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Re: Any ideas on faster TLS ?

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Alex <alex@rivadpm.com> writes:
>The problem is surely the double indirection. For a "standard" variable, 
>I generate
>
>variable x  ok
>: foo 10 x ! ;  ok
>  ok
>see foo
>: foo ( ? -- ? )
>\ foo is defined in (console)
>\ call compiled
>\ code=$41CA5E len=10 type=130
>( $0 )    mov     dword { $80537C } $A  \ C7057C5380000A000000
>( $A )    ret                           \ C3 ( end ) ok
>
>whereas a TLS variable based on a register (eg EBX) (a "user" variable) 
>generates (the offset of Y up the TLS is $8C)
>
>0 user y  ok
>: bar 10 y ! ;  ok
>see bar
>: bar ( ? -- ? )
>\ bar is defined in (console)
>\ call compiled
>\ code=$41CAC8 len=10 type=129
>( $0 )    mov     dword { $8C ebx } $A  \ C7838C0000000A000000
>( $A )    ret                           \ C3 ( end ) ok
>
>If the TLS is addressed by FS: then it would be (and this code hand 
>cranked, because my Forth doesn't generate this);
>
>code tls  ok
>mov ecx { fs: $e10 } \ first TLS slot in the TEB at $E10  ok
>mov { ecx $8c } $a   \ offset $8C for the TLS var  ok[1]
>next;  ok
>see tls
>code tls ( ? -- ? )
>\ tls is defined in (console)
>\ call compiled
>\ code=$41CA4F len=17 type=129
>( $0 )    mov     ecx dword { fs: $E10 }   \ 648B0D100E0000
>( $7 )    mov     dword { $8C ecx } $A     \ C7818C0000000A000000
>( $11 )   ret                              \ C3 ( end ) ok
>
>That's a lot more expensive; every reference is now twice through memory 
>and with an FS segreg overide on one of the.

It's not clear why you would code it like this.  I looked what gcc
generates for

volatile __thread long a;

int main()
{
  long l=0;
  long i;
  for (i=0; i<100000000; i++)
    a=10;
  return 0;
}

and the inner loop becomes

   5: movq   $0xa,%fs:0x0
  12: sub    $0x1,%rax
  16: jne    5 <main+0x5>

(the 0x0 is later filled with something sensible by the linker).  For
this loop I see 1.00-1.17 cycles/iteration on a Haswell
(1 cycle/iteration is the limit, because the Haswell can only perform
one store per cycle).

Interestingly, without the "__thread" this becomes often slower in 64-bit
mode, with the inner loop looking as follows:

   5: movq   $0xa,0x0(%rip)        # 10 <main+0x10>
  10: sub    $0x1,%rax
  14: jne    5 <main+0x5>

With this I see 1.07-1.43 cycles per iteration.

The large variation in performance for both variants is interesting.

- anton
-- 
M. Anton Ertl  http://www.complang.tuwien.ac.at/anton/home.html
comp.lang.forth FAQs: http://www.complang.tuwien.ac.at/forth/faq/toc.html
     New standard: http://www.forth200x.org/forth200x.html
   EuroForth 2016: http://www.euroforth.org/ef16/

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Thread

Any ideas on faster TLS ? Ron Aaron <rambamist@gmail.com> - 2017-02-27 07:30 +0200
  Re: Any ideas on faster TLS ? Andrew Haley <andrew29@littlepinkcloud.invalid> - 2017-02-27 04:01 -0600
    Re: Any ideas on faster TLS ? Ron Aaron <rambamist@gmail.com> - 2017-02-27 13:19 +0200
      Re: Any ideas on faster TLS ? Alex <alex@rivadpm.com> - 2017-02-27 12:56 +0000
        Re: Any ideas on faster TLS ? Ron Aaron <rambamist@gmail.com> - 2017-02-27 15:10 +0200
        Re: Any ideas on faster TLS ? albert@cherry.spenarnc.xs4all.nl (Albert van der Horst) - 2017-02-27 17:08 +0100
      Re: Any ideas on faster TLS ? Andrew Haley <andrew29@littlepinkcloud.invalid> - 2017-02-27 08:06 -0600
        Re: Any ideas on faster TLS ? Ron Aaron <rambamist@gmail.com> - 2017-02-27 16:10 +0200
          Re: Any ideas on faster TLS ? Andrew Haley <andrew29@littlepinkcloud.invalid> - 2017-02-27 09:31 -0600
            Re: Any ideas on faster TLS ? Ron Aaron <rambamist@gmail.com> - 2017-02-28 07:05 +0200
            Re: Any ideas on faster TLS ? Paul Rubin <no.email@nospam.invalid> - 2017-02-27 23:15 -0800
              Re: Any ideas on faster TLS ? Andrew Haley <andrew29@littlepinkcloud.invalid> - 2017-02-28 04:02 -0600
                Re: Any ideas on faster TLS ? Bernd Paysan <bernd@net2o.de> - 2017-03-11 19:58 +0000
      Re: Any ideas on faster TLS ? anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2017-03-01 12:36 +0000
        Re: Any ideas on faster TLS ? Ron Aaron <rambamist@gmail.com> - 2017-03-01 15:11 +0200
          Re: Any ideas on faster TLS ? Alex <alex@rivadpm.com> - 2017-03-03 01:18 +0000
            Re: Any ideas on faster TLS ? anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2017-03-03 12:55 +0000
              Re: Any ideas on faster TLS ? Alex <alex@rivadpm.com> - 2017-03-03 14:59 +0000
                Re: Any ideas on faster TLS ? anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2017-03-03 16:15 +0000
          Optimizing (was: Any ideas on faster TLS ?) Ron Aaron <rambamist@gmail.com> - 2017-03-17 13:14 +0200
  Re: Any ideas on faster TLS ? rickman <gnuarm@gmail.com> - 2017-02-28 01:13 -0500
    Re: Any ideas on faster TLS ? Paul Rubin <no.email@nospam.invalid> - 2017-02-27 22:48 -0800
      Re: Any ideas on faster TLS ? rickman <gnuarm@gmail.com> - 2017-02-28 03:50 -0500

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