Index: llvm/lib/Target/AMDGPU/AMDGPUCallLowering.cpp
===================================================================
--- llvm/lib/Target/AMDGPU/AMDGPUCallLowering.cpp
+++ llvm/lib/Target/AMDGPU/AMDGPUCallLowering.cpp
@@ -627,6 +627,13 @@
     CCInfo.AllocateReg(ImplicitBufferPtrReg);
   }
 
+  // FIXME: This probably isn't defined for mesa
+  if (Info->hasFlatScratchInit() && !Subtarget.isAmdPalOS()) {
+    Register FlatScratchInitReg = Info->addFlatScratchInit(*TRI);
+    MF.addLiveIn(FlatScratchInitReg, &AMDGPU::SGPR_64RegClass);
+    CCInfo.AllocateReg(FlatScratchInitReg);
+  }
+
   SmallVector<ArgInfo, 32> SplitArgs;
   unsigned Idx = 0;
   unsigned PSInputNum = 0;
Index: llvm/test/CodeGen/AMDGPU/GlobalISel/flat-scratch-init.gfx.ll
===================================================================
--- /dev/null
+++ llvm/test/CodeGen/AMDGPU/GlobalISel/flat-scratch-init.gfx.ll
@@ -0,0 +1,45 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc -global-isel -amdgpu-enable-flat-scratch -mtriple=amdgcn-mesa-mesa3d -mcpu=gfx900 < %s | FileCheck -check-prefix=MESA %s
+; RUN: llc -global-isel -amdgpu-enable-flat-scratch -mtriple=amdgcn-amd-amdpal -mcpu=gfx900 < %s | FileCheck -check-prefix=PAL %s
+
+; Test that the initialization for flat_scratch doesn't crash.  PAL
+; doesn't add a user SGPR for initializing flat_scratch, mesa does
+; (although this probably isn't actually defined).
+
+define amdgpu_ps void @amdgpu_ps() {
+; MESA-LABEL: amdgpu_ps:
+; MESA:       ; %bb.0:
+; MESA-NEXT:    s_add_u32 flat_scratch_lo, s2, s4
+; MESA-NEXT:    s_getreg_b32 s0, hwreg(HW_REG_SH_MEM_BASES, 0, 16)
+; MESA-NEXT:    s_addc_u32 flat_scratch_hi, s3, 0
+; MESA-NEXT:    s_lshl_b32 s0, s0, 16
+; MESA-NEXT:    v_mov_b32_e32 v0, 4
+; MESA-NEXT:    v_mov_b32_e32 v1, s0
+; MESA-NEXT:    v_mov_b32_e32 v2, 0
+; MESA-NEXT:    flat_store_dword v[0:1], v2
+; MESA-NEXT:    s_waitcnt vmcnt(0)
+; MESA-NEXT:    s_endpgm
+;
+; PAL-LABEL: amdgpu_ps:
+; PAL:       ; %bb.0:
+; PAL-NEXT:    s_getpc_b64 s[2:3]
+; PAL-NEXT:    s_mov_b32 s2, s0
+; PAL-NEXT:    s_load_dwordx2 s[2:3], s[2:3], 0x0
+; PAL-NEXT:    v_mov_b32_e32 v0, 4
+; PAL-NEXT:    v_mov_b32_e32 v2, 0
+; PAL-NEXT:    s_waitcnt lgkmcnt(0)
+; PAL-NEXT:    s_and_b32 s3, s3, 0xffff
+; PAL-NEXT:    s_add_u32 flat_scratch_lo, s2, s0
+; PAL-NEXT:    s_getreg_b32 s0, hwreg(HW_REG_SH_MEM_BASES, 0, 16)
+; PAL-NEXT:    s_addc_u32 flat_scratch_hi, s3, 0
+; PAL-NEXT:    s_lshl_b32 s0, s0, 16
+; PAL-NEXT:    v_mov_b32_e32 v1, s0
+; PAL-NEXT:    flat_store_dword v[0:1], v2
+; PAL-NEXT:    s_waitcnt vmcnt(0)
+; PAL-NEXT:    s_endpgm
+  %alloca = alloca i32, addrspace(5)
+  %cast = addrspacecast i32 addrspace(5)* %alloca to i32*
+  store volatile i32 0, i32* %cast
+  ret void
+}
+