This PR implements the ability to convert floats stored in vfloat4 vectors to fp16 bit patterns stored in vint4 vectors. Scalar versions are also provided as overloaded functions of the same name, for sake of convenience.
The code has been refactored to take advantage of the vectorization opportunities this gives, in particular using NEON (Arm) and F16C (x86-64) ISA support for float<>fp16 conversion. The F16C support is tied to the AVX2 enable config option, it is not a separate enable. In cases where the soft-float code is not needed, code size reduces by ~5KB.
This phase introduces the first phase of the API split between the API front-end and the CODEC backed. However, some aspects of the final design are not yet supported:
This build only only supports automatic multi-threading; no user owned thread pools are supported.
Some memory allocations still tied to the compression pass, not the context, so reusing the context will have higher than planned CPU overhead due to the repeated memory allocation (although the impact is probably small unless compression small image).
The input image layout is still the legacy format and must be pre-padded by the caller for any pass using the area-based perceptual metrics.
Peter Harris