Re: Let's make PostgreSQL multi-threaded

Hi,

On 2023-06-08 16:47:48 +0300, Konstantin Knizhnik wrote:
> Actually TLS not not more expensive then accessing struct fields (at least
> at x86 platform), consider the following program:

It really depends on the OS and the architecture, not just the
architecture. And even on x86-64 Linux, the fact that you're using the segment
offset in the address calculation means you can't use the more complicated
addressing modes for other reasons. And plenty instructions, e.g. most (?) SSE
instructions, won't be able to use that kind of addressing directly.

Even just compiling your, example you can see that with gcc -O2 you get
considerably faster code with the non-TLS version.

As a fairly extreme example, here's the mingw -O3 compiled code:

use_struct:
        movq    xmm1, QWORD PTR .LC0[rip]
        movq    xmm0, QWORD PTR [rcx]
        add     DWORD PTR 8[rcx], 1
        paddd   xmm0, xmm1
        movq    QWORD PTR [rcx], xmm0
        ret
use_tls:
        sub     rsp, 40
        lea     rcx, __emutls_v.a[rip]
        call    __emutls_get_address
        lea     rcx, __emutls_v.b[rip]
        add     DWORD PTR [rax], 1
        call    __emutls_get_address
        lea     rcx, __emutls_v.c[rip]
        add     DWORD PTR [rax], 1
        call    __emutls_get_address
        add     DWORD PTR [rax], 1
        add     rsp, 40
        ret

Greetings,

Andres Freund