434bf16084
Implement double precision exp function correctly rounded for all rounding modes. Using 4 stages: - Range reduction: reduce to `exp(x) = 2^hi * 2^mid1 * 2^mid2 * exp(lo)`. - Use 64 + 64 LUT for 2^mid1 and 2^mid2, and use cubic Taylor polynomial to approximate `(exp(lo) - 1) / lo` in double precision. Relative error in this step is bounded by 1.5 * 2^-63. - If the rounding test fails, use degree-6 Taylor polynomial to approximate `exp(lo)` in double-double precision. Relative error in this step is bounded by 2^-99. - If the rounding test still fails, use degree-7 Taylor polynomial to compute `exp(lo)` in ~128-bit precision. Reviewed By: zimmermann6 Differential Revision: https://reviews.llvm.org/D158551
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