x float> %a3, <4 x float> <float undef, float 4.000000e+00, float 5.000000e+00, float undef>
+; CHECK: shufflevector <4 x float> %a3, <4 x float> <float undef, float 4.000000e+00, float 5.000000e+00, float undef>, <4 x i32> <i32 0, i32 5, i32 6, i32 3>
   %a0 = insertelement <4 x float> undef, float %f, i32 0
   %a1 = insertelement <4 x float> %a0, float 1.000000e+00, i32 1
   %a2 = insertelement <4 x float> %a1, float 2.000000e+00, i32 2
diff --git a/llvm/test/Transforms/InstCombine/vec_shuffle.ll b/llvm/test/Transforms/InstCombine/vec_shuffle.ll
index 8f78c2e..a7f9fcf 100644
--- a/llvm/test/Transforms/InstCombine/vec_shuffle.ll
+++ b/llvm/test/Transforms/InstCombine/vec_shuffle.ll
@@ -153,3 +153,38 @@ define <8 x i8> @test12a(<8 x i8> %tmp6, <8 x i8> %tmp2) nounwind {
   ret <8 x i8> %tmp3
 }
 
+; We should form a shuffle out of a select with constant condition.
+define <4 x i16> @test13a(<4 x i16> %lhs, <4 x i16> %rhs) {
+; CHECK: @test13a
+; CHECK-NEXT: shufflevector <4 x i16> %lhs, <4 x i16> %rhs, <4 x i32> <i32 0, i32 5, i32 2, i32 7>
+; CHECK-NEXT: ret
+  %A = select <4 x i1> <i1 true, i1 false, i1 true, i1 false>,
+           <4 x i16> %lhs, <4 x i16> %rhs
+  ret <4 x i16> %A
+}
+
+define <4 x i16> @test13b(<4 x i16> %lhs, <4 x i16> %rhs) {
+; CHECK: @test13b
+; CHECK-NEXT: ret <4 x i16> %lhs
+  %A = select <4 x i1> <i1 true, i1 undef, i1 true, i1 true>,
+           <4 x i16> %lhs, <4 x i16> %rhs
+  ret <4 x i16> %A
+}
+
+define <4 x i16> @test13c(<4 x i16> %lhs, <4 x i16> %rhs) {
+; CHECK: @test13c
+; CHECK-NEXT: shufflevector <4 x i16> %lhs, <4 x i16> %rhs, <4 x i32> <i32 0, i32 undef, i32 2, i32 7>
+; CHECK-NEXT: ret
+  %A = select <4 x i1> <i1 true, i1 undef, i1 true, i1 false>,
+           <4 x i16> %lhs, <4 x i16> %rhs
+  ret <4 x i16> %A
+}
+
+define <4 x i16> @test13d(<4 x i16> %lhs, <4 x i16> %rhs) {
+; CHECK: @test13d
+; CHECK: select
+; CHECK-NEXT: ret
+  %A = select <4 x i1> <i1 true, i1 icmp ugt (<4 x i16>(<4 x i16>, <4 x i16>)* @test13a, <4 x i16>(<4 x i16>, <4 x i16>)* @test13b), i1 true, i1 false>,
+           <4 x i16> %lhs, <4 x i16> %rhs
+  ret <4 x i16> %A
+}
-- 
2.7.4