6 ms·

Is there a way to make the compiler list the applied optimizations for a specific function, in e.g. the .lst output? Would be very helpful when the compiler is

by m_eiman 10y ago

Is there a way to make the compiler list the applied optimizations for a specific function, in e.g. the .lst output? Would be very helpful when the compiler is doing something creative and you can't figure out how to make it do what you intended.

adrianN 10y ago
You could start by not relying on undefined behavior ;)
- mpweiher 10y ago
  Compiler writers could/should start by not treating undefined behavior as blank checks. This is so idiotic, and no, it's not a bug in Cap'n'Proto, it is very definitely a bug in the compiler (and/or the current version of the C spec. if it actually allows this).
  - frederikvs 10y ago
    The C standard allows the compiler to do this, so it's definitely not a bug in the compiler.
    - mpweiher 10y ago
      As I wrote earlier: I had a C compiler before there was C standard. So by your definition, this compiler could not have bugs? As an exercise, try to express the difference between "standards-compliant" and "bug-free".
      - kutkloon7 10y ago
        I think the main point is that if there is a standard, an implementation should comply. If there is no standard, you have to use intuition and common sense to say what's a bug. Different people may have different expectations, so bugs become subjective. Ideally, there is a water-tight standard. In this case every deviation from the standard is a bug and vice versa.
        mpweiher 10y ago
        > if there is a standard, an implementation should comply Yes. However, we are talking specifically about the parts where the standard is undefined. The standard most emphatically does not require a compiler to do these crazy optimizations that remove safety critical code.
  - unwind 10y ago
    So rewriting code in order to "do what I mean" makes sense, since not doing what you mean is a bug in the language itself? Not sure that's a very productive approach. The spec is what it is, and it's of course rather deliberately done the way it is in order to make it possible for compilers to optimize and extract more performance.
    - mpweiher 10y ago
      > "do what I mean" Well, the code is right there, there is no magic "do what I mean" needed. Just don't magically remove code that I wrote. > spec is [..] rather deliberately done the way it is in order to make it possible for compilers to optimize and extract more performance Yes, and it is wrong. Breaking existing programs with magic that simply removes code that is clearly there in order to eke out a bit of performance is wrong.
      - masklinn 10y ago
        > Well, the code is right there, there is no magic "do what I mean" needed. Just don't magically remove code that I wrote. You can already do that: use a non-optimising compiler (tcc and the like) or compile without optimisations.
      - staticassertion 10y ago
        > Just don't magically remove code that I wrote. Compile with O0. > Yes, and it is wrong. Breaking existing programs with magic that simply removes code that is clearly there in order to eke out a bit of performance is wrong. Well welcome to writing C? It's been this way for a long time and that won't change - UB is critical to performance and doesn't always lead to unsafety. If you're programming in C, don't blame the compiler when your UB leads to remote code exec, you should have known what you were getting into.
        mpweiher 10y ago
        > Well welcome to writing C? I've been writing C for 30 years. > UB is critical to performance Not really, no. See "What every compiler writer should know about programmers or “Optimization” based on undefined behaviour hurts performance" (referenced elsewhere in this thread) http://www.complang.tuwien.ac.at/kps2015/proceedings/KPS_2015_submission_29.pdf http://www.complang.tuwien.ac.at/kps2015/proceedings/KPS_201...
- patrec 10y ago
  You mean like approximately zero percent of non-trivial C or C++ programs in actual existence?
  - adrianN 10y ago
    You could of course switch to Ada.
    - masklinn 10y ago
      Except Ada has UB (they used to call them bounded errors, but the semantics are basically identical), and reading the musings of ADAists[0] > I would say, having looked at hundreds of thousands of lines of Ada, that I have had a disappointingly frequent experience in finding the UNCHECKED_CONVERSION generic used in production Ada code. > I agree that this is often a mark of poor craftsmanship and unhesitatingly discourage its use, but it is there [0] https://www.cs.york.ac.uk/hise/safety-critical-archive/2011/0216.html https://www.cs.york.ac.uk/hise/safety-critical-archive/2011/...
      - adrianN 10y ago
        If finding potential UB in Ada programs is as simple as grepping for unchecked_conversion, then that's a big step forward compared to C, don't you think?
        masklinn 10y ago
        1. your claim was essentially that ADA = no UB, so that's hardly relevant 2. that's not Ada's only UB, the 2005 spec has 35 or 36, which granted is an improvement over C's circa 200, but a far cry from being UB free
        adrianN 10y ago
        The claim is that it's impossible to avoid UB in real world C, but not impossible in real world $SAFERLANGUAGE, with Ada as an example. Any language that allows you to poke around in memory (a feature you need for embedded systems) will contain unsafe constructs. The question is whether you can tell by looking whether a particular piece of code is safe or not.
chrisseaton 10y ago
You can ask Clang to print the compiler IR after each optimisation, but you would need to know that you were looking for something before that was useful.
mbel 10y ago
With clang you can use -Rpass family of flags to get information about missed and used optimization opportunities. Although it will not print absolutely everything (trivial optimizations will be omitted). Other compilers have other options, ICC for example is able to generate optimization reports.

by m_eiman 10y ago

adrianN 10y ago

You could start by not relying on undefined behavior ;)

mpweiher 10y ago
Compiler writers could/should start by not treating undefined behavior as blank checks. This is so idiotic, and no, it's not a bug in Cap'n'Proto, it is very definitely a bug in the compiler (and/or the current version of the C spec. if it actually allows this).
- frederikvs 10y ago
  The C standard allows the compiler to do this, so it's definitely not a bug in the compiler.
  - mpweiher 10y ago
    As I wrote earlier: I had a C compiler before there was C standard. So by your definition, this compiler could not have bugs? As an exercise, try to express the difference between "standards-compliant" and "bug-free".
    - kutkloon7 10y ago
      I think the main point is that if there is a standard, an implementation should comply. If there is no standard, you have to use intuition and common sense to say what's a bug. Different people may have different expectations, so bugs become subjective. Ideally, there is a water-tight standard. In this case every deviation from the standard is a bug and vice versa.
      - mpweiher 10y ago
        > if there is a standard, an implementation should comply Yes. However, we are talking specifically about the parts where the standard is undefined. The standard most emphatically does not require a compiler to do these crazy optimizations that remove safety critical code.
- unwind 10y ago
  So rewriting code in order to "do what I mean" makes sense, since not doing what you mean is a bug in the language itself? Not sure that's a very productive approach. The spec is what it is, and it's of course rather deliberately done the way it is in order to make it possible for compilers to optimize and extract more performance.
  - mpweiher 10y ago
    > "do what I mean" Well, the code is right there, there is no magic "do what I mean" needed. Just don't magically remove code that I wrote. > spec is [..] rather deliberately done the way it is in order to make it possible for compilers to optimize and extract more performance Yes, and it is wrong. Breaking existing programs with magic that simply removes code that is clearly there in order to eke out a bit of performance is wrong.
    - masklinn 10y ago
      > Well, the code is right there, there is no magic "do what I mean" needed. Just don't magically remove code that I wrote. You can already do that: use a non-optimising compiler (tcc and the like) or compile without optimisations.
    - staticassertion 10y ago
      > Just don't magically remove code that I wrote. Compile with O0. > Yes, and it is wrong. Breaking existing programs with magic that simply removes code that is clearly there in order to eke out a bit of performance is wrong. Well welcome to writing C? It's been this way for a long time and that won't change - UB is critical to performance and doesn't always lead to unsafety. If you're programming in C, don't blame the compiler when your UB leads to remote code exec, you should have known what you were getting into.
      - mpweiher 10y ago
        > Well welcome to writing C? I've been writing C for 30 years. > UB is critical to performance Not really, no. See "What every compiler writer should know about programmers or “Optimization” based on undefined behaviour hurts performance" (referenced elsewhere in this thread) http://www.complang.tuwien.ac.at/kps2015/proceedings/KPS_2015_submission_29.pdf http://www.complang.tuwien.ac.at/kps2015/proceedings/KPS_201...
patrec 10y ago
You mean like approximately zero percent of non-trivial C or C++ programs in actual existence?
- adrianN 10y ago
  You could of course switch to Ada.
  - masklinn 10y ago
    Except Ada has UB (they used to call them bounded errors, but the semantics are basically identical), and reading the musings of ADAists[0] > I would say, having looked at hundreds of thousands of lines of Ada, that I have had a disappointingly frequent experience in finding the UNCHECKED_CONVERSION generic used in production Ada code. > I agree that this is often a mark of poor craftsmanship and unhesitatingly discourage its use, but it is there [0] https://www.cs.york.ac.uk/hise/safety-critical-archive/2011/0216.html https://www.cs.york.ac.uk/hise/safety-critical-archive/2011/...
    - adrianN 10y ago
      If finding potential UB in Ada programs is as simple as grepping for unchecked_conversion, then that's a big step forward compared to C, don't you think?
      - masklinn 10y ago
        1. your claim was essentially that ADA = no UB, so that's hardly relevant 2. that's not Ada's only UB, the 2005 spec has 35 or 36, which granted is an improvement over C's circa 200, but a far cry from being UB free
        adrianN 10y ago
        The claim is that it's impossible to avoid UB in real world C, but not impossible in real world $SAFERLANGUAGE, with Ada as an example. Any language that allows you to poke around in memory (a feature you need for embedded systems) will contain unsafe constructs. The question is whether you can tell by looking whether a particular piece of code is safe or not.

chrisseaton 10y ago

You can ask Clang to print the compiler IR after each optimisation, but you would need to know that you were looking for something before that was useful.

mbel 10y ago

With clang you can use -Rpass family of flags to get information about missed and used optimization opportunities. Although it will not print absolutely everything (trivial optimizations will be omitted). Other compilers have other options, ICC for example is able to generate optimization reports.