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1) pre-shift GCM last4 to use 32-bit shift On 32-bit architectures like Aarch32, RV32, Xtensa, shifting a 64-bit variable by 32-bits is free, since it changes the register representing half of the 64-bit var. Pre-shift the last4 array to take advantage of this. 2) unroll first GCM iteration The first loop of gcm_mult() is different from the others. By unrolling it separately from the others, the other iterations may take advantage of the zero-overhead loop construct, in addition to saving a conditional branch in the loop.