AMDGPU: Error on calls from graphics shaders

[arsenm]

Summary

In principle nothing should stop these from working, but
work is necessary to create an ABI for dealing with the stack
related registers.

[msearles]

LGTM

[arsenm]

r335829

lib/Target/AMDGPU/SIISelLowering.cpp

return lowerUnhandledCall(CLI, InVals,
                               "unsupported indirect call to function ");
   }
 
   if (IsTailCall && MF.getTarget().Options.GuaranteedTailCallOpt) {
     return lowerUnhandledCall(CLI, InVals,
                               "unsupported required tail call to function ");
   }
 
+  if (AMDGPU::isShader(MF.getFunction().getCallingConv())) {
+    // Note the issue is with the CC of the calling function, not of the call
+    // itself.
+    return lowerUnhandledCall(CLI, InVals,
+                          "unsupported call from graphics shader of function ");
+  }
+
   // The first 4 bytes are reserved for the callee's emergency stack slot.
   const unsigned CalleeUsableStackOffset = 4;
 
   if (IsTailCall) {
     IsTailCall = isEligibleForTailCallOptimization(
       Callee, CallConv, IsVarArg, Outs, OutVals, Ins, DAG);
     if (!IsTailCall && CLI.CS && CLI.CS.isMustTailCall()) {
       report_fatal_error("failed to perform tail call elimination on a call "

test/CodeGen/AMDGPU/unsupported-calls.ll

 }
 
 ; GCN: :0:0: in function test_indirect_call void (void ()*): unsupported indirect call to function <unknown>
 ; R600: in function test_indirect_call{{.*}}: unsupported call to function <unknown>
 define void @test_indirect_call(void()* %fptr) {
   call void %fptr()
   ret void
 }
+
+; GCN: :0:0: in function test_call_from_shader i32 (): unsupported call from graphics shader of function defined_function
+; R600: in function test_call{{.*}}: unsupported call to function defined_function
+define amdgpu_ps i32 @test_call_from_shader() {
+  %call = call i32 @defined_function(i32 0)
+  ret i32 %call
+}