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Re: code quality, was Good practical language and OS agnostic text?

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BartC schrieb:
> "Hans-Peter Diettrich" <DrDiettrich1@aol.com> wrote in message
>> Life is too short for writing an full-blown heavily-optimizing
>> production compiler from scratch, including its whole RTL.
>
> Especially when there might only be difference of 2 or 3 times between
> performance of the best and worst code.
>
> My own compiler for x86-32 generates pretty awful code, and on a small
> handful of mostly numeric benchmarks, it averages out about 2.5 x as
> slow as gcc on it's highest optimisation setting. But, gcc often
> recognises these benchmarks as doing nothing useful, so removes whole
> sections of code!

I never trust benchmarks, in detail when supplied by compiler vendors :-]

When we had several workstations for evaluation, the AIX workstation
came with a benchmark program executing in about 3 seconds on that
machine. On other machines the time ranged from 8 minutes to more than
an hour.

When I added a printf after the loop in the benchmark code, it also took
about 8 minutes! The benchmark turned out to test the compiler *default*
optimization settings, where the HP station was so incredibly slow
because it had turned off *any* optimization by default.

 From that experience I also learned what optimization means in
practice. A dumb compiler, or with all optimizations disabled, may fetch
every expression operand from memory, and write back every intermediate
result (SSA?), which is nice when debugging some code. Thus *register
allocation* turned out to be a basic optimization of high value - I
wonder how somebody could ever design a CPU (with more than 8 bit
registers) with as few registers as available on most Intel processors.
Dead code elimination and moving loop-invariant computations out of
loops are the next optimizations to consider. And CSE...

Another compiler, for an M68000, turned out to be a very quick&dirty
port of an M6800 compiler. According to "an int is a pointer, a pointer
is an int" it kept all operands in the M68K *address* registers, so that
every logical or arithmetic operation had to be done in a subroutine,
which moved the operands from A0 and A1 into *data* registers, evaluated
and stored back the result into A0. The caller had to move the operands
into A0 and A1 before, of course. In my test programs every single C
statement resulted in about 3 pages of assembler code! [1]

But despite that horrible code generation, this compiler came with a
very powerful graphics and window software, with amazing runtime
behaviour. This observation again relativates the need for optimization,
at least in GUI applications.


[1] This compiler, and my rudimentary knowledge of C and compilers and
runtime libraries, made me write my first C decompiler in the late 80's.
Of course in Basic, which was the language I had used on all my
homecomputers before. Hereby I learned more and more about bad and good
practices in compiler writing, encountering any number of workarounds
for adopting old 8-bit compilers to 16/32 bit code and memory layouts.

DoDi

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Good practical language and OS agnostic text? compilers@is-not-my.name - 2012-04-17 21:28 +0000
  Re: Good practical language and OS agnostic text? Philip Herron <redbrain@gcc.gnu.org> - 2012-04-18 14:25 +0100
    Re: Good practical language and OS agnostic text? compilers@is-not-my.name - 2012-04-19 16:32 +0000
      Re: Good practical language and OS agnostic text? arnold@skeeve.com (Aharon Robbins) - 2012-04-20 03:58 +0000
        Re: Good practical language and OS agnostic text? compilers@is-not-my.name - 2012-04-22 10:10 +0000
      Re: Good practical language and OS agnostic text? "BartC" <bc@freeuk.com> - 2012-04-20 09:45 +0100
        Re: Good practical language and OS agnostic text? "Jonathan Thornburg" <jthorn@astro.indiana.edu> - 2012-04-21 15:04 +0000
  Re: Good practical language and OS agnostic text? BGB <cr88192@hotmail.com> - 2012-04-18 08:39 -0700
    Re: Good practical language and OS agnostic text? compilers@is-not-my.name - 2012-04-19 17:32 +0000
  Re: Good practical language and OS agnostic text? Alain Ketterlin <alain@dpt-info.u-strasbg.fr> - 2012-04-18 18:24 +0200
    Re: Good practical language and OS agnostic text? Hans-Peter Diettrich <DrDiettrich1@aol.com> - 2012-04-19 13:53 +0200
      Re: Good practical language and OS agnostic text? compilers@is-not-my.name - 2012-04-21 03:07 +0000
      Re: Good practical language and OS agnostic text? "BartC" <bc@freeuk.com> - 2012-04-21 12:01 +0100
        Re: code quality, was Good practical language and OS agnostic text? Hans-Peter Diettrich <DrDiettrich1@aol.com> - 2012-04-22 12:41 +0200
    Re: Good practical language and OS agnostic text? compilers@is-not-my.name - 2012-04-19 11:31 +0000
      Re: Good practical language and OS agnostic text? "Jonathan Thornburg" <jthorn@astro.indiana.edu> - 2012-04-20 16:19 +0000
  Re: Good practical language and OS agnostic text? "Derek M. Jones" <derek@knosof.co.uk> - 2012-04-18 18:16 +0100
    Re: Good practical language and OS agnostic text? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2012-04-18 22:43 +0000
      Re: Good practical language and OS agnostic text? BGB <cr88192@hotmail.com> - 2012-04-19 00:05 -0700
      Re: Good practical language and OS agnostic text? compilers@is-not-my.name - 2012-04-19 11:31 +0000
    Re: Good practical language and OS agnostic text? compilers@is-not-my.name - 2012-04-19 16:32 +0000
  Re: Good practical language and OS agnostic text? compilers@is-not-my.name - 2012-04-18 19:30 +0000
    Re: Good practical language and OS agnostic text? "BartC" <bc@freeuk.com> - 2012-04-19 18:43 +0100
  Re: Good practical language and OS agnostic text? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2012-04-18 20:29 +0000
    Re: Good practical language and OS agnostic text? Hans-Peter Diettrich <DrDiettrich1@aol.com> - 2012-04-19 14:20 +0200
    Re: Good practical language and OS agnostic text? compilers@is-not-my.name - 2012-04-19 19:05 +0000
      Re: Good practical language and OS agnostic text? Uli Kusterer <ulimakesacompiler@googlemail.com> - 2012-04-21 11:30 +0200
  Re: Good practical language and OS agnostic text? Roberto Waltman <usenet@rwaltman.com> - 2012-04-18 22:00 -0400
    Re: Good practical language and OS agnostic text? compilers@is-not-my.name - 2012-04-19 11:31 +0000
      Re: Good practical language and OS agnostic text? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2012-04-20 07:02 +0000
        Re: Good practical language and OS agnostic text? compilers@is-not-my.name - 2012-04-22 11:10 +0000
          Re: Good practical language and OS agnostic text? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2012-04-22 23:56 +0000
        Re: PL/360, was Good practical language and OS agnostic text? ArarghMail204@Arargh.com - 2012-04-24 19:13 -0500
  Re: Good practical language and OS agnostic text? Bakul Shah <usenet@bitblocks.com> - 2012-04-18 21:15 -0700
    Re: Good practical language and OS agnostic text? compilers@is-not-my.name - 2012-04-20 16:06 +0000
  Re: Good practical language and OS agnostic text? torbenm@diku.dk (Torben Ægidius Mogensen) - 2012-04-19 14:58 +0200
    Re: Good practical language and OS agnostic text? compilers@is-not-my.name - 2012-04-20 16:06 +0000
  Re: Good practical language and OS agnostic text? "Joe Schmo" <askmeforit@myisp.com> - 2012-04-21 02:53 -0600
    Re: Writing parsers, was Good practical language and OS agnostic text? Uli Kusterer <ulimakesacompiler@googlemail.com> - 2012-04-22 16:18 +0200
    Re: Good practical language and OS agnostic text? compilers@is-not-my.name - 2012-04-23 19:12 +0000
  Re: Good practical language and OS agnostic text? Uli Kusterer <ulimakesacompiler@googlemail.com> - 2012-04-21 11:22 +0200
    Re: Good practical language and OS agnostic text? BGB <cr88192@hotmail.com> - 2012-04-21 18:58 -0700
      Re: writing interpreters, was Good practical language and OS agnostic text? Uli Kusterer <ulimakesacompiler@googlemail.com> - 2012-04-22 12:53 +0200
        Re: writing interpreters, was Good practical language and OS agnostic text? BGB <cr88192@hotmail.com> - 2012-04-22 12:29 -0700
      Re: generating bytecode, was Good practical language and OS agnostic text? Uli Kusterer <ulimakesacompiler@googlemail.com> - 2012-04-22 13:12 +0200
      Re: Recursive descent parsing and optimization, was Good practical language and OS agnostic text? "BartC" <bc@freeuk.com> - 2012-04-22 12:51 +0100
        Re: Recursive descent parsing and optimization, was Good practical language and OS agnostic text? "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> - 2012-04-22 18:18 +0200
          Re: Recursive descent parsing and optimization, was Good practical language and OS agnostic text? "Bartc" <bartc@freeuk.com> - 2012-04-23 10:59 +0100
        Re: Recursive descent parsing and optimization, was Good practical language and OS agnostic text? BGB <cr88192@hotmail.com> - 2012-04-22 13:45 -0700
    Re: Good practical language and OS agnostic text? compilers@is-not-my.name - 2012-04-22 22:11 +0000
      Re: Good practical language and OS agnostic text? "BartC" <bc@freeuk.com> - 2012-04-23 18:41 +0100
  Re: Good practical language and OS agnostic text? basile@starynkevitch.net - 2012-05-02 22:16 -0700
    Re: Good practical language and OS agnostic text? Johann 'Myrkraverk' Oskarsson <johann@2ndquadrant.com> - 2012-06-06 16:52 +0000

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