Function bits

KittyMemoryEx/KittyAsm.cpp:13–24

namespace KittyAsm
{
    /// Extracts bits [hi:lo] of a word. The one place the shift/mask pair is
    /// written, so an off-by-one in a field extraction has a single home.
    uint32_t bits(uint32_t v, int hi, int lo)
    {
        // An inverted range or an out-of-word `lo` selects nothing; a `hi` past
        // the top of the word is clamped to it, so bits(v, 40, 8) is every bit
        // from 8 up rather than an error. Every in-tree call site passes
        // constants inside the word, but this is public API and the guard costs
        // one comparison on a path the compiler folds away for those constants.
        if (hi < lo || lo < 0 || lo > 31)
            return 0u;

        // Clamped before `width` is computed, not after: `hi` arrives unbounded
        // from a caller, and hi - lo + 1 below overflows int - itself undefined
        // behaviour - for a hi anywhere near INT_MAX, before the width>=32 case