[SR-1134] LFR: Ability to pack Structs with certain named types (like C / C++)

[swift-ci]

Previous ID	SR-1134
Radar	None
Original Reporter	hitstergtd (JIRA User)
Type	New Feature
Status	In Progress
Resolution

Additional Detail from JIRA

Votes	2
Component/s	Compiler
Labels	New Feature, AfterVersion3, LanguageFeatureRequest
Assignee	hitstergtd (JIRA)
Priority	Medium

md5: 5d89999f6f4efe7f2b7ae8a8b285acb9

relates to:

• SR-1451 html
• SR-7809 Define layout algorithm for structs

Issue Description:

It would be good to provide the Swift language/compiler the ability to pack certain type of Structs, similar to the __packed attribute provided by C/C++ compilers.

1. This would certainly further the goal of using Swift as a systems programming language.

2. Currently, one would have to write such code in C and then interface with it from Swift.

3. The feature could be restricted to Structs which only contain certain named types.

I am not an expert on compilers or language design so I apologise if I am trivialising the matter.

Even if the above is not desirable, at the very least, I hope this issue stirs discussion and thought on the topic of being able to use Swift as a systems programming language, and how we could possibly ensure the runtime and standard library can be made to work with it accordingly.

[swift-ci]

Comment by Hitster GTD (JIRA)

For reference:

1. https://github.com/nst/BinUtils/

2. http://stackoverflow.com/questions/24139149/how-do-i-create-a-packed-data-structure-in-swift

3. http://stackoverflow.com/questions/13688625/how-to-declare-packed-struct-without-padding-for-llvm

4. https://llvm.org/svn/llvm-project/cfe/branches/eh-experiment/test/Sema/struct-packed-align.c

5. http://www.catb.org/esr/structure-packing/

[belkadan]

This would need to go through the Swift Evolution Process.

[swift-ci]

Comment by Hitster GTD (JIRA)

@belkadan, thanks. I have just re-reviewed the guidelines. Thankfully it is not already a commonly rejected proposal. I will draft a proposal for submission to the swift-evolution@ mailing list.

[swift-ci]

Comment by Hitster GTD (JIRA)

Draft Proposal in progress.

[swift-ci]

Comment by Hitster GTD (JIRA)

@belkadan, I have submitted a draft proposal for comments to the swift-evolution@ mailing list.

[swift-ci]

Comment by Hitster GTD (JIRA)

Summarising feedback received from @lattner:

1. Good suggestion, but has too many moving parts for it to be implemented in Swift v3

2. Document draft proposal at bugs.swift.org

3. The Swift Evolution Process (swift-evolution@), encourages focus on Swift v3 to minimise distractions (rightly so)

This leaves room for a potential implementation after Swift v3; I would like to keep SR-1134 open to document the draft proposal and for anyone else interested in contributing to it as well.