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Subject: Re: More speedups for tuple deformation
From: Chao Li <li.evan.chao@gmail.com>
In-Reply-To: <CAApHDvqhbJU_-yF3Hbf4VhX33qXtpeYv3MsvMPDMcDwGGLr9ZQ@mail.gmail.com>
Date: Tue, 20 Jan 2026 12:32:55 +0800
Cc: PostgreSQL Developers <pgsql-hackers@lists.postgresql.org>
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To: David Rowley <dgrowleyml@gmail.com>
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> On Jan 20, 2026, at 08:11, David Rowley <dgrowleyml@gmail.com> wrote:
>=20
> On Mon, 19 Jan 2026 at 18:48, Chao Li <li.evan.chao@gmail.com> wrote:
>> I reviewed the patch and traced some basic workflows. But I haven=E2=80=
=99t done a load test to compare performance differences with and =
without this patch, I will do that if I get some bandwidth later. Here =
comes some review comments:
>>=20
>> 1 - tupmacs.h
>> ```
>> +       /* Create a mask with all bits beyond natts's bit set to off =
*/
>> +       mask =3D 0xFF & ((((uint8) 1) << (natts & 7)) - 1);
>> +       byte =3D (~bits[lastByte]) & mask;
>> ```
>>=20
>> When I read the code, I got an impression bits[lastByte] might =
overflow when natts % 8 =3D=3D 0, so I traced the code, then I realized =
that, this function is only called when a row has null values, so that, =
when reaching here, natts % 8 !=3D 0, otherwise it should return earlier =
within the for loop.
>=20
> It certainly is possible to get to that part of the code when natts is
> a multiple of 8 and the tuple contains NULLs after that (we may not be
> deforming the entire tuple). The code you quoted that's setting "mask"
> in that case will produce a zero mask, resulting in not finding any
> NULLs. I don't quite see any risk of overflowing any of the types
> here.  If natts is 16 then effectively the code does 0xFF & ((1 << 0)
> - 1); so no overflow. Just left shift by 0 bits and bitwise AND with
> zero, resulting in the mask becoming zero.
>=20
> How about if I write the comment as follows?
>=20
> /*
> * Create a mask with all bits beyond natts's bit set to off.  The code
> * below will generate a zero mask when natts & 7 =3D=3D 0.  When that =
happens
> * all bytes that need to be checked were done so in the loop above.  =
The
> * code below will create an empty mask and end up returning natts.  =
This
> * has been done to avoid having to write a special case to check if =
we've
> * covered all bytes already.
> */
>=20

I=E2=80=99m sorry I didn=E2=80=99t express myself clearly, maybe I =
should have used =E2=80=9COOB=E2=80=9D rather than =E2=80=9Coverflow". =
My real concern is about out-of-boundary read of bits[lastByte] when =
natts&7=3D=3D0.

Say, natts is 16, then bits is 2 bytes long; lastByte =3D 16>>3 =3D 2, =
so bits[2] is a OOB read.

If first_null_attr() is only called when hasnulls=3D=3Dtrue, then it =
will never hit the OOB point, because it will return early from the =
=E2=80=9Cfor=E2=80=9D loop. In the current patch, which is true, so the =
OOB should never happen.

However, I don=E2=80=99t see any comment mentions something like =
=E2=80=9Cfirst_null_attr() should only be called when hasnulls is true. =
If in future one calls first_null_attr() in a situation where hasnulls =
=3D=3D false, then the OOB will be triggered.

The comment you added explains that even if OOB happens, no matter what =
value is hold by bits[lastByte], because mask is 0, the final result is =
still correct, which is true, but OOB is still a concern. If the bits =
array happens to end exactly at the edge of a memory page, the OOB read =
bits[lastByte] may trigger a segment fault; and valgrind may detect the =
OOB and complain about it.

So, my original comment was that, we should at least add something to =
the header comment to mention =E2=80=9Cfirst_null_attr() should only be =
called when hasnulls is true. If we can add an Assert to ensure hasnulls =
is true, that would be even better.

But if we want first_null_attr() to be safe no matter hasnulls is true =
or false, I think we should avoid the OOB.

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/