From: "daniel.domjan via ruby-core" <ruby-core@...>
Date: 2025-11-17T11:15:31+00:00
Subject: [ruby-core:123827] [Ruby Misc#21690] Inconsistent `rb_popcount64()` definition

Issue #21690 has been reported by daniel.domjan (D�niel Domj�n).

----------------------------------------
Misc #21690: Inconsistent `rb_popcount64()` definition
https://bugs.ruby-lang.org/issues/21690

* Author: daniel.domjan (D�niel Domj�n)
* Status: Open
----------------------------------------
The `rb_popcount64()` function is defined both in `internal/bits.h` and `parser_bits.h`, but the definition of these functions is different on the `#else` branch.

```c
// internal/bits.h
static inline unsigned int
rb_popcount64(uint64_t x)
{
...
#else
    x = (x & 0x5555555555555555) + (x >> 1 & 0x5555555555555555);
    x = (x & 0x3333333333333333) + (x >> 2 & 0x3333333333333333);
    x = (x & 0x0707070707070707) + (x >> 4 & 0x0707070707070707);
    x = (x & 0x000f000f000f000f) + (x >> 8 & 0x000f000f000f000f);
    x = (x & 0x0000001f0000001f) + (x >>16 & 0x0000001f0000001f);
    x = (x & 0x000000000000003f) + (x >>32 & 0x000000000000003f);
    return (unsigned int)x;

#endif
}
```

```c
// parser_bits.h
static inline unsigned int
rb_popcount64(uint64_t x)
{
...
#else
    x = (x & 0x5555555555555555) + (x >> 1 & 0x5555555555555555);
    x = (x & 0x3333333333333333) + (x >> 2 & 0x3333333333333333);
    x = (x & 0x0707070707070707) + (x >> 4 & 0x0707070707070707);
    x = (x & 0x001f001f001f001f) + (x >> 8 & 0x001f001f001f001f);
    x = (x & 0x0000003f0000003f) + (x >>16 & 0x0000003f0000003f);
    x = (x & 0x000000000000007f) + (x >>32 & 0x000000000000007f);
    return (unsigned int)x;

#endif
}
```

```diff
    x = (x & 0x5555555555555555) + (x >> 1 & 0x5555555555555555);
    x = (x & 0x3333333333333333) + (x >> 2 & 0x3333333333333333);
    x = (x & 0x0707070707070707) + (x >> 4 & 0x0707070707070707);
-   x = (x & 0x000f000f000f000f) + (x >> 8 & 0x000f000f000f000f);
+   x = (x & 0x001f001f001f001f) + (x >> 8 & 0x001f001f001f001f);
-   x = (x & 0x0000001f0000001f) + (x >>16 & 0x0000001f0000001f);
+   x = (x & 0x0000003f0000003f) + (x >>16 & 0x0000003f0000003f);
-   x = (x & 0x000000000000003f) + (x >>32 & 0x000000000000003f);
+   x = (x & 0x000000000000007f) + (x >>32 & 0x000000000000007f);
```

IIUC, this is a modified [hamming weight](https://en.wikipedia.org/wiki/Hamming_weight) based on [this SO answer](https://stackoverflow.com/a/73300535), so both implementations give the same result, but it's a bit confusing at first look, so I flagged it as `Misc` instead of `Bug`.



-- 
https://bugs.ruby-lang.org/
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