On Tue, Jun 20, 2023, at 3:43 PM, Nadav Amit wrote:
>> 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]”.
Indeed. I also didn't notice that -Wall wasn't set. Oops.
