diff --git a/llvm/lib/Target/AArch64/GISel/AArch64LegalizerInfo.h b/llvm/lib/Target/AArch64/GISel/AArch64LegalizerInfo.h
--- a/llvm/lib/Target/AArch64/GISel/AArch64LegalizerInfo.h
+++ b/llvm/lib/Target/AArch64/GISel/AArch64LegalizerInfo.h
@@ -57,6 +57,7 @@
                                 LegalizerHelper &Helper) const;
   bool legalizeCTTZ(MachineInstr &MI, LegalizerHelper &Helper) const;
   bool legalizeMemOps(MachineInstr &MI, LegalizerHelper &Helper) const;
+  bool legalizeFCopySign(MachineInstr &MI, LegalizerHelper &Helper) const;
   const AArch64Subtarget *ST;
 };
 } // End llvm namespace.
diff --git a/llvm/lib/Target/AArch64/GISel/AArch64LegalizerInfo.cpp b/llvm/lib/Target/AArch64/GISel/AArch64LegalizerInfo.cpp
--- a/llvm/lib/Target/AArch64/GISel/AArch64LegalizerInfo.cpp
+++ b/llvm/lib/Target/AArch64/GISel/AArch64LegalizerInfo.cpp
@@ -814,6 +814,11 @@
   getActionDefinitionsBuilder({G_LROUND, G_LLROUND})
       .legalFor({{s64, s32}, {s64, s64}});
 
+  // TODO: Custom legalization for vector types.
+  // TODO: Custom legalization for mismatched types.
+  // TODO: s16 support.
+  getActionDefinitionsBuilder(G_FCOPYSIGN).customFor({{s32, s32}, {s64, s64}});
+
   getLegacyLegalizerInfo().computeTables();
   verify(*ST.getInstrInfo());
 }
@@ -856,6 +861,8 @@
   case TargetOpcode::G_MEMMOVE:
   case TargetOpcode::G_MEMSET:
     return legalizeMemOps(MI, Helper);
+  case TargetOpcode::G_FCOPYSIGN:
+    return legalizeFCopySign(MI, Helper);
   }
 
   llvm_unreachable("expected switch to return");
@@ -1438,3 +1445,63 @@
 
   return false;
 }
+
+bool AArch64LegalizerInfo::legalizeFCopySign(MachineInstr &MI,
+                                             LegalizerHelper &Helper) const {
+  MachineIRBuilder &MIRBuilder = Helper.MIRBuilder;
+  MachineRegisterInfo &MRI = *MIRBuilder.getMRI();
+  Register Dst = MI.getOperand(0).getReg();
+  LLT DstTy = MRI.getType(Dst);
+  assert(DstTy.isScalar() && "Only expected scalars right now!");
+  const unsigned DstSize = DstTy.getSizeInBits();
+  assert((DstSize == 32 || DstSize == 64) && "Unexpected dst type!");
+  assert(MRI.getType(MI.getOperand(2).getReg()) == DstTy &&
+         "Expected homogeneous types!");
+
+  // We want to materialize a mask with the high bit set.
+  uint64_t EltMask;
+  LLT VecTy;
+
+  // TODO: s16 support.
+  switch (DstSize) {
+  default:
+    llvm_unreachable("Unexpected type for G_FCOPYSIGN!");
+  case 64: {
+    // AdvSIMD immediate moves cannot materialize out mask in a single
+    // instruction for 64-bit elements. Instead, materialize zero and then
+    // negate it.
+    EltMask = 0;
+    VecTy = LLT::fixed_vector(2, DstTy);
+    break;
+  }
+  case 32:
+    EltMask = 0x80000000ULL;
+    VecTy = LLT::fixed_vector(4, DstTy);
+    break;
+  }
+
+  // Widen In1 and In2 to 128 bits. We want these to eventually become
+  // INSERT_SUBREGs.
+  auto Undef = MIRBuilder.buildUndef(VecTy);
+  auto Zero = MIRBuilder.buildConstant(DstTy, 0);
+  auto Ins1 = MIRBuilder.buildInsertVectorElement(
+      VecTy, Undef, MI.getOperand(1).getReg(), Zero);
+  auto Ins2 = MIRBuilder.buildInsertVectorElement(
+      VecTy, Undef, MI.getOperand(2).getReg(), Zero);
+
+  // Construct the mask.
+  auto Mask = MIRBuilder.buildConstant(VecTy, EltMask);
+  if (DstSize == 64)
+    Mask = MIRBuilder.buildFNeg(VecTy, Mask);
+
+  auto Sel = MIRBuilder.buildInstr(AArch64::G_BIT, {VecTy}, {Ins1, Ins2, Mask});
+
+  // Build an unmerge whose 0th elt is the original G_FCOPYSIGN destination. We
+  // want this to eventually become an EXTRACT_SUBREG.
+  SmallVector<Register, 2> DstRegs(1, Dst);
+  for (unsigned I = 1, E = VecTy.getNumElements(); I < E; ++I)
+    DstRegs.push_back(MRI.createGenericVirtualRegister(DstTy));
+  MIRBuilder.buildUnmerge(DstRegs, Sel);
+  MI.eraseFromParent();
+  return true;
+}
diff --git a/llvm/test/CodeGen/AArch64/GlobalISel/legalize-fcopysign.mir b/llvm/test/CodeGen/AArch64/GlobalISel/legalize-fcopysign.mir
new file mode 100644
--- /dev/null
+++ b/llvm/test/CodeGen/AArch64/GlobalISel/legalize-fcopysign.mir
@@ -0,0 +1,56 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -mtriple=aarch64 -run-pass=legalizer -verify-machineinstrs %s -o - | FileCheck %s
+
+...
+---
+name:            legalize_s32
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    liveins: $s0, $s1
+    ; CHECK-LABEL: name: legalize_s32
+    ; CHECK: liveins: $s0, $s1
+    ; CHECK: %val:_(s32) = COPY $s0
+    ; CHECK: %sign:_(s32) = COPY $s1
+    ; CHECK: [[DEF:%[0-9]+]]:_(<4 x s32>) = G_IMPLICIT_DEF
+    ; CHECK: [[C:%[0-9]+]]:_(s32) = G_CONSTANT i32 0
+    ; CHECK: [[IVEC:%[0-9]+]]:_(<4 x s32>) = G_INSERT_VECTOR_ELT [[DEF]], %val(s32), [[C]](s32)
+    ; CHECK: [[IVEC1:%[0-9]+]]:_(<4 x s32>) = G_INSERT_VECTOR_ELT [[DEF]], %sign(s32), [[C]](s32)
+    ; CHECK: [[C1:%[0-9]+]]:_(s32) = G_CONSTANT i32 -2147483648
+    ; CHECK: [[BUILD_VECTOR:%[0-9]+]]:_(<4 x s32>) = G_BUILD_VECTOR [[C1]](s32), [[C1]](s32), [[C1]](s32), [[C1]](s32)
+    ; CHECK: [[BIT:%[0-9]+]]:_(<4 x s32>) = G_BIT [[IVEC]], [[IVEC1]], [[BUILD_VECTOR]]
+    ; CHECK: %fcopysign:_(s32), %10:_(s32), %11:_(s32), %12:_(s32) = G_UNMERGE_VALUES [[BIT]](<4 x s32>)
+    ; CHECK: $s0 = COPY %fcopysign(s32)
+    ; CHECK: RET_ReallyLR implicit $s0
+    %val:_(s32) = COPY $s0
+    %sign:_(s32) = COPY $s1
+    %fcopysign:_(s32) = G_FCOPYSIGN %val, %sign(s32)
+    $s0 = COPY %fcopysign(s32)
+    RET_ReallyLR implicit $s0
+
+...
+---
+name:            legalize_s64
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    liveins: $d0, $d1
+    ; CHECK-LABEL: name: legalize_s64
+    ; CHECK: liveins: $d0, $d1
+    ; CHECK: %val:_(s64) = COPY $d0
+    ; CHECK: %sign:_(s64) = COPY $d1
+    ; CHECK: [[DEF:%[0-9]+]]:_(<2 x s64>) = G_IMPLICIT_DEF
+    ; CHECK: [[C:%[0-9]+]]:_(s64) = G_CONSTANT i64 0
+    ; CHECK: [[IVEC:%[0-9]+]]:_(<2 x s64>) = G_INSERT_VECTOR_ELT [[DEF]], %val(s64), [[C]](s64)
+    ; CHECK: [[IVEC1:%[0-9]+]]:_(<2 x s64>) = G_INSERT_VECTOR_ELT [[DEF]], %sign(s64), [[C]](s64)
+    ; CHECK: [[BUILD_VECTOR:%[0-9]+]]:_(<2 x s64>) = G_BUILD_VECTOR [[C]](s64), [[C]](s64)
+    ; CHECK: [[FNEG:%[0-9]+]]:_(<2 x s64>) = G_FNEG [[BUILD_VECTOR]]
+    ; CHECK: [[BIT:%[0-9]+]]:_(<2 x s64>) = G_BIT [[IVEC]], [[IVEC1]], [[FNEG]]
+    ; CHECK: %fcopysign:_(s64), %10:_(s64) = G_UNMERGE_VALUES [[BIT]](<2 x s64>)
+    ; CHECK: $d0 = COPY %fcopysign(s64)
+    ; CHECK: RET_ReallyLR implicit $d0
+    %val:_(s64) = COPY $d0
+    %sign:_(s64) = COPY $d1
+    %fcopysign:_(s64) = G_FCOPYSIGN %val, %sign(s64)
+    $d0 = COPY %fcopysign(s64)
+    RET_ReallyLR implicit $d0
diff --git a/llvm/test/CodeGen/AArch64/GlobalISel/legalizer-info-validation.mir b/llvm/test/CodeGen/AArch64/GlobalISel/legalizer-info-validation.mir
--- a/llvm/test/CodeGen/AArch64/GlobalISel/legalizer-info-validation.mir
+++ b/llvm/test/CodeGen/AArch64/GlobalISel/legalizer-info-validation.mir
@@ -487,8 +487,8 @@
 # DEBUG-NEXT: .. type index coverage check SKIPPED: user-defined predicate detected
 # DEBUG-NEXT: .. imm index coverage check SKIPPED: user-defined predicate detected
 # DEBUG-NEXT: G_FCOPYSIGN (opcode {{[0-9]+}}): 2 type indices
-# DEBUG-NEXT: .. type index coverage check SKIPPED: no rules defined
-# DEBUG-NEXT: .. imm index coverage check SKIPPED: no rules defined
+# DEBUG-NEXT: .. the first uncovered type index: 2, OK
+# DEBUG-NEXT: .. the first uncovered imm index: 0, OK
 # DEBUG-NEXT: G_IS_FPCLASS (opcode {{[0-9]+}}): 2 type indices, 0 imm indices
 # DEBUG-NEXT: .. type index coverage check SKIPPED: no rules defined
 # DEBUG-NEXT: .. imm index coverage check SKIPPED: no rules defined