Just compute it as 2^(y*log2(x)).
There is a x86 instruction FYL2X to compute y*log2(x) and a x86 instruction F2XM1 to do exponentiation. F2XM1 requires an argument in [-1,1] range, so you'd have to add some code in between to extract the integer part and the remainder, exponentiate the remainder, use FSCALE to scale the result by an appropriate power of 2.
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