> On Jun 20, 2023, at 3:32 PM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>
>> // out needs to be zeroed first
>> void unpack(struct uncompressed *out, const u64 *in, const struct
>> bitblock *blocks, int nblocks)
>> {
>> u64 *out_as_words = (u64*)out;
>> for (int i = 0; i < nblocks; i++) {
>> const struct bitblock *b;
>> out_as_words[b->target] |= (in[b->source] & b->mask) <<
>> b->shift;
>> }
>> }
>>
>> void apply_offsets(struct uncompressed *out, const struct uncompressed *offsets)
>> {
>> out->a += offsets->a;
>> out->b += offsets->b;
>> out->c += offsets->c;
>> out->d += offsets->d;
>> out->e += offsets->e;
>> out->f += offsets->f;
>> }
>>
>> Which generates nice code: https://godbolt.org/z/3fEq37hf5
>
> Thinking about this a bit more, I think the only real performance issue with my code is that it does 12 read-xor-write operations in memory, which all depend on each other in horrible ways.
If you compare the generated code, just notice that you forgot to initialize b in unpack() in this version.
I presume you wanted it to say "b = &blocks[i]”.
