Use the .drectve section for exporting symbols from dlls on Windows #142568

bjorn3 · 2025-06-16T09:24:31Z

While it would be reasonable to expect the Windows linker to handle linker args in the .drectve section identical to cli arguments, as it turns out exporting weak symbols only works when the /EXPORT is in the .drectve section, not when it is a linker argument or when a .DEF file is used.

Necessary for and extracted out of #142366.

Thanks @dpaoliello for figuring out this weird quirk of link.exe!

rustbot · 2025-06-16T09:24:35Z

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rustbot · 2025-06-16T09:24:36Z

Some changes occurred in compiler/rustc_codegen_ssa

cc @WaffleLapkin

compiler/rustc_codegen_ssa/src/back/linker.rs

bjorn3 · 2025-06-16T13:29:19Z

@bors2 try jobs=x86_64-msvc-1,x86_64-msvc-2

rust-bors · 2025-06-16T13:29:23Z

⌛ Trying commit 5e2efb6 with merge 7bae343…

To cancel the try build, run the command @bors2 try cancel.

Use the .drectve section for exporting symbols from dlls on Windows While it would be reasonable to expect the Windows linker to handle linker args in the .drectve section identical to cli arguments, as it turns out exporting weak symbols only works when the /EXPORT is in the .drectve section, not when it is a linker argument or when a .DEF file is used. Necessary for and extracted out of #142366. Thanks `@dpaoliello` for figuring out this weird quirk of link.exe! try-job: x86_64-msvc-1 try-job: x86_64-msvc-2

rust-bors · 2025-06-16T14:51:23Z

💔 Test failed

CI ❌

bjorn3 · 2025-06-16T15:29:16Z

@bors2 try jobs=x86_64-msvc-1,x86_64-msvc-2

rust-bors · 2025-06-16T15:29:20Z

⌛ Trying commit c353c7d with merge 3dadabc…

To cancel the try build, run the command @bors2 try cancel.

Use the .drectve section for exporting symbols from dlls on Windows While it would be reasonable to expect the Windows linker to handle linker args in the .drectve section identical to cli arguments, as it turns out exporting weak symbols only works when the /EXPORT is in the .drectve section, not when it is a linker argument or when a .DEF file is used. Necessary for and extracted out of #142366. Thanks `@dpaoliello` for figuring out this weird quirk of link.exe! try-job: x86_64-msvc-1 try-job: x86_64-msvc-2

dpaoliello

LGTM.

One overall design question: is it supported for folks to run the linker themselves AND specify their own def file? A couple of my changes broke the Chromium and Fuschia folks, so I'm a little paranoid now about changing anything to do with linking that may affect them.

dpaoliello · 2025-06-16T16:23:01Z

compiler/rustc_codegen_ssa/src/back/linker.rs

@@ -1116,10 +1122,6 @@ impl<'a> Linker for MsvcLinker<'a> {
            // straight to exports.
            writeln!(f, "LIBRARY")?;
            writeln!(f, "EXPORTS")?;
-            for symbol in symbols {


I don't believe that we need the def file any more, we should be able to get away with passing /DLL to the linker.

In the mingw docs I read something about the presence of a .DEF file disabling auto-export of certain symbols. I don't know if the same applies to MSVC, so I added it here just to be safe.

compiler/rustc_codegen_ssa/src/back/linker.rs

bjorn3 · 2025-06-16T16:58:23Z

One overall design question: is it supported for folks to run the linker themselves AND specify their own def file?

This PR on it's own has no effect at all on people running the linker themself. Both the .DEF file and the symbols.o file are only generated when rustc itself runs the linker.

As for a future PR that will use weak definitions, those symbols are not meant to be exported from a staticlib anyway. The only case that could break is if someone (Chromium) tries to link rlibs together into a dll and wants to export all symbols, which we don't officially support anyway. The Chromium people have had to adaot their build system for rustc changes anyway and I expect it to not all that hard to adapt to said future weak symbol usage too.

rust-bors · 2025-06-16T17:09:05Z

💔 Test failed

CI ❌

dpaoliello · 2025-06-16T17:32:23Z

💔 Test failed

CI ❌

The test executable failed with 0xc0000135 which is STATUS_DLL_NOT_FOUND

Looks like this is a test for shaking out unused functions: is the compiler supposed to do that, or is it relying on the linker?

bjorn3 · 2025-06-17T09:00:00Z

@bors2 try jobs=x86_64-msvc-1,x86_64-msvc-2

rust-bors · 2025-06-17T09:00:05Z

⌛ Trying commit 8c88ff4 with merge e97fbdf…

To cancel the try build, run the command @bors2 try cancel.

Use the .drectve section for exporting symbols from dlls on Windows While it would be reasonable to expect the Windows linker to handle linker args in the .drectve section identical to cli arguments, as it turns out exporting weak symbols only works when the /EXPORT is in the .drectve section, not when it is a linker argument or when a .DEF file is used. Necessary for and extracted out of #142366. Thanks `@dpaoliello` for figuring out this weird quirk of link.exe! try-job: x86_64-msvc-1 try-job: x86_64-msvc-2

rust-bors · 2025-06-17T11:30:12Z

☀️ Try build successful (CI)
Build commit: e97fbdf (e97fbdf63e618534ac0929eef0229de9fb477dad, parent: 55d436467c351b56253deeba209ae2553d1c243f)

While it would be reasonable to expect the Windows linker to handle linker args in the .drectve section identical to cli arguments, as it turns out exporting weak symbols only works when the /EXPORT is in the .drectve section, not when it is a linker argument or when a .DEF file is used.

bjorn3 · 2025-06-17T11:42:13Z

Squashed everything and reverted some now unnecessary changes.

@rustbot ready

rustbot assigned wesleywiser Jun 16, 2025

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Jun 16, 2025

bjorn3 force-pushed the windows_symbols_o_export branch from e63c76b to 94ec88c Compare June 16, 2025 09:25

bjorn3 commented Jun 16, 2025

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bjorn3 commented Jun 16, 2025

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dpaoliello approved these changes Jun 16, 2025

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bjorn3 force-pushed the windows_symbols_o_export branch from 8c88ff4 to fc88eae Compare June 17, 2025 09:01

bjorn3 force-pushed the windows_symbols_o_export branch from fc88eae to e6aa201 Compare June 17, 2025 11:41

bjorn3 mentioned this pull request Jun 17, 2025

[WIP] Use weak aliases for the allocator shim #142366

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Use the .drectve section for exporting symbols from dlls on Windows #142568

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Use the .drectve section for exporting symbols from dlls on Windows #142568

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