Re: Remove unnecessary static type qualifiers

On Tue, Apr 8, 2025 at 4:29 PM Japin Li <japinli(at)hotmail(dot)com> wrote:
>
>
> Hi, all
>
> When I read the libpq source code, I found unnecessary static type qualifiers
> in PQsetClientEncoding().
>
> diff --git a/src/interfaces/libpq/fe-connect.c b/src/interfaces/libpq/fe-connect.c
> index 0258d9ace3c..300ddfffd55 100644
> --- a/src/interfaces/libpq/fe-connect.c
> +++ b/src/interfaces/libpq/fe-connect.c
> @@ -7717,7 +7717,7 @@ int
> PQsetClientEncoding(PGconn *conn, const char *encoding)
> {
> char qbuf[128];
> - static const char query[] = "set client_encoding to '%s'";
> + const char query[] = "set client_encoding to '%s'";
> PGresult *res;
> int status;
>

I doubt that, if you remove the *static*, it will allocate more memory
on stack and need more instructions to copy the string to that area.

You can check the assembly to prove this.

Here is what I got on macos M3 chip.

with static:

0000000000015710 <_PQsetClientEncoding>:
15710: d10383ff sub sp, sp, #0xe0
15714: a90d7bfd stp x29, x30, [sp, #0xd0]
15718: 910343fd add x29, sp, #0xd0
1571c: f00002a8 adrp x8, 0x6c000 <_write+0x6c000>
15720: f941a908 ldr x8, [x8, #0x350]
15724: f9400108 ldr x8, [x8]
15728: f81f83a8 stur x8, [x29, #-0x8]
1572c: f9001fe0 str x0, [sp, #0x38]
15730: f9001be1 str x1, [sp, #0x30]
15734: f9401fe8 ldr x8, [sp, #0x38]
15738: f1000108 subs x8, x8, #0x0
1573c: 1a9f17e8 cset w8, eq
15740: 37000108 tbnz w8, #0x0, 0x15760 <_PQsetClientEncoding+0x50>
15744: 14000001 b 0x15748 <_PQsetClientEncoding+0x38>
15748: f9401fe8 ldr x8, [sp, #0x38]
1574c: b941f908 ldr w8, [x8, #0x1f8]

without:

00000000000156ec <_PQsetClientEncoding>:
156ec: d10443ff sub sp, sp, #0x110
156f0: a90f6ffc stp x28, x27, [sp, #0xf0]
156f4: a9107bfd stp x29, x30, [sp, #0x100]
156f8: 910403fd add x29, sp, #0x100
156fc: f00002a8 adrp x8, 0x6c000 <_write+0x6c000>
15700: f941a908 ldr x8, [x8, #0x350]
15704: f9400108 ldr x8, [x8]
15708: f81e83a8 stur x8, [x29, #-0x18]
1570c: f9001be0 str x0, [sp, #0x30]
15710: f90017e1 str x1, [sp, #0x28]
15714: f00001c9 adrp x9, 0x50000 <_write+0x50000>
15718: 913ded29 add x9, x9, #0xf7b
1571c: 3dc00120 ldr q0, [x9]
15720: 910103e8 add x8, sp, #0x40
15724: 3d8013e0 str q0, [sp, #0x40]
15728: 3cc0c120 ldur q0, [x9, #0xc]
1572c: 3c80c100 stur q0, [x8, #0xc]
15730: f9401be8 ldr x8, [sp, #0x30]
15734: f1000108 subs x8, x8, #0x0
15738: 1a9f17e8 cset w8, eq
1573c: 37000108 tbnz w8, #0x0, 0x1575c <_PQsetClientEncoding+0x70>
15740: 14000001 b 0x15744 <_PQsetClientEncoding+0x58>
15744: f9401be8 ldr x8, [sp, #0x30]
15748: b941f908 ldr w8, [x8, #0x1f8]

>
>
> --
> Regrads,
> Japin Li
>
>

--
Regards
Junwang Zhao