Index: llvm/lib/Target/AMDGPU/AMDGPUInstructionSelector.h
===================================================================
--- llvm/lib/Target/AMDGPU/AMDGPUInstructionSelector.h
+++ llvm/lib/Target/AMDGPU/AMDGPUInstructionSelector.h
@@ -92,6 +92,7 @@
   bool selectG_SZA_EXT(MachineInstr &I) const;
   bool selectG_CONSTANT(MachineInstr &I) const;
   bool selectG_FNEG(MachineInstr &I) const;
+  bool selectG_FABS(MachineInstr &I) const;
   bool selectG_AND_OR_XOR(MachineInstr &I) const;
   bool selectG_ADD_SUB(MachineInstr &I) const;
   bool selectG_UADDO_USUBO_UADDE_USUBE(MachineInstr &I) const;
Index: llvm/lib/Target/AMDGPU/AMDGPUInstructionSelector.cpp
===================================================================
--- llvm/lib/Target/AMDGPU/AMDGPUInstructionSelector.cpp
+++ llvm/lib/Target/AMDGPU/AMDGPUInstructionSelector.cpp
@@ -1969,6 +1969,48 @@
   return true;
 }
 
+// FIXME: This is a workaround for the same tablegen problems as G_FNEG
+bool AMDGPUInstructionSelector::selectG_FABS(MachineInstr &MI) const {
+  Register Dst = MI.getOperand(0).getReg();
+  const RegisterBank *DstRB = RBI.getRegBank(Dst, *MRI, TRI);
+  if (DstRB->getID() != AMDGPU::SGPRRegBankID ||
+      MRI->getType(Dst) != LLT::scalar(64))
+    return false;
+
+  Register Src = MI.getOperand(1).getReg();
+  MachineBasicBlock *BB = MI.getParent();
+  const DebugLoc &DL = MI.getDebugLoc();
+  Register LoReg = MRI->createVirtualRegister(&AMDGPU::SReg_32RegClass);
+  Register HiReg = MRI->createVirtualRegister(&AMDGPU::SReg_32RegClass);
+  Register ConstReg = MRI->createVirtualRegister(&AMDGPU::SReg_32RegClass);
+  Register OpReg = MRI->createVirtualRegister(&AMDGPU::SReg_32RegClass);
+
+  if (!RBI.constrainGenericRegister(Src, AMDGPU::SReg_64RegClass, *MRI) ||
+      !RBI.constrainGenericRegister(Dst, AMDGPU::SReg_64RegClass, *MRI))
+    return false;
+
+  BuildMI(*BB, &MI, DL, TII.get(AMDGPU::COPY), LoReg)
+    .addReg(Src, 0, AMDGPU::sub0);
+  BuildMI(*BB, &MI, DL, TII.get(AMDGPU::COPY), HiReg)
+    .addReg(Src, 0, AMDGPU::sub1);
+  BuildMI(*BB, &MI, DL, TII.get(AMDGPU::S_MOV_B32), ConstReg)
+    .addImm(0x7fffffff);
+
+  // Clear sign bit.
+  // TODO: Should this used S_BITSET0_*?
+  BuildMI(*BB, &MI, DL, TII.get(AMDGPU::S_AND_B32), OpReg)
+    .addReg(HiReg)
+    .addReg(ConstReg);
+  BuildMI(*BB, &MI, DL, TII.get(AMDGPU::REG_SEQUENCE), Dst)
+    .addReg(LoReg)
+    .addImm(AMDGPU::sub0)
+    .addReg(OpReg)
+    .addImm(AMDGPU::sub1);
+
+  MI.eraseFromParent();
+  return true;
+}
+
 static bool isConstant(const MachineInstr &MI) {
   return MI.getOpcode() == TargetOpcode::G_CONSTANT;
 }
@@ -2609,6 +2651,10 @@
     if (selectImpl(I, *CoverageInfo))
       return true;
     return selectG_FNEG(I);
+  case TargetOpcode::G_FABS:
+    if (selectImpl(I, *CoverageInfo))
+      return true;
+    return selectG_FABS(I);
   case TargetOpcode::G_EXTRACT:
     return selectG_EXTRACT(I);
   case TargetOpcode::G_MERGE_VALUES:
Index: llvm/lib/Target/AMDGPU/SIInstructions.td
===================================================================
--- llvm/lib/Target/AMDGPU/SIInstructions.td
+++ llvm/lib/Target/AMDGPU/SIInstructions.td
@@ -1255,7 +1255,7 @@
   (S_OR_B32 SReg_32:$src, (S_MOV_B32 (i32 0x80008000))) // Set sign bit
 >;
 
-// FIXME: The implicit-def of scc from S_[X]OR_B32 is mishandled
+// FIXME: The implicit-def of scc from S_[X]OR/AND_B32 is mishandled
  // def : GCNPat <
 //   (fneg (f64 SReg_64:$src)),
 //   (REG_SEQUENCE SReg_64,
@@ -1276,6 +1276,17 @@
 //     sub1)
 // >;
 
+// FIXME: Use S_BITSET0_B32/B64?
+// def : GCNPat <
+//   (fabs (f64 SReg_64:$src)),
+//   (REG_SEQUENCE SReg_64,
+//     (i32 (EXTRACT_SUBREG SReg_64:$src, sub0)),
+//     sub0,
+//     (S_AND_B32 (i32 (EXTRACT_SUBREG SReg_64:$src, sub1)),
+//                (i32 (S_MOV_B32 (i32 0x7fffffff)))),
+//     sub1)
+// >;
+
 } // End let AddedComplexity = 1
 
 def : GCNPat <
Index: llvm/test/CodeGen/AMDGPU/GlobalISel/inst-select-fabs.mir
===================================================================
--- llvm/test/CodeGen/AMDGPU/GlobalISel/inst-select-fabs.mir
+++ llvm/test/CodeGen/AMDGPU/GlobalISel/inst-select-fabs.mir
@@ -200,9 +200,13 @@
     liveins: $sgpr0_sgpr1
     ; GCN-LABEL: name: fabs_s64_ss
     ; GCN: liveins: $sgpr0_sgpr1
-    ; GCN: [[COPY:%[0-9]+]]:sgpr(s64) = COPY $sgpr0_sgpr1
-    ; GCN: [[FABS:%[0-9]+]]:sgpr(s64) = G_FABS [[COPY]]
-    ; GCN: S_ENDPGM 0, implicit [[FABS]](s64)
+    ; GCN: [[COPY:%[0-9]+]]:sreg_64 = COPY $sgpr0_sgpr1
+    ; GCN: [[COPY1:%[0-9]+]]:sreg_32 = COPY [[COPY]].sub0
+    ; GCN: [[COPY2:%[0-9]+]]:sreg_32 = COPY [[COPY]].sub1
+    ; GCN: [[S_MOV_B32_:%[0-9]+]]:sreg_32 = S_MOV_B32 2147483647
+    ; GCN: [[S_AND_B32_:%[0-9]+]]:sreg_32 = S_AND_B32 [[COPY2]], [[S_MOV_B32_]], implicit-def $scc
+    ; GCN: [[REG_SEQUENCE:%[0-9]+]]:sreg_64 = REG_SEQUENCE [[COPY1]], %subreg.sub0, [[S_AND_B32_]], %subreg.sub1
+    ; GCN: S_ENDPGM 0, implicit [[REG_SEQUENCE]]
     %0:sgpr(s64) = COPY $sgpr0_sgpr1
     %1:sgpr(s64) = G_FABS %0
     S_ENDPGM 0, implicit %1
@@ -249,3 +253,50 @@
     %1:vgpr(s64) = G_FABS %0
     S_ENDPGM 0, implicit %1
 ...
+
+# Make sure the source register is constrained
+---
+name: fabs_s64_vv_no_src_constraint
+legalized: true
+regBankSelected: true
+tracksRegLiveness: true
+
+body: |
+  bb.0:
+    liveins: $vgpr0_vgpr1
+    ; GCN-LABEL: name: fabs_s64_vv_no_src_constraint
+    ; GCN: liveins: $vgpr0_vgpr1
+    ; GCN: [[DEF:%[0-9]+]]:vreg_64 = IMPLICIT_DEF
+    ; GCN: [[V_MOV_B32_e32_:%[0-9]+]]:vgpr_32 = V_MOV_B32_e32 2147483647, implicit $exec
+    ; GCN: [[COPY:%[0-9]+]]:vgpr_32 = COPY [[DEF]].sub1
+    ; GCN: [[V_AND_B32_e64_:%[0-9]+]]:vgpr_32 = V_AND_B32_e64 [[COPY]], [[V_MOV_B32_e32_]], implicit $exec
+    ; GCN: [[COPY1:%[0-9]+]]:vgpr_32 = COPY [[DEF]].sub0
+    ; GCN: [[REG_SEQUENCE:%[0-9]+]]:vreg_64 = REG_SEQUENCE [[COPY1]], %subreg.sub0, [[V_AND_B32_e64_]], %subreg.sub1
+    ; GCN: S_ENDPGM 0, implicit [[REG_SEQUENCE]]
+    %0:vgpr(s64) = IMPLICIT_DEF
+    %1:vgpr(s64) = G_FABS %0:vgpr(s64)
+    S_ENDPGM 0, implicit %1
+...
+
+---
+name: fabs_s64_ss_no_src_constraint
+legalized: true
+regBankSelected: true
+tracksRegLiveness: true
+
+body: |
+  bb.0:
+    liveins: $sgpr0_sgpr1
+    ; GCN-LABEL: name: fabs_s64_ss_no_src_constraint
+    ; GCN: liveins: $sgpr0_sgpr1
+    ; GCN: [[DEF:%[0-9]+]]:sreg_64 = IMPLICIT_DEF
+    ; GCN: [[COPY:%[0-9]+]]:sreg_32 = COPY [[DEF]].sub0
+    ; GCN: [[COPY1:%[0-9]+]]:sreg_32 = COPY [[DEF]].sub1
+    ; GCN: [[S_MOV_B32_:%[0-9]+]]:sreg_32 = S_MOV_B32 2147483647
+    ; GCN: [[S_AND_B32_:%[0-9]+]]:sreg_32 = S_AND_B32 [[COPY1]], [[S_MOV_B32_]], implicit-def $scc
+    ; GCN: [[REG_SEQUENCE:%[0-9]+]]:sreg_64 = REG_SEQUENCE [[COPY]], %subreg.sub0, [[S_AND_B32_]], %subreg.sub1
+    ; GCN: S_ENDPGM 0, implicit [[REG_SEQUENCE]]
+    %0:sgpr(s64) = IMPLICIT_DEF
+    %1:sgpr(s64) = G_FABS %0:sgpr(s64)
+    S_ENDPGM 0, implicit %1
+...