Simple as in #once INCLUDE_GUARD_STRING_YOU_CANNOT_OMIT?

On Fri, Aug 30, 2024 at 11:04 AM Tiago Freire via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> I agree, bit-wise comparison is not the way to go, that’s why no compiler
> does this.
>
> There’s no need to over-engineer this. Keep it simple.
>
>
>
>
>
> *From:* Std-Proposals <std-proposals-bounces_at_[hidden]> *On Behalf
> Of *Gašper Ažman via Std-Proposals
> *Sent:* Friday, August 30, 2024 11:51 AM
> *To:* marcinjaczewski86_at_[hidden]
> *Cc:* Gašper Ažman <gasper.azman_at_[hidden]>;
> std-proposals_at_[hidden]; Tom Honermann <tom_at_[hidden]>
> *Subject:* Re: [std-proposals] Revising #pragma once
>
>
>
> If you knew up-front you wouldn't do it :).
>
>
>
> This happens though. There are people who generate code - whole include
> trees - and for plugins they end up looking very very similar or identical.
> And then as the build engineer my users complain that "their build doesn't
> work on the cloud build environment" and I have to somehow *find their bug*.
>
>
>
> Think about it. How the hell do you diagnose this silent noninclusion?
>
>
>
> On Fri, Aug 30, 2024 at 9:30 AM Marcin Jaczewski <
> marcinjaczewski86_at_[hidden]> wrote:
>
> pt., 30 sie 2024 o 01:20 Gašper Ažman via Std-Proposals
> <std-proposals_at_[hidden]> napisał(a):
> >
> > Darlings,
> >
> > byte-identical is just plain incorrect. Consider.
> >
> > [library1/public.hpp]
> > #pragma once
> > #include "utils.hpp"
> >
> > [library1/utils.hpp]
> > class lib1 {};
> >
> > library2/public.hpp
> > #pragma once
> > #include "utils.hpp"
> >
> > [library2/utils.hpp]
> > class lib2 {};
> >
> > [main.cpp]
> > #include "library1/public.hpp"
> > #include "library2/public.hpp" # boom, library2/utils.hpp does not get
> included
> >
> > same-contents also means same-relative-include-trees. Congratulations.
> >
>
> Question is do you know upfront that files are problematic?
> Then we could do something like this:
> a) we only consider normalized paths
> b) use have option to add path mappings that are used in path comparison
>
> Like:
>
> We have folders `libs/fooV1` and `libs/fooV2`. Both folders are symlinks
> but by default when compiler load `libs/fooV1/bar.h` and `libs/fooV2/bar.h`
> the compiler considers them as separate files even when they are the same
> file.
> Now we add compiler flag `-PI libs/fooV1:libs/fooV2` (similar to `-I`) and
> now
> when compiler load `libs/fooV1/foo.h` he consider it as if he load
> `libs/fooV2/bar.h` and thus next loading of `libs/fooV2/bar.h` will be
> blocked.
>
> And this should be ver dumb process it could be cases when
> `libs/fooV1/bar.h`
> and `libs/fooV2/bar.h` are unrelated but if you make specific maping it
> will
> override even diffrnet files. This could be useful to hacking some broken
> includes like:
>
> ```
> -PI hack/foo/bar.h:libs/fooV2/bar.h
> ```
>
> and we can in our files do:
>
> ```
> #include "hack/foo/bar.h"
> #include "libs/fooV2/foo.h" // it will ignore `#include "bar.h"`
> ```
>
> Could mechnism like this work on your build system?
>
> > On Fri, Aug 30, 2024 at 12:15 AM Jeremy Rifkin via Std-Proposals <
> std-proposals_at_[hidden]> wrote:
> >>
> >> > In this very thread there are examples showing why taking only the
> content into account doesn't work but it was brushed off as "that can be
> fixed".
> >>
> >> I'm sorry you feel I have brushed any presented examples off. I have
> >> found them all immensely helpful for consideration. It's hard for me
> >> to imagine times when you'd want the same include-guarded content
> >> included twice, however I found the example of a "main header"
> >> compelling. The example of a header that only defines macros you undef
> >> is also not impractical.
> >>
> >> However, there are also compelling examples for filesystem identity.
> >> Mainly the multiple mount point issue.
> >>
> >> I think both can be reasonable, however, I have been trying to
> >> understand the most probable failure modes. While I originally
> >> proposed a content-based definition, I do think a filesystem-based
> >> definition is closer to current semantics and expectations.
> >>
> >> Jeremy
> >>
> >> On Thu, Aug 29, 2024 at 5:24 PM Breno Guimarães via Std-Proposals
> >> <std-proposals_at_[hidden]> wrote:
> >> >
> >> > To add to that, the whole idea is to standardize standard practice.
> If the first thing you do is to change spec to something else, then you're
> not standardizing standard practice, you are adding a new feature that
> inconveniently clashes with an existing one.
> >> >
> >> > In this very thread there are examples showing why taking only the
> content into account doesn't work but it was brushed off as "that can be
> fixed".
> >> >
> >> > None of this make sense to me.
> >> >
> >> >
> >> > Em qui., 29 de ago. de 2024 18:59, Tiago Freire via Std-Proposals <
> std-proposals_at_[hidden]> escreveu:
> >> >>
> >> >> Again, hashing content... totally unnecessary.
> >> >>
> >> >> There's no need to identify "same content" which as far as I can see
> can be defeated by modifications that don't change the interpretation, like
> spaces, which although not technically a violation of "same content" it
> clearly defeats the intent.
> >> >>
> >> >> An include summons a resource, a pragma once bars that resources
> from bey re-summonable. That's it. File paths should be more than enough.
> >> >>
> >> >> I'm unconvinced that the "bad cases" are not just a product of bad
> build architecture, if done properly a compiler should never be presented
> with multiple alternatives of the same file. And putting such requirements
> on compilers puts an unnecessary burden on developers to support a scenario
> that it is that is arguably bad practice.
> >> >>
> >> >> The argument is "prgma once" is supported everywhere it is good, we
> should make it official in the standard, effectively no change to a
> compiler should occur as a consequence.
> >> >> If a change needs to occur, then in fact "your version" of what you
> mean by "pragma once" is actually "not supported" by all the major
> compilers.
> >> >>
> >> >> Current compiler support of "pragma once" and it's usage on cross
> platform projects have a particular way of dealing with dependencies in
> mind. That workflow works. It's pointless to have this discussion if you
> don't understand that flow, and you shouldn't tailor the tool to a workflow
> that doesn't exist to the detriment of all.
> >> >>
> >> >>
> >> >> ________________________________
> >> >> From: Std-Proposals <std-proposals-bounces_at_[hidden]> on
> behalf of Jeremy Rifkin via Std-Proposals <std-proposals_at_[hidden]>
> >> >> Sent: Thursday, August 29, 2024 9:56:18 PM
> >> >> To: Tom Honermann <tom_at_[hidden]>
> >> >> Cc: Jeremy Rifkin <rifkin.jer_at_[hidden]>;
> std-proposals_at_[hidden] <std-proposals_at_[hidden]>
> >> >> Subject: Re: [std-proposals] Revising #pragma once
> >> >>
> >> >> Performance should be fine if using a content definition. An
> implementation can do inode/path checks against files it already knows of,
> as a fast path. The first time a file is #included it’s just a hash+table
> lookup to decide whether to continue.
> >> >>
> >> >> Regarding the filesystem definition vs content definition question,
> while I think a content-based definition is robust I can see there is FUD
> about it and also an argument about current practice being a
> filesystem-based definition. It may just be best to approach this as
> filesystem uniqueness to the implementation’s ability, with a requirement
> that symbolic links/hard links are handled. This doesn’t cover the case of
> multiple mount points, but we’ve discussed that that’s impossible with
> #pragma once without using contents instead.
> >> >>
> >> >> Jeremy
> >> >>
> >> >> On Thu, Aug 29, 2024 at 13:06 Tom Honermann <tom_at_[hidden]>
> wrote:
> >> >>>
> >> >>> On 8/28/24 12:32 AM, Jeremy Rifkin via Std-Proposals wrote:
> >> >>>
> >> >>> Another question is whether the comparison should be post
> translation
> >> >>> phase 1.
> >> >>>
> >> >>> I gave this some thought while drafting the proposal. I think it
> comes
> >> >>> down to whether the intent is single inclusion of files or single
> >> >>> inclusion of contents.
> >> >>>
> >> >>> Indeed. The proposal currently favors the "same contents" approach
> and offers the following wording.
> >> >>>
> >> >>> A preprocessing directive of the form
> >> >>> # pragma once new-line
> >> >>> shall cause no subsequent #include directives to perform
> replacement for a file with text contents identical to this file.
> >> >>>
> >> >>> The wording will have to define what it means for contents to be
> identical. Options include:
> >> >>>
> >> >>> The files must be byte-for-byte identical. This makes source file
> encoding observable (which I would be strongly against).
> >> >>> The files must encode the same character sequence post translation
> phase 1. This makes comparisons potentially expensive.
> >> >>>
> >> >>> Note that the "same contents" approach obligates an implementation
> to consider every previously encountered file for every #include directive.
> An inode based optimization can help to determine if a file was previously
> encountered based on identity, but it doesn't help to reduce the costs when
> a file that was not previously seen is encountered.
> >> >>>
> >> >>>
> >> >>> Tom.
> >> >>>
> >> >>> Jeremy
> >> >>>
> >> >>> On Tue, Aug 27, 2024 at 3:39 PM Tom Honermann via Std-Proposals
> >> >>> <std-proposals_at_[hidden]> wrote:
> >> >>>
> >> >>> On 8/27/24 4:10 PM, Thiago Macieira via Std-Proposals wrote:
> >> >>>
> >> >>> On Tuesday 27 August 2024 12:35:17 GMT-7 Andrey Semashev via
> Std-Proposals
> >> >>> wrote:
> >> >>>
> >> >>> The fact that gcc took the approach to compare file contents I
> consider
> >> >>> a poor choice, and not an argument to standardize this
> implementation.
> >> >>>
> >> >>> Another question is whether a byte comparison of two files of the
> same size is
> >> >>> expensive for compilers.
> >> >>>
> >> >>> #once ID doesn't need to compare the entire file.
> >> >>>
> >> >>> Another question is whether the comparison should be post
> translation
> >> >>> phase 1. In other words, whether differently encoded source files
> that
> >> >>> decode to the same sequence of code points are considered the same
> file
> >> >>> (e.g., a Windows-1252 version and a UTF-8 version). Standard C++
> does
> >> >>> not currently allow source file encoding to be observable but a
> #pragma
> >> >>> once implementation that only compares bytes would make such
> differences
> >> >>> observable.
> >> >>>
> >> >>> Tom.
> >> >>>
> >> >>> --
> >> >>> Std-Proposals mailing list
> >> >>> Std-Proposals_at_[hidden]
> >> >>> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
> >> >>
> >> >>
> >> >> --
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> >> >
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