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Even slower than elsif?

Why is sub eins with else slower here than sub zwei with elsif ?

 #!/usr/bin/env perl use warnings; use 5.012; use Benchmark qw(:all); my $d = 0; my $c = 2; sub eins { if ( $c == 1) { $d = 1; } else { $d = 2; } } sub zwei { if ( $c == 1) { $d = 1; } elsif ( $c == 2 ) { $d = 2; } } sub drei { $d = 1; $d = 2 if $c == 2; } cmpthese( -5, { eins => sub{ eins() }, zwei => sub{ zwei() }, drei => sub{ drei() }, } ); 
  Rate eins drei zwei eins 4167007/s -- -1% -16% drei 4207631/s 1% -- -15% zwei 4972740/s 19% 18% -- Rate eins drei zwei eins 4074356/s -- -8% -16% drei 4428649/s 9% -- -9% zwei 4854964/s 19% 10% -- Rate eins drei zwei eins 3455697/s -- -6% -19% drei 3672628/s 6% -- -14% zwei 4250826/s 23% 16% -- Rate eins drei zwei eins 2832634/s -- -8% -19% drei 3088931/s 9% -- -12% zwei 3503197/s 24% 13% -- Rate eins zwei drei eins 3053821/s -- -17% -26% zwei 3701601/s 21% -- -10% drei 4131128/s 35% 12% -- Rate eins drei zwei eins 3033041/s -- -2% -12% drei 3092511/s 2% -- -10% zwei 3430837/s 13% 11% -- 
 Summary of my perl5 (revision 5 version 16 subversion 0) configuration: Platform: osname=linux, osvers=3.1.10-1.9-desktop, archname=x86_64-linux uname='linux linux1 3.1.10-1.9-desktop #1 smp preempt thu apr 5 18:48:38 utc 2012 (4a97ec8) x86_64 x86_64 x86_64 gnulinux ' config_args='-de' hint=recommended, useposix=true, d_sigaction=define useithreads=undef, usemultiplicity=undef useperlio=define, d_sfio=undef, uselargefiles=define, usesocks=undef use64bitint=define, use64bitall=define, uselongdouble=undef usemymalloc=n, bincompat5005=undef Compiler: cc='cc', ccflags ='-fno-strict-aliasing -pipe -fstack-protector -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64', optimize='-O2', cppflags='-fno-strict-aliasing -pipe -fstack-protector -I/usr/local/include' ccversion='', gccversion='4.6.2', gccosandvers='' intsize=4, longsize=8, ptrsize=8, doublesize=8, byteorder=12345678 d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=16 ivtype='long', ivsize=8, nvtype='double', nvsize=8, Off_t='off_t', lseeksize=8 alignbytes=8, prototype=define Linker and Libraries: ld='cc', ldflags =' -fstack-protector -L/usr/local/lib' libpth=/usr/local/lib /lib/../lib64 /usr/lib/../lib64 /lib /usr/lib /lib64 /usr/lib64 /usr/local/lib64 libs=-lnsl -lndbm -lgdbm -ldb -ldl -lm -lcrypt -lutil -lc -lgdbm_compat perllibs=-lnsl -ldl -lm -lcrypt -lutil -lc libc=/lib/libc-2.14.1.so, so=so, useshrplib=false, libperl=libperl.a gnulibc_version='2.14.1' Dynamic Linking: dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-Wl,-E' cccdlflags='-fPIC', lddlflags='-shared -O2 -L/usr/local/lib -fstack-protector' Characteristics of this binary (from libperl): Compile-time options: HAS_TIMES PERLIO_LAYERS PERL_DONT_CREATE_GVSV PERL_MALLOC_WRAP PERL_PRESERVE_IVUV USE_64_BIT_ALL USE_64_BIT_INT USE_LARGE_FILES USE_LOCALE USE_LOCALE_COLLATE USE_LOCALE_CTYPE USE_LOCALE_NUMERIC USE_PERLIO USE_PERL_ATOF Built under linux Compiled at May 24 2012 20:53:15 %ENV: PERL_HTML_DISPLAY_COMMAND="/usr/bin/firefox -new-window %s" @INC: /usr/local/lib/perl5/site_perl/5.16.0/x86_64-linux /usr/local/lib/perl5/site_perl/5.16.0 /usr/local/lib/perl5/5.16.0/x86_64-linux /usr/local/lib/perl5/5.16.0 .
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3 answers

[This is an answer for everyone, but it is useful information that does not match the comment. ]

First, let's look at the compiled form side by side, If $c == 2 , the execution path of "zwei" is a pure superset of "eins". (Marked with a "*".)

 *1 <0> enter *1 <0> enter *2 <;> nextstate(main 4 -e:2) v:{ *2 <;> nextstate(main 4 -e:2) v:{ *3 <#> gvsv[*c] s *3 <#> gvsv[*c] s *4 <$> const[IV 1] s *4 <$> const[IV 1] s *5 <2> eq sK/2 *5 <2> eq sK/2 *6 <|> cond_expr(other->7) vK/1 *6 <|> cond_expr(other->7) vK/1 7 <0> enter v 7 <0> enter v 8 <;> nextstate(main 1 -e:3) v:{ 8 <;> nextstate(main 1 -e:3) v:{ 9 <$> const[IV 1] s 9 <$> const[IV 1] s a <#> gvsv[*d] sa <#> gvsv[*d] s b <2> sassign vKS/2 b <2> sassign vKS/2 c <@> leave vKP c <@> leave vKP goto d goto d *e <#> gvsv[*c] s *f <$> const[IV 2] s *g <2> eq sK/2 *h <|> and(other->i) vK/1 *e <0> enter v *i <0> enter v *f <;> nextstate(main 2 -e:6) v:{ *j <;> nextstate(main 2 -e:6) v:{ *g <$> const[IV 2] s *k <$> const[IV 2] s *h <#> gvsv[*d] s *l <#> gvsv[*d] s *i <2> sassign vKS/2 *m <2> sassign vKS/2 *j <@> leave vKP *n <@> leave vKP *d <@> leave[1 ref] vKP/REFC *d <@> leave[1 ref] vKP/REFC

The fact is that I can reproduce your results! (v5.16.0 for x86_64-linux-thread-multi)

  Rate drei eins zwei drei 8974033/s -- -3% -19% eins 9263260/s 3% -- -16% zwei 11034175/s 23% 19% -- Rate drei eins zwei drei 8971868/s -- -1% -21% eins 9031677/s 1% -- -20% zwei 11333871/s 26% 25% --

This is not entirely different (it may be the result of caching the processor), and it plays between different launches (so this is not another application that affects the benchmark). I'm at a dead end.

During the iteration, it takes 22 ns (1/9031677 s - 1/11333871 s) more to do 4 fewer operations. I expect it to take about 100 ns less.

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You will need to check the generated opcode for your specific Perl version. Interestingly, when launched from the debugger, the result is different:

  Win64, Activeperl 5.14.2, from debugger environment: Rate zwei eins drei zwei 130806/s -- -0% -1% eins 130957/s 0% -- -0% drei 131612/s 1% 1% --

Without a debugger, eins is faster (as you would expect from the code):

  Win64, Activeperl 5.14.2 : Rate drei zwei eins drei 3402015/s -- -5% -13% zwei 3585171/s 5% -- -8% eins 3916856/s 15% 9% --

Actually, I did not find a system here where zwei faster than eins :

  Linux x64, Perl 5.12.1: Rate drei zwei eins drei 2439279/s -- -13% -15% zwei 2797316/s 15% -- -3% eins 2875184/s 18% 3% --

Adding

After reading the ikegami post, I tried Strawberry 5.16 on Win64, and voilá:

  Perl v5.16.0, MSWin32-x64-multi-t Rate eins drei zwei eins 3954005/s -- -3% -10% drei 4084178/s 3% -- -7% zwei 4406707/s 11% 8% --

here we go, elsif/zwei faster .

So, this seems to be a 5.16.0 related issue?

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I think the best answer might be: "Who cares?"

This is micro benchmarking of the most premature kind. If profiling detects that the biggest reason for slowing down real code is to use else instead of elsif , then I have a very thin hat that I would not want to lose, but which I would devour in the blink of an eye.

Personally, am I writing $d = ($c == 1) ? 1 : 2 $d = ($c == 1) ? 1 : 2 and not caring about its performance.

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Source: https://habr.com/ru/post/917772/

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