Date: Thu, 29 Aug 2024 21:59:31 +0000
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]<mailto: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]><mailto: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]<mailto:Std-Proposals_at_[hidden]>
https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
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]<mailto: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]><mailto: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]<mailto:Std-Proposals_at_[hidden]>
https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
Received on 2024-08-29 21:59:35