In the deoptimization process, now we only save SIMD128 registers (the full state of the floating-point registers), we need a way to access the double registers.  In ia32 and x64, we use d[0], and in ARM, we use d[0] for the lower double register and d[1] for the upper register as ARM SIMD128(Neon) registers are aliased with double(VFP) registers.