Re: [PATCH] Small optimization across postgres (remove strlenduplicate usage)

On 4/19/20 10:29 PM, Ranier Vilela wrote:
> Em dom., 19 de abr. de 2020 às 16:33, Tomas Vondra 
> <tomas.vondra@2ndquadrant.com <mailto:tomas.vondra@2ndquadrant.com>> 
> escreveu:
> 
>     On Sun, Apr 19, 2020 at 11:24:38AM -0300, Ranier Vilela wrote:
>      >Hi,
>      >strlen it is one of the low fruits that can be harvested.
>      >What is your opinion?
>      >
> 
>     That assumes this actually affects/improves performance, without any
>     measurements proving that. Considering large number of the places you
>     modified are related to DDL (CreateComment, ChooseIndexColumnNames, ...)
>     or stuff that runs only once or infrequently (like the changes in
>     PostmasterMain or libpqrcv_get_senderinfo). Likewise, it seems entirely
>     pointless to worry about strlen() overhead e.g. in fsync_parent_path
>     which is probably dominated by I/O.
> 
> With code as interconnected as postgres, it is difficult to say that a 
> function, which calls strlen, repeatedly, would not have any gain.
> Regarding the functions, I was just being consistent, trying to remove 
> all occurrences, even where, there is very little gain.

At least gcc 9.3 optimizes "strlen(s) == 0" to "s[0] == '\0'", even at 
low optimization levels. I tried it out with https://godbolt.org/.

Maybe some of the others cases are performance improvements, I have not 
checked your patch in details, but strlen() == 0 is easily handled by 
the compiler.

C code:

int f1(char *str) {
     return strlen(str) == 0;
}

int f2(char *str) {
     return str[0] == '\0';
}

Assembly generated with default flags:

f1:
         pushq   %rbp
         movq    %rsp, %rbp
         movq    %rdi, -8(%rbp)
         movq    -8(%rbp), %rax
         movzbl  (%rax), %eax
         testb   %al, %al
         sete    %al
         movzbl  %al, %eax
         popq    %rbp
         ret
f2:
         pushq   %rbp
         movq    %rsp, %rbp
         movq    %rdi, -8(%rbp)
         movq    -8(%rbp), %rax
         movzbl  (%rax), %eax
         testb   %al, %al
         sete    %al
         movzbl  %al, %eax
         popq    %rbp
         ret

Assembly generated with -O2.

f1:
         xorl    %eax, %eax
         cmpb    $0, (%rdi)
         sete    %al
         ret
f2:
         xorl    %eax, %eax
         cmpb    $0, (%rdi)
         sete    %al
         ret

Andreas