[PATCH] D76096: [clang] allow const structs to be constant expressions in initializer lists

[Richard Smith - zygoloid via Phabricator via cfe-commits]

rsmith added a comment.

In D76096#1921842 <https://reviews.llvm.org/D76096#1921842>, @nickdesaulniers wrote:

> > The performance implications of deleting those lines is the complicated part.
>
> Where does compile time performance suffer from this? I guess if we have massive array initializers, or large struct definitions, or deeply nested struct definitions, it might take time to recursively evaluate if all members are constant expressions; but isn't that what I want as a developer, to offload the calculations to compile time rather than runtime?  Or is the cost way too significant?


It's exactly what you suspect: very large global arrays initialized from constant data. For those, it's substantially more efficient for CodeGen to walk the AST representation (the `InitListExpr`) and directly generate an IR constant than it is to create an `APValue` representation of the array. (`APValue` is not especially space-efficient, and the extra copying and data shuffling can be quite slow.) We saw a significant compile time regression on a couple of LNT benchmarks without these early bailouts for C.


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