This doesn't work.

Author: linuxrocks123

llvm/lib/Target/AMDGPU/AMDGPUISelLowering.cpp

@@ -18,6 +18,7 @@
 #include "AMDGPUMachineFunction.h"
 #include "AMDGPUMemoryUtils.h"
 #include "SIMachineFunctionInfo.h"
+#include "SIRegisterInfo.h"
 #include "llvm/CodeGen/Analysis.h"
 #include "llvm/CodeGen/GlobalISel/GISelValueTracking.h"
 #include "llvm/CodeGen/MachineFrameInfo.h"
@@ -5291,15 +5292,20 @@ SDValue AMDGPUTargetLowering::expandABS(SDNode *N, SelectionDAG &CurDAG,
   assert(N->getOpcode() == ISD::ABS &&
          "Tried to select abs with non-abs opcode.");
 
-  if (N->getValueSizeInBits(0) != 16 || IsNegative)
+  if (N->getValueSizeInBits(0) != 16 || getRegClassFor(N->getSimpleValueType(0)) != &AMDGPU::SReg_32RegClass)
     return TargetLowering::expandABS(N, CurDAG, IsNegative);
 
   SDValue Src = N->getOperand(0);
   SDLoc DL(Src);
 
   SDValue SExtSrc = CurDAG.getSExtOrTrunc(Src, DL, MVT::i32);
   SDValue ExtAbs = CurDAG.getNode(ISD::ABS, DL, MVT::i32, SExtSrc);
-  return CurDAG.getNode(ISD::TRUNCATE, DL, MVT::i16, ExtAbs);
+  SDValue TruncResult = CurDAG.getNode(ISD::TRUNCATE, DL, MVT::i16, ExtAbs);
+  
+  if (!IsNegative)
+    return TruncResult;
+  return CurDAG.getNode(ISD::SUB, DL, MVT::i16,
+                        CurDAG.getConstant(0, DL, MVT::i16), TruncResult);
 }
 
 SDValue AMDGPUTargetLowering::PerformDAGCombine(SDNode *N,

llvm/test/CodeGen/AMDGPU/s_abs_i16.ll

@@ -0,0 +1,22 @@
+; RUN: llc -mtriple=amdgcn-- -mcpu=gfx900 < %s | FileCheck %s
+
+define amdgpu_ps i16 @abs_i16(i16 inreg %arg) {
+; CHECK-LABEL: abs_i16:
+; CHECK: %bb.0:
+; CHECK-NEXT: s_sext_i32_i16 s0, s0
+; CHECK-NEXT: s_abs_i32 s0, s0
+
+  %res = call i16 @llvm.abs.i16(i16 %arg, i1 false)
+  ret i16 %res
+}
+
+define amdgpu_ps i16 @abs_i16_neg(i16 inreg %arg) {
+; CHECK-LABEL: abs_i16_neg:
+; CHECK: ; %bb.0:
+; CHECK-NEXT: s_sext_i32_i16 s0, s0
+; CHECK-NEXT: s_abs_i32 s0, s0
+; CHECK-NEXT: s_sub_i32 s0, 0, s0
+  %res1 = call i16 @llvm.abs.i16(i16 %arg, i1 false)
+  %res2 = sub i16 0, %res1
+  ret i16 %res2
+}