Re: Optmize bitmapword macros calc (src/backend/nodes/bitmapset.c)

On Mon, Jan 29, 2024 at 01:30:47PM -0300, Ranier Vilela wrote:
> IMO I believe that bitmapset can obtain an optimization in the calculation
> of the WORDNUM and BITNUM macros.
>
> As you know, in bitmapset, negative members are not allowed.
>
> if (x < 0)
> elog(ERROR, "negative bitmapset member not allowed");
>
> Then, allow the compiler to optimize and do the calculations in unsigned.

I'm currently +0.1 for this change. I don't see any huge problem with
trimming a few instructions, but I'm dubious there's any measurable impact.
However, a cycle saved is a cycle earned...

-#define WORDNUM(x) ((x) / BITS_PER_BITMAPWORD)
-#define BITNUM(x) ((x) % BITS_PER_BITMAPWORD)
+#define WORDNUM(x) ((bitmapword)(x) / BITS_PER_BITMAPWORD)
+#define BITNUM(x) ((bitmapword)(x) % BITS_PER_BITMAPWORD)

I'm curious why we'd cast to bitmapword and not straight to uint32. I
don't think the intent is that callers will provide a bitmapword to these
macros. I also wonder if it's worth asserting that x is >= 0 before
casting here.

--
Nathan Bossart
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