Imperative compilers (and the lack of it)

Article: Bulldozer Benchmarks, What Do They Mean?
By: ? (0xe2.0x9a.0x9b.delete@this.gmail.com), April 10, 2011 5:30 am
Room: Moderated Discussions
Paul (no@thanks.com) on 4/10/11 wrote:
---------------------------
>? (0xe2.0x9a.0x9b@gmail.com) on 4/10/11 wrote:
>---------------------------
>>Bottom line: if any of the compilers would just *blindly* follow the instructions
>>given by the programmer, it would actually generate optimal code in the case of
>>this particular function. But no, that's not the way how compilers work - they are
>>trying to be "smart" - but unfortunately, using those "smart" algorithms can be
>worse than using no algorithm at all !!!
>
>However most of the time compilers are asked to compile low quality code where
>blindly following what was asked for would lead to horrendously slow code.

I agree that compilers have rightful place in the world and are needed. It would be for example very time consuming and error-prone to do register allocation by hand, function inlining by hand, etc.

But I am reluctant to admit that existing production compilers can raise the quality of code from a performance perspective. The difference in the run-time performance of a code generated by a simple optimizing compiler and a code generated by an "advanced" optimizing compiler like GCC/ICC/etc is not very large. The difference is only around 1.3 or something.

It isn't very far from truth that there haven't been any major advancements in compiler technology in the last 10-20 years. If you compare the old Watcom C compiler with the newest GCC 4.6.0 (assuming you succeed to compile the code with both of the compilers), do you think you will measure a *significant* difference in performance of the generated code? I seriously doubt it.

>For example
>I recently encountered some code which performed the same multiplication 6 times
>in side a loop. The compiler was smart enough to notice this this, precompute it and pull it out of the loop.

Well, yes. But unless the compiler was able to determine the number of loop iterations (statically, or by profiling a previous run of the program), it was a heuristic decision. In the case of your program, the heuristic decision was probably the correct decision.

So, you were lucky. However, in other cases you might not be so lucky. Pulling the 6 expressions (A*B) out of the loop means that the program will compute (A*B) before it determines how many iterations the loop has. If the loop body is typically executed 0 times, the compiler's heuristic would have produced slower code.

The heuristic could also fail if some or all of the 6 expressions (A*B) were located in the bodies of if-then-else statements.

>Now you can argue as much as you like about programmers writing optimal code to
>start off with but the fact of the matter is that most code is crap which is why
>we have compilers which try to make the most of the code and honking great big dynamic
>frontends on processors which try to make the most of what the compiler spits out.
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New article: Bulldozer benchmark analysisDavid Kanter2011/03/30 02:27 AM
  New article: Bulldozer benchmark analysisNaN2011/03/30 07:47 AM
    New article: Bulldozer benchmark analysisDavid Kanter2011/03/30 09:25 AM
      New article: Bulldozer benchmark analysisdc2011/03/30 12:07 PM
      New article: Bulldozer benchmark analysisFoo_2011/03/30 12:17 PM
        New article: Bulldozer benchmark analysisDavid Kanter2011/03/30 01:34 PM
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            BD won't "kick in" until 2.5GHzDavid Kanter2011/03/31 09:21 AM
              He is an AMDZone lunaticNIKOLAS2011/04/01 06:40 AM
                See how he behaves hereDavid Kanter2011/04/01 08:08 AM
                He is an AMDZone lunatichorsefly2011/04/02 05:46 AM
                  This is not relevantDavid Kanter2011/04/02 09:16 AM
                  Post removedDavid Kanter2011/04/02 12:39 PM
          New article: Bulldozer benchmark analysisanonymous2011/04/04 05:08 AM
      New article: Bulldozer benchmark analysisinterested2011/04/01 03:28 PM
        New article: Bulldozer benchmark analysisDavid Kanter2011/04/01 03:53 PM
          New article: Bulldozer benchmark analysisNaN2011/04/02 07:12 AM
            New article: Bulldozer benchmark analysisDavid Kanter2011/04/02 09:05 AM
              New article: Bulldozer benchmark analysisNaN2011/04/02 09:24 AM
                New article: Bulldozer benchmark analysisDavid Kanter2011/04/02 12:36 PM
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                    AMD's Open64 compiler ?David Kanter2011/04/04 08:19 AM
                      AMD's Open64 compiler ?Richard Cownie2011/04/04 09:52 AM
                        AMD's Open64 compiler ?David Kanter2011/04/04 01:20 PM
                          AMD's Open64 compiler ?none2011/04/04 02:25 PM
                        AMD's Open64 compiler ?gallier22011/04/04 11:06 PM
                          thanks, very interesting (NT)Richard Cownie2011/04/05 03:44 AM
                          AMD's Open64 compiler ?Andreas Stiller2011/04/05 08:23 AM
                        AMD's Open64 compiler ?Vincent Diepeveen2011/04/06 01:11 AM
                      AMD's Open64 compiler ?Vincent Diepeveen2011/04/06 01:01 AM
                        AMD's Open64 compiler ?none2011/04/06 01:41 AM
                          AMD's Open64 compiler ?Vincent Diepeveen2011/04/07 02:02 PM
                            AMD's Open64 compiler ?Brett2011/04/07 04:55 PM
                              AMD's Open64 compiler ?Megol2011/04/09 11:03 AM
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                                            Clang+LLVMLinus Torvalds2011/04/13 04:40 PM
                                              Clang+LLVManon2011/04/13 07:31 PM
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                                                    Clang+LLVMJouni Osmala2011/04/14 09:54 PM
                                                  Clang+LLVM?2011/04/14 11:56 PM
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                                                      Clang+LLVM?2011/04/15 10:11 PM
                                                        Clang+LLVMLinus Torvalds2011/04/16 08:10 AM
                                                          Clang+LLVMLinus Torvalds2011/04/16 08:32 AM
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                                                            Clang+LLVMEduardoS2011/04/17 12:38 PM
                                                              Clang+LLVMLinus Torvalds2011/04/17 02:00 PM
                                                                Competing for speed?2011/04/17 10:57 PM
                                                                  Competing for speedLinus Torvalds2011/04/18 09:07 AM
                                                                    Competing for speedanon2011/04/18 05:38 PM
                                                                      Competing for speedLinus Torvalds2011/04/18 07:24 PM
                                                                        Competing for speedanon2011/04/19 05:05 AM
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                                                                            Competing for speedLinus Torvalds2011/04/22 11:09 AM
                                                                    Competing for speed?2011/04/18 11:45 PM
                                                                      Competing for speedLinus Torvalds2011/04/19 06:41 AM
                                                                        Competing for speedLinus Torvalds2011/04/19 07:46 AM
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                                                                          Oh ... the ironyLinus Torvalds2011/04/19 01:49 PM
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                                                                              What's the speedup?Linus Torvalds2011/04/20 06:48 AM
                                                                            Oh ... the irony?2011/04/23 05:52 AM
                                                                          Oh ... the ironyDougal2011/04/20 06:23 AM
                                                                            Oh ... the irony?2011/04/25 12:05 PM
                                                                              Oh ... the ironyDougal2011/04/26 05:22 AM
                                                                                Oh ... the irony?2011/04/27 12:14 AM
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                                                    Clang+LLVManon2011/04/16 03:01 PM
                                                      Clang+LLVMRob Thorpe2011/04/18 04:53 AM
                                                        Clang+LLVManon2011/04/18 05:28 PM
                                        Clang+LLVMPaul2011/04/14 10:43 AM
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                              AMD's Open64 compiler ?Paul2011/04/10 02:50 AM
                                C++ front-endsDavid Kanter2011/04/10 06:29 AM
                                  C++ front-endsPaul2011/04/10 07:19 AM
                                    C++ front-endsanonymous2011/04/10 07:28 AM
                                      C++ front-endsnone2011/04/10 07:36 AM
                                      C++ front-endsPaul2011/04/10 07:44 AM
                                    C++ front-endsMichael S2011/04/10 07:47 AM
                                      C++ front-endsnone2011/04/10 07:51 AM
                                      C++ front-endsRob Thorpe2011/04/10 11:13 AM
                                        C++ front-endsPaul2011/04/11 10:07 AM
                                AMD's Open64 compiler ?Konrad Schwarz2011/04/14 01:06 AM
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                                          Testing Clang + LLVMahnoun2011/04/20 03:13 PM
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                                        Testing Clang + LLVMnone2011/04/21 02:36 AM
                                        Testing Clang + LLVMahnoun2011/04/21 11:02 AM
                                          Testing Clang + LLVMMichael S2011/04/21 12:01 PM
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                              Imperative compilers (and the lack of it)?2011/04/10 12:36 AM
                                Imperative compilers (and the lack of it)?2011/04/10 12:43 AM
                                Imperative compilers (and the lack of it)Paul2011/04/10 02:37 AM
                                  Imperative compilers (and the lack of it)?2011/04/10 05:30 AM
                                  Imperative compilers (and the lack of it)Vincent Diepeveen2011/04/10 06:20 AM
                                  Imperative compilers (and the lack of it)Gabriele Svelto2011/04/11 08:24 AM
                                  Imperative compilers (and the lack of it)Michael S2011/04/12 12:12 AM
                                    Imperative compilers (and the lack of it)?2011/04/12 01:41 AM
                                      Imperative compilers (and the lack of it)EduardoS2011/04/12 03:13 PM
                                        U+269BMichael S2011/04/13 12:45 AM
                                          U+269B?2011/04/13 03:52 AM
                                            U+269BBrendan2011/04/20 07:18 AM
                                              U+269BMichael S2011/04/20 09:11 AM
                                                U+269BHeath Provost2011/04/20 10:26 AM
                                                  You are right, Heath. Sorry, Brannon.Michael S2011/04/20 10:42 AM
                                                    Sorry again, Brendan (NT)Michael S2011/04/20 10:43 AM
                                                    Testing western non-us letters.Jouni Osmala2011/04/20 09:41 PM
                                                      Those show up fine to me (NT)David Kanter2011/04/21 07:48 AM
                                                      Testing western non-us letters.Heath Provost2011/04/21 11:15 AM
                                                        Testing western non-us letters.EduardoS2011/04/22 02:14 PM
                                                          Testing western non-us letters.Heath Provost2011/04/22 03:21 PM
                                                            Testing western non-us letters.EduardoS2011/04/22 04:24 PM
                                                            We actually upgraded to 2008 R2David Kanter2011/04/22 10:23 PM
                                                              We actually upgraded to 2008 R2EduardoS2011/04/22 10:33 PM
                                                                We actually upgraded to 2008 R2rwessel2011/04/22 11:11 PM
                                                                  We actually upgraded to 2008 R2David Kanter2011/04/22 11:28 PM
                                                                    We actually upgraded to 2008 R2Ricardo B2011/04/23 09:43 AM
                                                                      We actually upgraded to 2008 R2David Kanter2011/04/23 10:31 AM
                                                                    We actually upgraded to 2008 R2sparkplug2011/04/23 08:25 PM
                                                                      We actually upgraded to 2008 R2David Kanter2011/04/24 03:36 PM
                                                                        We actually upgraded to 2008 R2anon2011/04/24 06:32 PM
                                                                          We actually upgraded to 2008 R2David Kanter2011/04/24 11:12 PM
                                                                        We actually upgraded to 2008 R2EduardoS2011/04/24 08:10 PM
                                                                          We actually upgraded to 2008 R2David Kanter2011/04/24 11:04 PM
                                                                        We actually upgraded to 2008 R2anonymous2011/04/24 08:44 PM
                                                                          We actually upgraded to 2008 R2David Kanter2011/04/24 10:57 PM
                                                                        File size optimizationsjrl2011/04/25 02:03 AM
                                                                          File size optimizationsRicardo B2011/04/25 07:14 AM
                                                                            File size optimizationsjrl2011/04/25 05:52 PM
                                                                          File size optimizationsDavid Kanter2011/04/25 07:49 AM
                                                                            File size optimizationsRicardo B2011/04/25 08:22 AM
                                                                              ContinuationRicardo B2011/04/25 08:25 AM
                                                                              File size optimizationsChristian Packmann2011/04/25 09:56 AM
                                                                                File size optimizationsEduardoS2011/04/25 11:57 AM
                                                                                  File size optimizationsRicardo B2011/04/25 01:01 PM
                                                                            File size optimizationsEduardoS2011/04/25 12:01 PM
                                                                            Javascript is at the root of the latency problemjrl2011/04/25 06:03 PM
                                                                          File size optimizationsEduardoS2011/04/25 12:14 PM
                                                                            File size optimizationsrwessel2011/04/25 12:49 PM
                                                                              File size optimizationsEduardoS2011/04/25 12:57 PM
                                                                            File size optimizationsjrl2011/04/25 05:54 PM
                                                                              A simple approachSilent2011/04/25 09:24 PM
                                                                                Show Thread exists but the default is Yes (NT)Silent2011/04/25 09:26 PM
                                                                          File size optimizationsHeath Provost2011/04/25 02:02 PM
                                    Imperative compilers (and the lack of it)Azazel2011/04/12 08:23 PM
                                Imperative compilers (and the lack of it)none2011/04/10 06:00 AM
                                  Imperative compilers (and the lack of it)dmsc2011/04/10 06:39 AM
                                  Imperative compilers (and the lack of it)EduardoS2011/04/10 12:55 PM
                                compiler failure rateshobold2011/04/10 10:55 AM
                                  compiler failure rates?2011/04/10 11:13 PM
                                    compiler failure rateshobold2011/04/11 05:07 AM
                                Imperative compilers (and the lack of it)EduardoS2011/04/10 12:55 PM
                                  Imperative compilers (and the lack of it)?2011/04/10 11:13 PM
                                    Imperative compilers (and the lack of it)Konrad Schwarz2011/04/14 01:28 AM
                                      Imperative compilers (and the lack of it)gallier22011/04/14 04:47 AM
                                      Imperative compilers (and the lack of it)EduardoS2011/04/14 02:04 PM
                                        Imperative compilers (and the lack of it)Konrad Schwarz2011/04/15 03:43 AM
                                Hard register problemsRob Thorpe2011/04/11 12:14 AM
                                  Hard register problemsMax2011/04/11 03:05 PM
                                    Hard register problemsRob Thorpe2011/04/11 03:28 PM
                                Imperative compilers (and the lack of it)gallier22011/04/11 09:26 AM
                                  I wonder why.j2011/04/11 10:39 PM
                                    I wonder why.?2011/04/11 11:44 PM
                                Imperative compilers (and the lack of it)nksingh2011/04/12 11:00 PM
                                  Imperative compilers (and the lack of it)?2011/04/12 11:23 PM
                              AMD's Open64 compiler ?Vincent Diepeveen2011/04/10 06:06 AM
                                AMD's Open64 compiler ?none2011/04/10 07:23 AM
                                  AMD's Open64 compiler ?Eugene Nalimov2011/04/11 03:57 PM
                                    AMD's Open64 compiler ?EduardoS2011/04/12 02:55 PM
                                AMD's Open64 compiler ?Megol2011/04/10 11:05 PM
                                  AMD's Open64 compiler ?Vincent Diepeveen2011/04/20 12:52 PM
                                    Yap transform? What is it? (NT)Michael S2011/04/21 09:04 AM
                                      Yap transform? What is it? (NT)Vincent Diepeveen2011/04/22 02:56 AM
                    AMD's Open64 compiler ?Vincent Diepeveen2011/04/05 12:02 PM
    New article: Bulldozer benchmark analysisNaN2011/03/30 05:24 PM
      New article: Bulldozer benchmark analysisChris2011/04/05 03:33 AM
  New article: Bulldozer benchmark analysisdc2011/03/30 08:09 AM
    New article: Bulldozer benchmark analysisMS2011/03/30 08:53 AM
      New article: Bulldozer benchmark analysisDaniel Bizo2011/03/31 12:20 AM
        New article: Bulldozer benchmark analysisanun2011/03/31 02:48 AM
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      New article: Bulldozer benchmark analysisanon2011/03/31 08:29 AM
        AMD's memory controllers are fineDavid Kanter2011/03/31 09:27 AM
          AMD's memory controllers are fineanon2011/04/05 11:24 AM
            AMD's memory controllers are fineanon2011/04/05 04:52 PM
              AMD's memory controllers are fineVincent Diepeveen2011/04/06 01:36 AM
                AMD's memory controllers are fineanon2011/04/06 06:59 AM
                  AMD's memory controllers are fineVincent Diepeveen2011/04/07 02:15 PM
                    AMD's memory controllers are fineVincent Diepeveen2011/04/07 02:27 PM
                      AMD's memory controllers are fineanon2011/04/07 02:57 PM
                    AMD's memory controllers are fineanon2011/04/07 02:54 PM
        New article: Bulldozer benchmark analysisAzazel2011/03/31 09:54 AM
        New article: Bulldozer benchmark analysisAlex2011/03/31 11:38 PM
      New article: Bulldozer benchmark analysisanon2011/04/01 03:23 AM
    Streaming conflicts can lead to performance degradationsgc2011/04/08 06:35 AM
  AMD's probe filterAndreas Stiller2011/04/05 08:37 AM
  a waste of timeMoritz2011/04/09 07:01 AM
    a waste of timehorsefly2011/04/09 03:34 PM
      a waste of timeDavid Kanter2011/04/09 06:56 PM
        a waste of timehorsefly2011/04/10 05:13 AM
          InvestorsDavid Kanter2011/04/10 06:27 AM
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