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Improve handling of dllexport #27416

Merged
merged 2 commits into from
Aug 11, 2015
Merged

Improve handling of dllexport #27416

merged 2 commits into from
Aug 11, 2015

Commits on Aug 11, 2015

  1. syntax: Add a new unstable #[linked_from] attribute 

    To correctly reexport statically included libraries from a DLL on Windows, the
compiler will soon need to have knowledge about what symbols are statically
included and which are not. To solve this problem a new unstable
`#[linked_from]` attribute is being added and recognized on `extern` blocks to
indicate which native library the symbols are coming from.

The compiler then keeps track of what the set of FFI symbols are that are
included statically. This information will be used in a future commit to
configure how we invoke the linker on Windows.
    @alexcrichton
    alexcrichton committed Aug 11, 2015
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  2. trans: Stop informing LLVM about dllexport 

    Rust's current compilation model makes it impossible on Windows to generate one
object file with a complete and final set of dllexport annotations. This is
because when an object is generated the compiler doesn't actually know if it
will later be included in a dynamic library or not. The compiler works around
this today by flagging *everything* as dllexport, but this has the drawback of
exposing too much.

Thankfully there are alternate methods of specifying the exported surface area
of a dll on Windows, one of which is passing a `*.def` file to the linker which
lists all public symbols of the dynamic library. This commit removes all
locations that add `dllexport` to LLVM variables and instead dynamically
generates a `*.def` file which is passed to the linker. This file will include
all the public symbols of the current object file as well as all upstream
libraries, and the crucial aspect is that it's only used when generating a
dynamic library. When generating an executable this file isn't generated, so all
the symbols aren't exported from an executable.

To ensure that statically included native libraries are reexported correctly,
the previously added support for the `#[linked_from]` attribute is used to
determine the set of FFI symbols that are exported from a dynamic library, and
this is required to get the compiler to link correctly.
    @alexcrichton
    alexcrichton committed Aug 11, 2015
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