Nifty sign extend

signextend.cpp

template< unsigned SrcSize, unsigned DstSize >
static inline uint32_t signExtend(uint32_t x)
{
    static_assert(SrcSize <= 32, "SrcSize cannot be larger than 32 bits.");
    static_assert(SrcSize <= 32, "DstSize cannot be larger than 32 bits.");
    static_assert(SrcSize < DstSize, "DstSize must be larger than SrcSize.");
    constexpr unsigned src_mask = (1 << SrcSize) - 1;
    constexpr unsigned extension = ((1 << (DstSize - SrcSize)) - 1) << SrcSize;
    uint32_t result = x & src_mask;
    uint32_t is_signed = (x & (1 << (SrcSize - 1))) >> (SrcSize - 1);
    result |= (extension * is_signed);
    return result;
}