always use a vptr (possibly with an empty type) in object representations #4517
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We considered a couple of other options for this: * #4515 Keep the `ElementIndex` numbering vptr-ignorant, and do +1 offsets as needed - seems subtle/easy to miss * #4517 Always have a zeroth element in the object representation, make it zero-size in the case of no-vptr - @zygoloid was concerned this would add overhead especially to stateless objects used in type-trait-like things. But currently moving forward with this direction - of initializing field indexes with an invalid value until the end of the class definition, then assigning field indexes during construction of the class's object representation struct type. This direction might reinforce/help avoid premature access to the object representation before the class is complete, and give a single place where class layout is done (at class completion) if we want to add more options there, such as class layout optimizations, etc. This patch still has problems with object initialization (that #4515 does not have/does address) but does address normal `obj.member` access correctly. --------- Co-authored-by: Richard Smith <[email protected]>
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Nov 28, 2024
We considered a couple of other options for this: * carbon-language#4515 Keep the `ElementIndex` numbering vptr-ignorant, and do +1 offsets as needed - seems subtle/easy to miss * carbon-language#4517 Always have a zeroth element in the object representation, make it zero-size in the case of no-vptr - @zygoloid was concerned this would add overhead especially to stateless objects used in type-trait-like things. But currently moving forward with this direction - of initializing field indexes with an invalid value until the end of the class definition, then assigning field indexes during construction of the class's object representation struct type. This direction might reinforce/help avoid premature access to the object representation before the class is complete, and give a single place where class layout is done (at class completion) if we want to add more options there, such as class layout optimizations, etc. This patch still has problems with object initialization (that carbon-language#4515 does not have/does address) but does address normal `obj.member` access correctly. --------- Co-authored-by: Richard Smith <[email protected]>
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This breaks a lot of tests and lowering (with a pre-existing crash, probably just due to the lack of lowering of the initializer (or lowering it in check into an Error builtin, specifically), that only happens currently in dynamic classes, but by putting a vptr (even a zero-length one) in every class means they all hit that crash) so the presubmit checks aren't clear, but that's not really the fault of this patch.