R600: Better fix for bug 20982

[arsenm]

Summary

Just do the left shift as unsigned to avoid the UB.

[arsenm]

r218092

lib/Target/R600/AMDGPUISelLowering.cpp

 static SDValue constantFoldBFE(SelectionDAG &DAG, IntTy Src0,
                                uint32_t Offset, uint32_t Width) {
   if (Width + Offset < 32) {
-    IntTy Result = (Src0 << (32 - Offset - Width)) >> (32 - Width);
+    uint32_t Shl = static_cast<uint32_t>(Src0) << (32 - Offset - Width);
+    IntTy Result = static_cast<IntTy>(Shl) >> (32 - Width);
     return DAG.getConstant(Result, MVT::i32);
   }
 

         // Avoid undefined left shift of a negative in the constant fold.
         // TODO: I'm not sure what the behavior of the hardware is, this should
         // probably follow that instead.
-        int32_t Val = CVal->getSExtValue();
-        if (Val < 0)
-          return SDValue();
-
         return constantFoldBFE<int32_t>(DAG,
-                                        Val,
+                                        CVal->getSExtValue(),
                                         OffsetVal,
                                         WidthVal);
       }

test/CodeGen/R600/llvm.AMDGPU.bfe.i32.ll

   ret void
 }
 
-; FIXME: This should fold to something
 ; FUNC-LABEL: @bfe_i32_constant_fold_test_16
-; SI: S_BFE_I32 [[SREG:s[0-9]+]], -1, 0x70001
-; SI: V_MOV_B32_e32 [[VREG:v[0-9]+]], [[SREG]]
+; SI-NOT: BFE
+; SI: V_MOV_B32_e32 [[VREG:v[0-9]+]], -1
 ; SI: BUFFER_STORE_DWORD [[VREG]],
 ; SI: S_ENDPGM
-
+; EG-NOT: BFE
 define void @bfe_i32_constant_fold_test_16(i32 addrspace(1)* %out) nounwind {
   %bfe_i32 = call i32 @llvm.AMDGPU.bfe.i32(i32 4294967295, i32 1, i32 7) nounwind readnone
   store i32 %bfe_i32, i32 addrspace(1)* %out, align 4