[X86] Teach the backend how to lower vector shift left into multiply rather than scalarizing it.

Instead of expanding a packed shift into a sequence of scalar shifts,
the backend now tries (when possible) to convert the vector shift into a
vector multiply.

Before this change, a shift of a MVT::v8i16 vector by a
build_vector of constants was always scalarized into a long sequence of "vector
extracts + scalar shifts + vector insert".
With this change, if there is SSE2 support, we emit a single vector multiply.

This change also affects SSE4.1, AVX, AVX2 shifts:
 - A shift of a MVT::v4i32 vector by a build_vector of non uniform constants
is now lowered when possible into a single SSE4.1 vector multiply.
 - Packed v16i16 shift left by constant build_vector are now expanded when
possible into a single AVX2 vpmullw.
This change also improves the lowering of AVX512f vector shifts.

Added test CodeGen/X86/vec_shift6.ll with some code examples that are affected
by this change.

llvm-svn: 201271

lib/Target/X86/X86ISelLowering.cpp

@@ -13156,6 +13156,39 @@ static SDValue LowerShift(SDValue Op, const X86Subtarget* Subtarget,
       return Op;
   }
 
+  // If possible, lower this packed shift into a vector multiply instead of
+  // expanding it into a sequence of scalar shifts.
+  // Do this only if the vector shift count is a constant build_vector.
+  if (Op.getOpcode() == ISD::SHL && 
+      (VT == MVT::v8i16 || VT == MVT::v4i32 ||
+       (Subtarget->hasInt256() && VT == MVT::v16i16)) &&
+      ISD::isBuildVectorOfConstantSDNodes(Amt.getNode())) {
+    SmallVector<SDValue, 8> Elts;
+    EVT SVT = VT.getScalarType();
+    unsigned SVTBits = SVT.getSizeInBits();
+    const APInt &One = APInt(SVTBits, 1);
+    unsigned NumElems = VT.getVectorNumElements();
+
+    for (unsigned i=0; i !=NumElems; ++i) {
+      SDValue Op = Amt->getOperand(i);
+      if (Op->getOpcode() == ISD::UNDEF) {
+        Elts.push_back(Op);
+        continue;
+      }
+
+      ConstantSDNode *ND = cast<ConstantSDNode>(Op);
+      const APInt &C = APInt(SVTBits, ND->getAPIntValue().getZExtValue());
+      uint64_t ShAmt = C.getZExtValue();
+      if (ShAmt >= SVTBits) {
+        Elts.push_back(DAG.getUNDEF(SVT));
+        continue;
+      }
+      Elts.push_back(DAG.getConstant(One.shl(ShAmt), SVT));
+    }
+    SDValue BV = DAG.getNode(ISD::BUILD_VECTOR, dl, VT, &Elts[0], NumElems);
+    return DAG.getNode(ISD::MUL, dl, VT, R, BV);
+  }
+
   // Lower SHL with variable shift amount.
   if (VT == MVT::v4i32 && Op->getOpcode() == ISD::SHL) {
     Op = DAG.getNode(ISD::SHL, dl, VT, Amt, DAG.getConstant(23, VT));

test/CodeGen/X86/avx-shift.ll

@@ -115,8 +115,8 @@ define <8 x i32> @vshift08(<8 x i32> %a) nounwind {
 ; PR15141
 ; CHECK: _vshift13:
 ; CHECK-NOT: vpsll
-; CHECK: vcvttps2dq
-; CHECK-NEXT: vpmulld
+; CHECK-NOT: vcvttps2dq
+; CHECK: vpmulld
 define <4 x i32> @vshift13(<4 x i32> %in) {
   %T = shl <4 x i32> %in, <i32 0, i32 1, i32 2, i32 4>
   ret <4 x i32> %T

test/CodeGen/X86/vec_shift6.ll

@@ -0,0 +1,134 @@
+; RUN: llc < %s -mtriple=x86_64-unknown-linux-gnu -mcpu=corei7 | FileCheck %s -check-prefix=CHECK -check-prefix=SSE
+; RUN: llc < %s -mtriple=x86_64-unknown-linux-gnu -mcpu=core-avx2 | FileCheck %s -check-prefix=CHECK -check-prefix=AVX2 -check-prefix=AVX2ONLY
+; RUN: llc < %s -mtriple=x86_64-unknown-linux-gnu -mcpu=knl | FileCheck %s -check-prefix=CHECK -check-prefix=AVX2 -check-prefix=AVX512
+
+
+; Verify that we don't scalarize a packed vector shift left of 16-bit
+; signed integers if the amount is a constant build_vector.
+; Check that we produce a SSE2 packed integer multiply (pmullw) instead.
+
+define <8 x i16> @test1(<8 x i16> %a) {
+  %shl = shl <8 x i16> %a, <i16 1, i16 1, i16 2, i16 3, i16 7, i16 0, i16 9, i16 11>
+  ret <8 x i16> %shl
+}
+; CHECK-LABEL: test1
+; CHECK: pmullw
+; CHECK-NEXT: ret
+
+
+define <8 x i16> @test2(<8 x i16> %a) {
+  %shl = shl <8 x i16> %a, <i16 0, i16 undef, i16 0, i16 0, i16 1, i16 undef, i16 -1, i16 1>
+  ret <8 x i16> %shl
+}
+; CHECK-LABEL: test2
+; CHECK: pmullw
+; CHECK-NEXT: ret
+
+
+; Verify that a vector shift left of 32-bit signed integers is simply expanded
+; into a SSE4.1 pmulld (instead of cvttps2dq + pmulld) if the vector of shift
+; counts is a constant build_vector.
+
+define <4 x i32> @test3(<4 x i32> %a) {
+  %shl = shl <4 x i32> %a, <i32 1, i32 -1, i32 2, i32 -3>
+  ret <4 x i32> %shl
+}
+; CHECK-LABEL: test3
+; CHECK-NOT: cvttps2dq
+; SSE: pmulld
+; AVX2: vpsllvd
+; CHECK-NEXT: ret
+
+
+define <4 x i32> @test4(<4 x i32> %a) {
+  %shl = shl <4 x i32> %a, <i32 0, i32 0, i32 1, i32 1>
+  ret <4 x i32> %shl
+}
+; CHECK-LABEL: test4
+; CHECK-NOT: cvttps2dq
+; SSE: pmulld
+; AVX2: vpsllvd
+; CHECK-NEXT: ret
+
+
+; If we have AVX/SSE2 but not AVX2, verify that the following shift is split
+; into two pmullw instructions. With AVX2, the test case below would produce
+; a single vpmullw.
+
+define <16 x i16> @test5(<16 x i16> %a) {
+  %shl = shl <16 x i16> %a, <i16 1, i16 1, i16 2, i16 3, i16 7, i16 0, i16 9, i16 11, i16 1, i16 1, i16 2, i16 3, i16 7, i16 0, i16 9, i16 11>
+  ret <16 x i16> %shl
+}
+; CHECK-LABEL: test5
+; SSE: pmullw
+; SSE-NEXT: pmullw
+; AVX2: vpmullw
+; AVX2-NOT: vpmullw
+; CHECK: ret
+
+
+; If we have AVX/SSE4.1 but not AVX2, verify that the following shift is split
+; into two pmulld instructions. With AVX2, the test case below would produce
+; a single vpsllvd instead.
+
+define <8 x i32> @test6(<8 x i32> %a) {
+  %shl = shl <8 x i32> %a, <i32 1, i32 1, i32 2, i32 3, i32 1, i32 1, i32 2, i32 3>
+  ret <8 x i32> %shl
+}
+; CHECK-LABEL: test6
+; SSE: pmulld
+; SSE-NEXT: pmulld
+; AVX2: vpsllvd
+; CHECK: ret
+
+
+; With AVX2 and AVX512, the test case below should produce a sequence of
+; two vpmullw instructions. On SSE2 instead, we split the shift in four
+; parts and then we convert each part into a pmullw.
+
+define <32 x i16> @test7(<32 x i16> %a) {
+  %shl = shl <32 x i16> %a, <i16 1, i16 1, i16 2, i16 3, i16 7, i16 0, i16 9, i16 11, i16 1, i16 1, i16 2, i16 3, i16 7, i16 0, i16 9, i16 11, i16 1, i16 1, i16 2, i16 3, i16 7, i16 0, i16 9, i16 11, i16 1, i16 1, i16 2, i16 3, i16 7, i16 0, i16 9, i16 11>
+  ret <32 x i16> %shl
+}
+; CHECK-LABEL: test7
+; SSE: pmullw
+; SSE-NEXT: pmullw
+; SSE-NEXT: pmullw
+; SSE-NEXT: pmullw
+; AVX2: vpmullw
+; AVX2-NEXT: vpmullw
+; CHECK: ret
+
+
+; Similar to test7; the difference is that with AVX512 support
+; we only produce a single vpsllvd/vpsllvq instead of a pair of vpsllvd/vpsllvq.
+
+define <16 x i32> @test8(<16 x i32> %a) {
+  %shl = shl <16 x i32> %a, <i32 1, i32 1, i32 2, i32 3, i32 1, i32 1, i32 2, i32 3, i32 1, i32 1, i32 2, i32 3, i32 1, i32 1, i32 2, i32 3>
+  ret <16 x i32> %shl
+}
+; CHECK-LABEL: test8
+; SSE: pmulld
+; SSE-NEXT: pmulld
+; SSE-NEXT: pmulld
+; SSE-NEXT: pmulld
+; AVX2ONLY: vpsllvd
+; AVX2ONLY-NEXT: vpsllvd
+; AVX512: vpsllvd
+; AVX512-NOT: vpsllvd
+; CHECK: ret
+
+
+; The shift from 'test9' gets scalarized if we don't have AVX2/AVX512f support.
+
+define <8 x i64> @test9(<8 x i64> %a) {
+  %shl = shl <8 x i64> %a, <i64 1, i64 1, i64 2, i64 3, i64 1, i64 1, i64 2, i64 3>
+  ret <8 x i64> %shl
+}
+; CHECK-LABEL: test9
+; AVX2ONLY: vpsllvq
+; AVX2ONLY-NEXT: vpsllvq
+; AVX512: vpsllvq
+; AVX512-NOT: vpsllvq
+; CHECK: ret
+