[SystemZ] Eliminate the need to create a zero vector by reusing the VPERM mask.

Try to avoid creating VGBMs by reusing the permutation mask if it contains a
zero. If the first byte was into (any byte of) a zero vector, then the first
byte of the mask can become zero and reused by putting the mask also as the
first operand. If there instead was a first-byte use of the other source
operand, then that zero index can be reused if the mask is placed as the
second operand.

Review: Ulrich Weigand

Differential Revision: https://reviews.llvm.org/D79925

diff --git a/llvm/lib/Target/SystemZ/SystemZISelLowering.cpp b/llvm/lib/Target/SystemZ/SystemZISelLowering.cpp
index 7a8b524..59896d6 100644 (file)
--- a/llvm/lib/Target/SystemZ/SystemZISelLowering.cpp
+++ b/llvm/lib/Target/SystemZ/SystemZISelLowering.cpp
@@ -4450,6 +4450,13 @@ static SDValue getPermuteNode(SelectionDAG &DAG, const SDLoc &DL,
   return Op;
 }
 
+static bool isZeroVector(SDValue N) {
+  if (N->getOpcode() == ISD::SPLAT_VECTOR)
+    if (auto *Op = dyn_cast<ConstantSDNode>(N->getOperand(0)))
+      return Op->getZExtValue() == 0;
+  return ISD::isBuildVectorAllZeros(N.getNode());
+}
+
 // Bytes is a VPERM-like permute vector, except that -1 is used for
 // undefined bytes.  Implement it on operands Ops[0] and Ops[1] using
 // VSLDB or VPERM.
@@ -4466,7 +4473,54 @@ static SDValue getGeneralPermuteNode(SelectionDAG &DAG, const SDLoc &DL,
                        Ops[OpNo1],
                        DAG.getTargetConstant(StartIndex, DL, MVT::i32));
 
-  // Fall back on VPERM.  Construct an SDNode for the permute vector.
+  // Fall back on VPERM.  Construct an SDNode for the permute vector.  Try to
+  // eliminate a zero vector by reusing any zero index in the permute vector.
+  unsigned ZeroVecIdx =
+    isZeroVector(Ops[0]) ? 0 : (isZeroVector(Ops[1]) ? 1 : UINT_MAX);
+  if (ZeroVecIdx != UINT_MAX) {
+    bool MaskFirst = true;
+    int ZeroIdx = -1;
+    for (unsigned I = 0; I < SystemZ::VectorBytes; ++I) {
+      unsigned OpNo = unsigned(Bytes[I]) / SystemZ::VectorBytes;
+      unsigned Byte = unsigned(Bytes[I]) % SystemZ::VectorBytes;
+      if (OpNo == ZeroVecIdx && I == 0) {
+        // If the first byte is zero, use mask as first operand.
+        ZeroIdx = 0;
+        break;
+      }
+      if (OpNo != ZeroVecIdx && Byte == 0) {
+        // If mask contains a zero, use it by placing that vector first.
+        ZeroIdx = I + SystemZ::VectorBytes;
+        MaskFirst = false;
+        break;
+      }
+    }
+    if (ZeroIdx != -1) {
+      SDValue IndexNodes[SystemZ::VectorBytes];
+      for (unsigned I = 0; I < SystemZ::VectorBytes; ++I) {
+        if (Bytes[I] >= 0) {
+          unsigned OpNo = unsigned(Bytes[I]) / SystemZ::VectorBytes;
+          unsigned Byte = unsigned(Bytes[I]) % SystemZ::VectorBytes;
+          if (OpNo == ZeroVecIdx)
+            IndexNodes[I] = DAG.getConstant(ZeroIdx, DL, MVT::i32);
+          else {
+            unsigned BIdx = MaskFirst ? Byte + SystemZ::VectorBytes : Byte;
+            IndexNodes[I] = DAG.getConstant(BIdx, DL, MVT::i32);
+          }
+        } else
+          IndexNodes[I] = DAG.getUNDEF(MVT::i32);
+      }
+      SDValue Mask = DAG.getBuildVector(MVT::v16i8, DL, IndexNodes);
+      SDValue Src = ZeroVecIdx == 0 ? Ops[1] : Ops[0];
+      if (MaskFirst)
+        return DAG.getNode(SystemZISD::PERMUTE, DL, MVT::v16i8, Mask, Src,
+                           Mask);
+      else
+        return DAG.getNode(SystemZISD::PERMUTE, DL, MVT::v16i8, Src, Mask,
+                           Mask);
+    }
+  }
+
   SDValue IndexNodes[SystemZ::VectorBytes];
   for (unsigned I = 0; I < SystemZ::VectorBytes; ++I)
     if (Bytes[I] >= 0)

diff --git a/llvm/test/CodeGen/SystemZ/vec-perm-14.ll b/llvm/test/CodeGen/SystemZ/vec-perm-14.ll
index 0cf3c6e..3678969 100644 (file)
--- a/llvm/test/CodeGen/SystemZ/vec-perm-14.ll
+++ b/llvm/test/CodeGen/SystemZ/vec-perm-14.ll
@@ -3,7 +3,7 @@
 ; Test that only one vperm of the vector compare is needed for both extracts.
 
 define void @fun() {
-; CHECK-LABEL: fun
+; CHECK-LABEL: fun:
 ; CHECK: vperm
 ; CHECK-NOT: vperm
 bb:
@@ -25,3 +25,74 @@ bb3:
 bb4:
   unreachable
 }
+
+; Test that a zero index in the permute vector is used instead of VGBM, with
+; a zero index into the other source operand.
+define <4 x i8> @fun1(<2 x i8> %arg) {
+; CHECK-LABEL:.LCPI1_0:
+; CHECK-NEXT:        .byte   1                       # 0x1
+; CHECK-NEXT:        .byte   18                      # 0x12
+; CHECK-NEXT:        .byte   0                       # 0x0
+; CHECK-NEXT:        .byte   18                      # 0x12
+; CHECK-NEXT:        .space  1
+; CHECK-NEXT:        .space  1
+; CHECK-NEXT:        .space  1
+; CHECK-NEXT:        .space  1
+; CHECK-NEXT:        .space  1
+; CHECK-NEXT:        .space  1
+; CHECK-NEXT:        .space  1
+; CHECK-NEXT:        .space  1
+; CHECK-NEXT:        .space  1
+; CHECK-NEXT:        .space  1
+; CHECK-NEXT:        .space  1
+; CHECK-NEXT:        .space  1
+; CHECK-NEXT:        .text
+; CHECK-NEXT:        .globl  fun1
+; CHECK-NEXT:        .p2align        4
+; CHECK-NEXT:        .type   fun1,@function
+; CHECK-NEXT: fun1:                                  # @fun1
+; CHECK-NEXT:        .cfi_startproc
+; CHECK-NEXT: # %bb.0:
+; CHECK-NEXT:        larl    %r1, .LCPI1_0
+; CHECK-NEXT:        vl      %v0, 0(%r1), 3
+; CHECK-NEXT:        vperm   %v24, %v24, %v0, %v0
+; CHECK-NEXT:        br      %r14
+   %res = shufflevector <2 x i8> %arg, <2 x i8> zeroinitializer,
+                        <4 x i32> <i32 1, i32 2, i32 0, i32 3>
+   ret <4 x i8> %res
+}
+
+; Same, but with the first byte indexing into an element of the zero vector.
+define <4 x i8> @fun2(<2 x i8> %arg) {
+; CHECK-LABEL:.LCPI2_0:
+; CHECK-NEXT:        .byte   0                       # 0x0
+; CHECK-NEXT:        .byte   17                      # 0x11
+; CHECK-NEXT:        .byte   17                      # 0x11
+; CHECK-NEXT:        .byte   0                       # 0x0
+; CHECK-NEXT:        .space  1
+; CHECK-NEXT:        .space  1
+; CHECK-NEXT:        .space  1
+; CHECK-NEXT:        .space  1
+; CHECK-NEXT:        .space  1
+; CHECK-NEXT:        .space  1
+; CHECK-NEXT:        .space  1
+; CHECK-NEXT:        .space  1
+; CHECK-NEXT:        .space  1
+; CHECK-NEXT:        .space  1
+; CHECK-NEXT:        .space  1
+; CHECK-NEXT:        .space  1
+; CHECK-NEXT:        .text
+; CHECK-NEXT:        .globl  fun2
+; CHECK-NEXT:        .p2align        4
+; CHECK-NEXT:        .type   fun2,@function
+; CHECK-NEXT:fun2:                                   # @fun2
+; CHECK-NEXT:        .cfi_startproc
+; CHECK-NEXT:# %bb.0:
+; CHECK-NEXT:        larl    %r1, .LCPI2_0
+; CHECK-NEXT:        vl      %v0, 0(%r1), 3
+; CHECK-NEXT:        vperm   %v24, %v0, %v24, %v0
+; CHECK-NEXT:        br      %r14
+   %res = shufflevector <2 x i8> %arg, <2 x i8> zeroinitializer,
+                        <4 x i32> <i32 3, i32 1, i32 1, i32 2>
+   ret <4 x i8> %res
+}