Index: include/llvm/CodeGen/GlobalISel/LegalizerHelper.h
===================================================================
--- include/llvm/CodeGen/GlobalISel/LegalizerHelper.h
+++ include/llvm/CodeGen/GlobalISel/LegalizerHelper.h
@@ -121,6 +121,7 @@
   void extractParts(unsigned Reg, LLT Ty, int NumParts,
                     SmallVectorImpl<unsigned> &VRegs);
 
+  LegalizeResult narrowScalarMul(MachineInstr &MI, unsigned TypeIdx, LLT Ty);
   LegalizeResult lowerBitCount(MachineInstr &MI, unsigned TypeIdx, LLT Ty);
 
   MachineRegisterInfo &MRI;
Index: include/llvm/CodeGen/GlobalISel/MachineIRBuilder.h
===================================================================
--- include/llvm/CodeGen/GlobalISel/MachineIRBuilder.h
+++ include/llvm/CodeGen/GlobalISel/MachineIRBuilder.h
@@ -1138,6 +1138,24 @@
     return buildInstr(TargetOpcode::G_MUL, {Dst}, {Src0, Src1}, Flags);
   }
 
+  MachineInstrBuilder buildUMulH(const DstOp &Dst, const SrcOp &Src0,
+                                 const SrcOp &Src1,
+                                 Optional<unsigned> Flags = None) {
+    return buildInstr(TargetOpcode::G_UMULH, {Dst}, {Src0, Src1}, Flags);
+  }
+
+  MachineInstrBuilder buildSMulH(const DstOp &Dst, const SrcOp &Src0,
+                                 const SrcOp &Src1,
+                                 Optional<unsigned> Flags = None) {
+    return buildInstr(TargetOpcode::G_SMULH, {Dst}, {Src0, Src1}, Flags);
+  }
+
+  MachineInstrBuilder buildShl(const DstOp &Dst, const SrcOp &Src0,
+                               const SrcOp &Src1,
+                               Optional<unsigned> Flags = None) {
+    return buildInstr(TargetOpcode::G_SHL, {Dst}, {Src0, Src1}, Flags);
+  }
+
   /// Build and insert \p Res = G_AND \p Op0, \p Op1
   ///
   /// G_AND sets \p Res to the bitwise and of integer parameters \p Op0 and \p
Index: lib/CodeGen/GlobalISel/LegalizerHelper.cpp
===================================================================
--- lib/CodeGen/GlobalISel/LegalizerHelper.cpp
+++ lib/CodeGen/GlobalISel/LegalizerHelper.cpp
@@ -344,6 +344,8 @@
     MI.eraseFromParent();
     return Legalized;
   }
+  case TargetOpcode::G_MUL:
+    return narrowScalarMul(MI, TypeIdx, NarrowTy);
   case TargetOpcode::G_EXTRACT: {
     if (TypeIdx != 1)
       return UnableToLegalize;
@@ -1620,6 +1622,51 @@
   }
 }
 
+LegalizerHelper::LegalizeResult
+LegalizerHelper::narrowScalarMul(MachineInstr &MI, unsigned TypeIdx, LLT NewTy) {
+  unsigned DstReg = MI.getOperand(0).getReg();
+  unsigned Src0 = MI.getOperand(1).getReg();
+  unsigned Src1 = MI.getOperand(2).getReg();
+  LLT Ty = MRI.getType(DstReg);
+  if (Ty.isVector())
+    return UnableToLegalize;
+
+  unsigned Size = Ty.getSizeInBits();
+  unsigned NewSize = Size / 2;
+  if (Size != 2 * NewSize)
+    return UnableToLegalize;
+
+  LLT HalfTy = LLT::scalar(NewSize);
+  // TODO: if HalfTy != NewTy, handle the breakdown all at once?
+
+  unsigned ShiftAmt = MRI.createGenericVirtualRegister(Ty);
+  unsigned Lo = MRI.createGenericVirtualRegister(HalfTy);
+  unsigned Hi = MRI.createGenericVirtualRegister(HalfTy);
+  unsigned ExtLo = MRI.createGenericVirtualRegister(Ty);
+  unsigned ExtHi = MRI.createGenericVirtualRegister(Ty);
+  unsigned ShiftedHi = MRI.createGenericVirtualRegister(Ty);
+
+  SmallVector<unsigned, 2> Src0Parts;
+  SmallVector<unsigned, 2> Src1Parts;
+
+  extractParts(Src0, HalfTy, 2, Src0Parts);
+  extractParts(Src1, HalfTy, 2, Src1Parts);
+
+  MIRBuilder.buildMul(Lo, Src0Parts[0], Src1Parts[0]);
+
+  // TODO: Use smulh or umulh depending on what the target has.
+  MIRBuilder.buildUMulH(Hi, Src0Parts[1], Src1Parts[1]);
+
+  MIRBuilder.buildConstant(ShiftAmt, NewSize);
+  MIRBuilder.buildAnyExt(ExtHi, Hi);
+  MIRBuilder.buildShl(ShiftedHi, ExtHi, ShiftAmt);
+
+  MIRBuilder.buildZExt(ExtLo, Lo);
+  MIRBuilder.buildOr(DstReg, ExtLo, ShiftedHi);
+  MI.eraseFromParent();
+  return Legalized;
+}
+
 LegalizerHelper::LegalizeResult
 LegalizerHelper::lowerBitCount(MachineInstr &MI, unsigned TypeIdx, LLT Ty) {
   unsigned Opc = MI.getOpcode();
Index: lib/Target/AMDGPU/AMDGPULegalizerInfo.cpp
===================================================================
--- lib/Target/AMDGPU/AMDGPULegalizerInfo.cpp
+++ lib/Target/AMDGPU/AMDGPULegalizerInfo.cpp
@@ -100,6 +100,7 @@
 
   getActionDefinitionsBuilder({G_MUL, G_UMULH, G_SMULH})
     .legalFor({S32})
+    .clampScalar(0, S32, S32)
     .scalarize(0);
 
   // FIXME: 64-bit ones only legal for scalar
Index: test/CodeGen/AMDGPU/GlobalISel/legalize-mul.mir
===================================================================
--- test/CodeGen/AMDGPU/GlobalISel/legalize-mul.mir
+++ test/CodeGen/AMDGPU/GlobalISel/legalize-mul.mir
@@ -38,3 +38,29 @@
     %2:_(<2 x s32>) = G_MUL %0, %1
     $vgpr0_vgpr1 = COPY %2
 ...
+
+---
+name: test_mul_s64
+body: |
+  bb.0:
+    liveins: $vgpr0_vgpr1, $vgpr2_vgpr3
+
+    ; CHECK-LABEL: name: test_mul_s64
+    ; CHECK: [[COPY:%[0-9]+]]:_(s64) = COPY $vgpr0_vgpr1
+    ; CHECK: [[COPY1:%[0-9]+]]:_(s64) = COPY $vgpr2_vgpr3
+    ; CHECK: [[UV:%[0-9]+]]:_(s32), [[UV1:%[0-9]+]]:_(s32) = G_UNMERGE_VALUES [[COPY]](s64)
+    ; CHECK: [[UV2:%[0-9]+]]:_(s32), [[UV3:%[0-9]+]]:_(s32) = G_UNMERGE_VALUES [[COPY1]](s64)
+    ; CHECK: [[MUL:%[0-9]+]]:_(s32) = G_MUL [[UV]], [[UV2]]
+    ; CHECK: [[UMULH:%[0-9]+]]:_(s32) = G_UMULH [[UV1]], [[UV3]]
+    ; CHECK: [[C:%[0-9]+]]:_(s64) = G_CONSTANT i64 32
+    ; CHECK: [[ANYEXT:%[0-9]+]]:_(s64) = G_ANYEXT [[UMULH]](s32)
+    ; CHECK: [[TRUNC:%[0-9]+]]:_(s32) = G_TRUNC [[C]](s64)
+    ; CHECK: [[SHL:%[0-9]+]]:_(s64) = G_SHL [[ANYEXT]], [[TRUNC]](s32)
+    ; CHECK: [[ZEXT:%[0-9]+]]:_(s64) = G_ZEXT [[MUL]](s32)
+    ; CHECK: [[OR:%[0-9]+]]:_(s64) = G_OR [[ZEXT]], [[SHL]]
+    ; CHECK: $vgpr0_vgpr1 = COPY [[OR]](s64)
+    %0:_(s64) = COPY $vgpr0_vgpr1
+    %1:_(s64) = COPY $vgpr2_vgpr3
+    %2:_(s64) = G_MUL %0, %1
+    $vgpr0_vgpr1 = COPY %2
+...