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do something more careful than eval_const_expr #1417

Closed kevina closed 12 years ago

kevina commented 12 years ago

Eval_const_expr is used in the front end in several places. This code quasi-duplicates stuff that trans should probably be doing (in Graydon's words).

Any evaluation of compile-time values should likely be moved to a pass somewhere in the middle end. I assume there is a way to get a compile-time value out of LLVM, but I'm not sure.

brson commented 12 years ago

I don't know how to manage that. trans is rightly letting LLVM do the same work that eval_const_expr does, but eval_const_expr is used for analysis passes that don't have access to LLVM.

graydon commented 12 years ago

Which analysis passes? I'm mostly concerned that the choice of what to evaluate in either place seems ad-hoc, and it's not clear which subsets of the expression language ought to be viable as constants in which context. We should get a clear picture of those. If the result is we do some eval in duplicate (or in 2 phases) that's ok.

brson commented 12 years ago

It's used by enum_variants to get the discriminant, which is used in many places. for example the discriminant is written to the metadata.

It's used in type checking, also to check properties of enum discriminants.

It also looks like it's used for reachability testing in alt expressions.