Initialization of memory and variables #257
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This proposal outlines a suggested design for initialization in Carbon. The early draft was developed in the document here: https://docs.google.com/document/d/1UkDu1Wo5qsmedgt-gZyINdOV5Qc8nICpltZ_TZfTo40/edit# Moving it here to begin the RFC process for this proposal.
chandlerc
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Moving this to RFC. I think I still have one TODO left in the document, but I think the early draft has largely converged and its in a good state for folks to review! |
josh11b
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Feb 4, 2021
josh11b
reviewed
Feb 4, 2021
Co-authored-by: josh11b <josh11b@users.noreply.github.com>
…247) This isn't reflected in the above text, not sure how it managed to remain. There is still a note about filing issues for open questions at https://github.com/carbon-language/carbon-lang/blob/trunk/docs/project/consensus_decision_making.md#formal-decision-content, but I think it's much clearer when it's part of the core team's decision, rather than during proposal iteration.
Per chandlerc's comment: First, this builds inside the Bazel tree rather than in the source repository. This ensures we start in a clean directory each time the repository rule is run again. We don't need to detect an existing build with this, and it will only be rebuilt when the workspace file or the repository rule implementation is changed. This can be forced by using: ``` bazel sync --configure ``` Second, we use an implicit dependency on the `WORKSPACE` file to locate the workspace directory automatically, and the `HEAD` file from the `llvm-project` submodule to trigger a rebuild if the submodule is updated. Third, teach the CMake script to try to use a system-installed `clang` if installed and not overridden by the `CC` environment variable. This is very different from the prior logic -- this is only used with the CMake build, and so should work with any system C++ compiler that can build Clang and LLVM. Lastly, this tweaks the CMake options to tune this build given that we now fully control it and it will only be used in this context. This still results in a 1.5gb build for me. =/ But its as small as I can make it really. It's a frustrating long list, but I couldn't find a more brief way of representing this. Co-authored-by: Chandler Carruth <chandlerc@gmail.com>
kcc
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Feb 10, 2021
mmdriley
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Feb 23, 2021
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Commented throughout.
I read this proposal chiefly from a safety angle. Having gotten to the end, I'm worried that I would be just as likely to write code in Carbon that reads uninitialized values as if I were writing C or C++.
Does that disagree with others' understanding? Did I miss the forest for the trees?
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Thanks! I think there are a couple of interesting threads left here -- at least the return of an unformed value one and the one around swap. |
zygoloid
reviewed
Jun 17, 2021
Co-authored-by: Richard Smith <richard@metafoo.co.uk>
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Thanks all for the great discussion. I've updated the document to be explicit and clear about all of the things we're precluding and added more rationale to the sections discussing those alternatives. PTAL! |
geoffromer
reviewed
Jun 18, 2021
josh11b
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Jun 18, 2021
Co-authored-by: Geoff Romer <gromer@google.com> Co-authored-by: josh11b <josh11b@users.noreply.github.com>
zygoloid
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Jun 21, 2021
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Jun 28, 2022
This proposal outlines a suggested design for initialization in Carbon. The early draft was developed in the document here: https://docs.google.com/document/d/1UkDu1Wo5qsmedgt-gZyINdOV5Qc8nICpltZ_TZfTo40/edit# Co-authored-by: josh11b <josh11b@users.noreply.github.com> Co-authored-by: Jon Meow <46229924+jonmeow@users.noreply.github.com> Co-authored-by: Richard Smith <richard@metafoo.co.uk> Co-authored-by: Geoff Romer <gromer@google.com>
zygoloid
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May 31, 2023
…-or-nothing. Disallow storing to subobjects of uninitialized objects.
jonmeow
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May 31, 2023
… checking (#2862) Per #257, we should be treating unformedness as all-or-nothing, rather than being a per-field or per-array-element property. Previously we initialized an array with no explicit initializer as containing a sequence of uninitialized values, but that led to crashes when attempting to access those values, as the checks for reading an uninitialized value only expected values to be uninitialized at the top level. Also, we had existing tests that attempt to store to an element of an uninitialized array. We now detect that and treat it as UB during evaluation, rather than crashing due to trying to perform field access into an uninitialized value. Finally, many of these problems can be detected statically, but the resolve_unformed pass wasn't catching them because it missed a few expression and declaration forms. Support for those cases has been added too. This causes the pass to recurse more often, and in particular our existing recursion test started hitting a stack overflow after this, so resolve_unformed now uses `RunWithExtraStack`. In passing, remove the need to explicitly tell `RunWithExtraStack` the return type, and infer it as the return type of the callable instead.
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This proposal outlines a suggested design for initialization in Carbon.
The early draft was developed in the document here:
https://docs.google.com/document/d/1UkDu1Wo5qsmedgt-gZyINdOV5Qc8nICpltZ_TZfTo40/edit#
Moving it here to begin the RFC process for this proposal.