Auto merge of #122790 - Zoxc:dllimp-rev, r=<try>

Apply dllimport in ThinLTO for -Z dylib-lto

This partially reverts #103353 by properly applying `dllimport` if  `-Z dylib-lto` is passed. That PR should probably fully be reverted as it looks quite sketchy. We don't know locally if the entire crate graph would be statically linked.

This should hopefully be sufficient to make ThinLTO work for rustc on Windows.

r? `@wesleywiser`

Author: bors

.github/workflows/ci.yml

@@ -438,7 +438,7 @@ jobs:
             os: windows-2019-8core-32gb
           - name: dist-x86_64-msvc
             env:
-              RUST_CONFIGURE_ARGS: "--build=x86_64-pc-windows-msvc --host=x86_64-pc-windows-msvc --target=x86_64-pc-windows-msvc --enable-full-tools --enable-profiler --set rust.codegen-units=1"
+              RUST_CONFIGURE_ARGS: "--build=x86_64-pc-windows-msvc --host=x86_64-pc-windows-msvc --target=x86_64-pc-windows-msvc --enable-full-tools --enable-profiler --set rust.codegen-units=1 --set rust.lto=thin"
               SCRIPT: python x.py build --set rust.debug=true opt-dist && PGO_HOST=x86_64-pc-windows-msvc ./build/x86_64-pc-windows-msvc/stage0-tools-bin/opt-dist windows-ci -- python x.py dist bootstrap --include-default-paths
               DIST_REQUIRE_ALL_TOOLS: 1
             os: windows-2019-8core-32gb
@@ -592,7 +592,6 @@ jobs:
   try:
     name: "try - ${{ matrix.name }}"
     env:
-      DIST_TRY_BUILD: 1
       CI_JOB_NAME: "${{ matrix.name }}"
       CARGO_REGISTRIES_CRATES_IO_PROTOCOL: sparse
       HEAD_SHA: "${{ github.event.pull_request.head.sha || github.sha }}"
@@ -610,10 +609,12 @@ jobs:
     strategy:
       matrix:
         include:
-          - name: dist-x86_64-linux
+          - name: dist-x86_64-msvc
             env:
-              CODEGEN_BACKENDS: "llvm,cranelift"
-            os: ubuntu-20.04-16core-64gb
+              RUST_CONFIGURE_ARGS: "--build=x86_64-pc-windows-msvc --host=x86_64-pc-windows-msvc --target=x86_64-pc-windows-msvc --enable-full-tools --enable-profiler --set rust.codegen-units=1 --set rust.lto=thin"
+              SCRIPT: python x.py build --set rust.debug=true opt-dist && PGO_HOST=x86_64-pc-windows-msvc ./build/x86_64-pc-windows-msvc/stage0-tools-bin/opt-dist windows-ci -- python x.py dist bootstrap --include-default-paths
+              DIST_REQUIRE_ALL_TOOLS: 1
+            os: windows-2019-8core-32gb
     defaults:
       run:
         shell: "${{ contains(matrix.os, 'windows') && 'msys2 {0}' || 'bash' }}"

compiler/rustc_codegen_llvm/src/consts.rs

@@ -302,15 +302,20 @@ impl<'ll> CodegenCx<'ll, '_> {
         }
 
         if !def_id.is_local() {
+            // Workaround an LLD bug (https://github.com/rust-lang/rust/issues/81408) with importing
+            // static symbols triggered by ThinLTO if we're not using -Z dylib-lto. Note this
+            // workaround may not be sound for crate graphs with dylibs.
+            let workaround_lld_bug =
+                self.tcx.sess.lto() == Lto::Thin && !self.tcx.sess.opts.unstable_opts.dylib_lto;
+
             let needs_dll_storage_attr = self.use_dll_storage_attrs && !self.tcx.is_foreign_item(def_id) &&
                 // Local definitions can never be imported, so we must not apply
                 // the DLLImport annotation.
                 !dso_local &&
                 // ThinLTO can't handle this workaround in all cases, so we don't
                 // emit the attrs. Instead we make them unnecessary by disallowing
                 // dynamic linking when linker plugin based LTO is enabled.
-                !self.tcx.sess.opts.cg.linker_plugin_lto.enabled() &&
-                self.tcx.sess.lto() != Lto::Thin;
+                !self.tcx.sess.opts.cg.linker_plugin_lto.enabled() && !workaround_lld_bug;
 
             // If this assertion triggers, there's something wrong with commandline
             // argument validation.

src/ci/github-actions/ci.yml

@@ -688,6 +688,7 @@ jobs:
                 --enable-full-tools
                 --enable-profiler
                 --set rust.codegen-units=1
+                --set rust.lto=thin
               SCRIPT: python x.py build --set rust.debug=true opt-dist && PGO_HOST=x86_64-pc-windows-msvc ./build/x86_64-pc-windows-msvc/stage0-tools-bin/opt-dist windows-ci -- python x.py dist bootstrap --include-default-paths
               DIST_REQUIRE_ALL_TOOLS: 1
             <<: *job-windows-8c
@@ -753,19 +754,25 @@ jobs:
     <<: *base-ci-job
     name: try - ${{ matrix.name }}
     env:
-      DIST_TRY_BUILD: 1
+#      DIST_TRY_BUILD: 1
       <<: [*shared-ci-variables, *prod-variables]
     if: github.event_name == 'push' && (((github.ref == 'refs/heads/try' || github.ref == 'refs/heads/try-perf') && github.repository == 'rust-lang-ci/rust') || ((github.ref == 'refs/heads/automation/bors/try') && github.repository == 'rust-lang/rust'))
     strategy:
       matrix:
         include:
-          - &dist-x86_64-linux
-            name: dist-x86_64-linux
+          - name: dist-x86_64-msvc
             env:
-              CODEGEN_BACKENDS: llvm,cranelift
-            <<: *job-linux-16c
-
-
+              RUST_CONFIGURE_ARGS: >-
+                --build=x86_64-pc-windows-msvc
+                --host=x86_64-pc-windows-msvc
+                --target=x86_64-pc-windows-msvc
+                --enable-full-tools
+                --enable-profiler
+                --set rust.codegen-units=1
+                --set rust.lto=thin
+              SCRIPT: python x.py build --set rust.debug=true opt-dist && PGO_HOST=x86_64-pc-windows-msvc ./build/x86_64-pc-windows-msvc/stage0-tools-bin/opt-dist windows-ci -- python x.py dist bootstrap --include-default-paths
+              DIST_REQUIRE_ALL_TOOLS: 1
+            <<: *job-windows-8c
   master:
     name: master
     runs-on: ubuntu-latest