[X86] Improved lowering of packed v8i16 vector shifts by non-constant count.

Before this patch, the backend sub-optimally expanded the non-constant shift
count of a v8i16 shift into a sequence of two 'movd' plus 'movzwl'.

With this patch the backend checks if the target features sse4.1. If so, then
it lets the shuffle legalizer deal with the expansion of the shift amount.

Example:
;;
define <8 x i16> @test(<8 x i16> %A, <8 x i16> %B) {
  %shamt = shufflevector <8 x i16> %B, <8 x i16> undef, <8 x i32> zeroinitializer
  %shl = shl <8 x i16> %A, %shamt
  ret <8 x i16> %shl
}
;;

Before (with -mattr=+avx):
  vmovd  %xmm1, %eax
  movzwl  %ax, %eax
  vmovd  %eax, %xmm1
  vpsllw  %xmm1, %xmm0, %xmm0
  retq

Now:
  vpxor  %xmm2, %xmm2, %xmm2
  vpblendw  $1, %xmm1, %xmm2, %xmm1
  vpsllw  %xmm1, %xmm0, %xmm0
  retq

llvm-svn: 223660

2 files changed, 26 insertions, 19 deletions

diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index 4c65a6ccd7e..6b4251df61a 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -16720,18 +16720,28 @@ static SDValue getTargetVShiftNode(unsigned Opc, SDLoc dl, MVT VT,
     case X86ISD::VSRAI: Opc = X86ISD::VSRA; break;
   }
 
-  // Need to build a vector containing shift amount.
-  // SSE/AVX packed shifts only use the lower 64-bit of the shift count.
-  SmallVector<SDValue, 4> ShOps;
-  ShOps.push_back(ShAmt);
-  if (SVT == MVT::i32) {
-    ShOps.push_back(DAG.getConstant(0, SVT));
+  const X86Subtarget &Subtarget =
+      DAG.getTarget().getSubtarget<X86Subtarget>();
+  if (Subtarget.hasSSE41() && ShAmt.getOpcode() == ISD::ZERO_EXTEND &&
+      ShAmt.getOperand(0).getSimpleValueType() == MVT::i16) {
+    // Let the shuffle legalizer expand this shift amount node.
+    SDValue Op0 = ShAmt.getOperand(0);
+    Op0 = DAG.getNode(ISD::SCALAR_TO_VECTOR, SDLoc(Op0), MVT::v8i16, Op0);
+    ShAmt = getShuffleVectorZeroOrUndef(Op0, 0, true, &Subtarget, DAG);
+  } else {
+    // Need to build a vector containing shift amount.
+    // SSE/AVX packed shifts only use the lower 64-bit of the shift count.
+    SmallVector<SDValue, 4> ShOps;
+    ShOps.push_back(ShAmt);
+    if (SVT == MVT::i32) {
+      ShOps.push_back(DAG.getConstant(0, SVT));
+      ShOps.push_back(DAG.getUNDEF(SVT));
+    }
     ShOps.push_back(DAG.getUNDEF(SVT));
-  }
-  ShOps.push_back(DAG.getUNDEF(SVT));
 
-  MVT BVT = SVT == MVT::i32 ? MVT::v4i32 : MVT::v2i64;
-  ShAmt = DAG.getNode(ISD::BUILD_VECTOR, dl, BVT, ShOps);
+    MVT BVT = SVT == MVT::i32 ? MVT::v4i32 : MVT::v2i64;
+    ShAmt = DAG.getNode(ISD::BUILD_VECTOR, dl, BVT, ShOps);
+  }
 
   // The return type has to be a 128-bit type with the same element
   // type as the input type.
diff --git a/llvm/test/CodeGen/X86/lower-vec-shift-2.ll b/llvm/test/CodeGen/X86/lower-vec-shift-2.ll
index 90505b6dd8f..770775d3242 100644
--- a/llvm/test/CodeGen/X86/lower-vec-shift-2.ll
+++ b/llvm/test/CodeGen/X86/lower-vec-shift-2.ll
@@ -11,9 +11,8 @@ define <8 x i16> @test1(<8 x i16> %A, <8 x i16> %B) {
 ; SSE2-NEXT:    retq
 ; AVX-LABEL: test1:
 ; AVX:       # BB#0
-; AVX-NEXT:    vmovd  %xmm1, %eax
-; AVX-NEXT:    movzwl  %ax, %eax
-; AVX-NEXT:    vmovd  %eax, %xmm1
+; AVX-NEXT:    vpxor  %xmm2, %xmm2, %xmm2
+; AVX-NEXT:    vpblendw  {{.*#+}} xmm1 = xmm1[0],xmm2[1,2,3,4,5,6,7]
 ; AVX-NEXT:    vpsllw  %xmm1, %xmm0, %xmm0
 ; AVX-NEXT:    retq
 entry:
@@ -66,9 +65,8 @@ define <8 x i16> @test4(<8 x i16> %A, <8 x i16> %B) {
 ; SSE2-NEXT:    retq
 ; AVX-LABEL: test4:
 ; AVX:       # BB#0
-; AVX-NEXT:    vmovd  %xmm1, %eax
-; AVX-NEXT:    movzwl  %ax, %eax
-; AVX-NEXT:    vmovd  %eax, %xmm1
+; AVX-NEXT:    vpxor  %xmm2, %xmm2, %xmm2
+; AVX-NEXT:    vpblendw  {{.*#+}} xmm1 = xmm1[0],xmm2[1,2,3,4,5,6,7]
 ; AVX-NEXT:    vpsrlw  %xmm1, %xmm0, %xmm0
 ; AVX-NEXT:    retq
 entry:
@@ -121,9 +119,8 @@ define <8 x i16> @test7(<8 x i16> %A, <8 x i16> %B) {
 ; SSE2-NEXT:    retq
 ; AVX-LABEL: test7:
 ; AVX:       # BB#0
-; AVX-NEXT:    vmovd  %xmm1, %eax
-; AVX-NEXT:    movzwl  %ax, %eax
-; AVX-NEXT:    vmovd  %eax, %xmm1
+; AVX-NEXT:    vpxor  %xmm2, %xmm2, %xmm2
+; AVX-NEXT:    vpblendw  {{.*#+}} xmm1 = xmm1[0],xmm2[1,2,3,4,5,6,7]
 ; AVX-NEXT:    vpsraw  %xmm1, %xmm0, %xmm0
 ; AVX-NEXT:    retq
 entry: