On Mon, Nov 9, 2020 at 2:41 PM William Breathitt Gray
[off-list ref] wrote:

On Mon, Nov 09, 2020 at 06:04:11PM +0530, Syed Nayyar Waris wrote:

One of my concerns is that we're incurring the latency two additional
conditional checks just to suppress a compiler warning about a case that
wouldn't occur in the actual use of bitmap_set_value(). I'm hoping
there's a way for us to suppress these warnings without adding onto the
latency of this function; given that bitmap_set_value() is intended to
be used in loops, conditionals here could significantly increase latency
in drivers.

At least for this caller, the size check would be a compile-time
constant that can be eliminated.

I wonder if array_index_nospec() might have the side effect of
suppressing these warnings for us. For example, would this work:

static inline void bitmap_set_value(unsigned long *map,
                                    unsigned long value,
                                    unsigned long start, unsigned long nbits)
{
        const unsigned long offset = start % BITS_PER_LONG;
        const unsigned long ceiling = round_up(start + 1, BITS_PER_LONG);
        const unsigned long space = ceiling - start;
        size_t index = BIT_WORD(start);

        value &= GENMASK(nbits - 1, 0);

        if (space >= nbits) {
                index = array_index_nospec(index, index + 1);

                map[index] &= ~(GENMASK(nbits - 1, 0) << offset);
                map[index] |= value << offset;
        } else {
                index = array_index_nospec(index, index + 2);

                map[index + 0] &= ~BITMAP_FIRST_WORD_MASK(start);
                map[index + 0] |= value << offset;
                map[index + 1] &= ~BITMAP_LAST_WORD_MASK(start + nbits);
                map[index + 1] |= value >> space;
        }
}

Or is this going to produce the same warning because we're not using an
explicit check against the map array size?

https://godbolt.org/z/fxnsG9

It still warns about the 'map[index + 1]' access: from all I can tell,
gcc mainly complains because it cannot rule out that 'space < nbits',
and then it knows the size of 'DECLARE_BITMAP(old, 64)' and finds
that if 'index + 0' is correct, then 'index + 1' overflows that array.

      Arnd

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