Hard register problems

Article: Bulldozer Benchmarks, What Do They Mean?
By: Rob Thorpe (rthorpe.delete@this.robertthorpeconsulting.com), April 11, 2011 3:28 pm
Room: Moderated Discussions
Max (max@a.com) on 4/11/11 wrote:
---------------------------
...
>Most functions don't need to use the ABI. Even without whole program compilation,
>the compiler can ignore the ABI for static functions which don't have their address taken.
>
>This is more about code size than performance, though. Anything performance critical should be inlined anyway.

Yes that's right.

The problem with compilers though is that doing things like this is often very tricky. That's why many compilers use the ABI form even when they don't need to.

This is how I think about it...

Suppose you're writing a compiler for a (mostly) general register machine like an x86. In that case you can fill up registers mostly as you like and spill in the places where performance matters the least. Only the odd instruction frustrates that (such as integer multiply), and even then a little inefficiency for those cases isn't a huge problem.

But, functions and blocks of code are a different ball game. In some ways they're like instructions, they do something and return things in register and memory. But they can be a whole lot more complicated than instructions. A compiler can switch around how a block of code or a non-ABI function uses register and memory inputs and outputs. But, it has a huge range of options. Picking the right one requires heuristics.

The compilers that we're considering here use simple heuristics, they open up stack-frame slots for all of the variables at the beginning of the function, then allocate some to registers in the function. Then at the end they put results in registers and stack frame slots. That's a reasonable way of working, that will be high performance for many functions though not this one. Some then modify this by omitting stack frame for leaf-function where everything can be kept in memory.

Something like the old problem of non-general register allocation arises again. The compiler has a choice to create several different forms for a block of code that aren't general from the interface point of view. It must then glue them together without too much waste.
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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
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                He is an AMDZone lunatichorsefly2011/04/02 05:46 AM
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                  Post removedDavid Kanter2011/04/02 12:39 PM
          New article: Bulldozer benchmark analysisanonymous2011/04/04 05:08 AM
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            New article: Bulldozer benchmark analysisDavid Kanter2011/04/02 09:05 AM
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                New article: Bulldozer benchmark analysisDavid Kanter2011/04/02 12:36 PM
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                          AMD's Open64 compiler ?none2011/04/04 02:25 PM
                        AMD's Open64 compiler ?gallier22011/04/04 11:06 PM
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                                      C++ front-endsnone2011/04/10 07:36 AM
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                                                U+269BHeath Provost2011/04/20 10:26 AM
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                                                    Sorry again, Brendan (NT)Michael S2011/04/20 10:43 AM
                                                    Testing western non-us letters.Jouni Osmala2011/04/20 09:41 PM
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                                                                            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
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                                                                            File size optimizationsEduardoS2011/04/25 12:01 PM
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                                                                          File size optimizationsEduardoS2011/04/25 12:14 PM
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                                                                              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
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                    AMD's Open64 compiler ?Vincent Diepeveen2011/04/05 12:02 PM
    New article: Bulldozer benchmark analysisNaN2011/03/30 05:24 PM
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        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
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  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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