Index: lib/Target/AMDGPU/AMDGPULegalizerInfo.h
===================================================================
--- lib/Target/AMDGPU/AMDGPULegalizerInfo.h
+++ lib/Target/AMDGPU/AMDGPULegalizerInfo.h
@@ -38,6 +38,9 @@
 
   bool legalizeAddrSpaceCast(MachineInstr &MI, MachineRegisterInfo &MRI,
                              MachineIRBuilder &MIRBuilder) const;
+  bool legalizeITOFP(MachineInstr &MI, MachineRegisterInfo &MRI,
+                     MachineIRBuilder &MIRBuilder, bool Signed) const;
+
 };
 } // End llvm namespace.
 #endif
Index: lib/Target/AMDGPU/AMDGPULegalizerInfo.cpp
===================================================================
--- lib/Target/AMDGPU/AMDGPULegalizerInfo.cpp
+++ lib/Target/AMDGPU/AMDGPULegalizerInfo.cpp
@@ -280,6 +280,7 @@
   getActionDefinitionsBuilder({G_SITOFP, G_UITOFP})
     .legalFor({{S32, S32}, {S64, S32}})
     .lowerFor({{S32, S64}})
+    .customFor({{S64, S64}})
     .scalarize(0);
 
   getActionDefinitionsBuilder({G_FPTOSI, G_FPTOUI})
@@ -672,6 +673,10 @@
   switch (MI.getOpcode()) {
   case TargetOpcode::G_ADDRSPACE_CAST:
     return legalizeAddrSpaceCast(MI, MRI, MIRBuilder);
+  case TargetOpcode::G_SITOFP:
+    return legalizeITOFP(MI, MRI, MIRBuilder, true);
+  case TargetOpcode::G_UITOFP:
+    return legalizeITOFP(MI, MRI, MIRBuilder, false);
   default:
     return false;
   }
@@ -828,3 +833,35 @@
   MI.eraseFromParent();
   return true;
 }
+
+bool AMDGPULegalizerInfo::legalizeITOFP(
+  MachineInstr &MI, MachineRegisterInfo &MRI,
+  MachineIRBuilder &B, bool Signed) const {
+  B.setInstr(MI);
+
+  unsigned Dst = MI.getOperand(0).getReg();
+  unsigned Src = MI.getOperand(1).getReg();
+
+  const LLT S64 = LLT::scalar(64);
+  const LLT S32 = LLT::scalar(32);
+
+  assert(MRI.getType(Src) == S64 && MRI.getType(Dst) == S64);
+
+  auto Unmerge = B.buildUnmerge({S32, S32}, Src);
+
+  auto CvtHi = Signed ?
+    B.buildSITOFP(S64, Unmerge.getReg(1)) :
+    B.buildUITOFP(S64, Unmerge.getReg(1));
+
+  auto CvtLo = B.buildUITOFP(S64, Unmerge.getReg(0));
+
+  auto ThirtyTwo = B.buildConstant(S32, 32);
+  auto LdExp = B.buildIntrinsic(Intrinsic::amdgcn_ldexp, {S64}, false)
+    .addUse(CvtHi.getReg(0))
+    .addUse(ThirtyTwo.getReg(0));
+
+  // TODO: Should this propagate fast-math-flags?
+  B.buildFAdd(Dst, LdExp, CvtLo);
+  MI.eraseFromParent();
+  return true;
+}
Index: test/CodeGen/AMDGPU/GlobalISel/legalize-sitofp.mir
===================================================================
--- test/CodeGen/AMDGPU/GlobalISel/legalize-sitofp.mir
+++ test/CodeGen/AMDGPU/GlobalISel/legalize-sitofp.mir
@@ -122,3 +122,23 @@
     %1:_(s32) = G_SITOFP %0
     $vgpr0 = COPY %1
 ...
+
+---
+name: test_sitofp_s64_to_s64
+body: |
+  bb.0:
+    liveins: $vgpr0_vgpr1
+
+    ; CHECK-LABEL: name: test_sitofp_s64_to_s64
+    ; CHECK: [[COPY:%[0-9]+]]:_(s64) = COPY $vgpr0_vgpr1
+    ; CHECK: [[UV:%[0-9]+]]:_(s32), [[UV1:%[0-9]+]]:_(s32) = G_UNMERGE_VALUES [[COPY]](s64)
+    ; CHECK: [[SITOFP:%[0-9]+]]:_(s64) = G_SITOFP [[UV1]](s32)
+    ; CHECK: [[UITOFP:%[0-9]+]]:_(s64) = G_UITOFP [[UV]](s32)
+    ; CHECK: [[C:%[0-9]+]]:_(s32) = G_CONSTANT i32 32
+    ; CHECK: [[INT:%[0-9]+]]:_(s64) = G_INTRINSIC intrinsic(@llvm.amdgcn.ldexp), [[SITOFP]](s64), [[C]](s32)
+    ; CHECK: [[FADD:%[0-9]+]]:_(s64) = G_FADD [[INT]], [[UITOFP]]
+    ; CHECK: $vgpr0_vgpr1 = COPY [[FADD]](s64)
+    %0:_(s64) = COPY $vgpr0_vgpr1
+    %1:_(s64) = G_SITOFP %0
+    $vgpr0_vgpr1 = COPY %1
+...
Index: test/CodeGen/AMDGPU/GlobalISel/legalize-uitofp.mir
===================================================================
--- test/CodeGen/AMDGPU/GlobalISel/legalize-uitofp.mir
+++ test/CodeGen/AMDGPU/GlobalISel/legalize-uitofp.mir
@@ -90,3 +90,23 @@
     %1:_(s32) = G_UITOFP %0
     $vgpr0 = COPY %1
 ...
+
+---
+name: test_uitofp_s64_to_s64
+body: |
+  bb.0:
+    liveins: $vgpr0_vgpr1
+
+    ; CHECK-LABEL: name: test_uitofp_s64_to_s64
+    ; CHECK: [[COPY:%[0-9]+]]:_(s64) = COPY $vgpr0_vgpr1
+    ; CHECK: [[UV:%[0-9]+]]:_(s32), [[UV1:%[0-9]+]]:_(s32) = G_UNMERGE_VALUES [[COPY]](s64)
+    ; CHECK: [[UITOFP:%[0-9]+]]:_(s64) = G_UITOFP [[UV1]](s32)
+    ; CHECK: [[UITOFP1:%[0-9]+]]:_(s64) = G_UITOFP [[UV]](s32)
+    ; CHECK: [[C:%[0-9]+]]:_(s32) = G_CONSTANT i32 32
+    ; CHECK: [[INT:%[0-9]+]]:_(s64) = G_INTRINSIC intrinsic(@llvm.amdgcn.ldexp), [[UITOFP]](s64), [[C]](s32)
+    ; CHECK: [[FADD:%[0-9]+]]:_(s64) = G_FADD [[INT]], [[UITOFP1]]
+    ; CHECK: $vgpr0_vgpr1 = COPY [[FADD]](s64)
+    %0:_(s64) = COPY $vgpr0_vgpr1
+    %1:_(s64) = G_UITOFP %0
+    $vgpr0_vgpr1 = COPY %1
+...