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 11:13 pm
Room: Moderated Discussions
EduardoS (no@spam.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.
>
>No, if they blindly follow the instructions given by the programmer they would
>create a prologue and an epilogue, like they did, they would have to be smart to
>realize that part of the function will be better without the prologue and epilogue.

I don't agree. We are talking about optimizing compilers. I gave the compiler precise information about the problem that the function "g_utf8_get_char" is trying to solve. The information provided by me to the compiler was:

(A) allocate me 1 variable (on the stack or in a register) and save all the registers that you need to save in order to allocate and initialize that variable

(B) consider the if-true path of the 1st if-then-else statement to be the most probable one

(C) in the if-false path of the 1st if-then-else statement, allocate me additional 4 variables (on the stack or in registers) that I am going to be using

All the "modern optimizing compilers" I tried (GCC, ICC, LLVM) have to some extent ignored all of these points. They actually, and on *purpose* (because that's how their "smart" algorithms work), forgot the information I gave there and decided that they know better. They chose to follow ungrounded fantasies, instead of following the information I provided.

ICC generated the best code, but unfortunately its register allocation algorithm and its stack management algorithm made suboptimal decisions.

For gods sake, GCC and ICC actually thought that converting a value of type "signed char" to type "unsigned char" is an actual operation for which you need to generate a CPU instruction! You have to agree that this is quite a shock.

If we compare the code generated by GCC/ICC/LLVM to the hypothetical code generated by an imperative optimizing C compiler that would be following my instructions, this almost brain-dead compiler would actually generate the best code. Maybe the generated code wouldn't be much faster on current OOO CPUs, but at least it is obvious that it would be the shortest.

NOTE: I realize that some of these issues have already been discussed here on RWT in the past, and maybe elsewhere in a bunch of other places. The shocking part is that after all the years that have passed, the situation has *not* improved. That's simply stagnation.

>>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 !!!
>
>They aren't trying to be smart, they are trying to put the prologue at beggining
>and the epilogue at the end, the problem here is that they aren't smart enough to
>realize the first part of the function doesn't need it.
>
>About the copy of eax/edx registers, we can't reach a conclusion why they did it
>without knowing how they generated the other part of the function, likely they "forgot"
>the first part is the most common.
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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
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                                          U+269B?2011/04/13 03:52 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
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                                                                            File size optimizationsRicardo B2011/04/25 08:22 AM
                                                                              ContinuationRicardo B2011/04/25 08:25 AM
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                                                                            File size optimizationsjrl2011/04/25 05:54 PM
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                                                                          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
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                                AMD's Open64 compiler ?none2011/04/10 07:23 AM
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                                    AMD's Open64 compiler ?EduardoS2011/04/12 02:55 PM
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