How Can I Efficiently Convert Between Double-Precision Floats and 64-bit Integers Using SSE, AVX, and AVX-512?

Double/int64 Conversions with SSE/AVX

The SSE2 instruction set provides intrinsics for converting vectors between single-precision floats and 32-bit integers, but equivalents do not exist for double-precision and 64-bit integers. AVX also lacks these conversions.

Limited AVX512 Support

AVX512 introduces intrinsics for converting to and from 64-bit integers, both signed and unsigned. However, AVX2 and earlier configurations lack such functionality.

Approximating Int64 Conversions

For AVX2 and less, there are methods to simulate double precision to/from uint64 and int64 conversions with specific value range limitations and rounding behavior.

Fast Double <-> uint64_t Conversion:

This method works for values within the range [0, 2^52). By adding a magic number and masking the upper bits, you can obtain an approximation of the uint64_t representation of the double.

Fast Double <-> int64_t Conversion:

Similar to the uint64_t version, this approximation works for values within [-2^51, 2^51].

Full Range Int64 <-> Double Conversion:

uint64_t -> double:

This method uses 5 instructions to represent the full int64 range as a double.

int64_t -> double:

This conversion requires 6 instructions and correctly rounds to the nearest representable double.

Conclusion

AVX512 provides direct support for double/int64 conversions, but for earlier configurations, the provided approximations offer efficient solutions with specific value range and rounding behavior constraints.

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