lower field access in the presence of a vptr #4515
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…s to be skipped over
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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 <richard@metafoo.co.uk>
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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 <richard@metafoo.co.uk>
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Add one to the field index when lowering, if there's a vptr that needs to be skipped over.