Em sex., 16 de jul. de 2021 às 12:37, Alvaro Herrera < alvhe...@alvh.no-ip.org> escreveu:
> On 2021-Jul-16, David Rowley wrote: > > > On Fri, 16 Jul 2021 at 20:35, Japin Li <japi...@hotmail.com> wrote: > > > > When I fix a bug about ALTER SUBSCRIPTION ... SET (slot_name) [1], > Ranier Vilela > > > > finds that ReplicationSlotValidateName() has redundant strlen() > call, Since it's > > > > not related to that problem, so I start a new thread to discuss it. > > > > I think this is a waste of time. The first strlen() call is just > > checking for an empty string. I imagine all compilers would just > > optimise that to checking if the first char is '\0'; > > I could find the following idioms > > 95 times: var[0] == '\0' > 146 times: *var == '\0' > 35 times: strlen(var) == 0 > > Resp. > git grep "[a-zA-Z_]*\[0\] == '\\\\0" > git grep "\*[a-zA-Z_]* == '\\\\0" > git grep "strlen([^)]*) == 0" > > See https://postgr.es/m/13847.1587332...@sss.pgh.pa.us about replacing > strlen with a check on first byte being zero. So still not Ranier's > patch, but rather the attached. I doubt this change is worth committing > on its own, though, since performance-wise it doesn't matter at all; if > somebody were to make it so that all "strlen(foo) == 0" occurrences were > changed to use the test on byte 0, that could be said to be establishing > a consistent style, which might be more pallatable. > Yeah, can be considered a refactoring. IMHO not in C is free. I do not think that this will improve 1% generally, but some function can gain. Another example I can mention is this little Lua code, in which I made comparisons between the generated asms, made some time ago. p[0] = luaS_newlstr(L, str, strlen(str)); with strlen (msvc 64 bits): inc r8 cmp BYTE PTR [r11+r8], 0 jne SHORT $LL19@luaS_new mov rdx, r11 mov rcx, rdi call luaS_newlstr without strlen (msvc 64 bits): mov r8, rsi mov rdx, r11 mov QWORD PTR [rbx+8], rax mov rcx, rdi call luaS_newlstr Of course, that doesn't mean anything about Postgres, but that I'm talking about using strlen. Clearly I can see that usage is not always free. If have some interest in actually changing that in Postgres, I can prepare a patch, where static analyzers claim it's performance loss. But without any reason to backpatch, it can only be considered as refactoring. regards, Ranier Vilela
