[lld] [LLD] [COFF] Preserve directives and export names from LTO objects (PR #78802)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-lld-coff

Author: Martin Storsjö (mstorsjo)

<details>
<summary>Changes</summary>

The export names are saved as StringRefs pointing into the COFF directives. In the case of LTO objects, this can be memory allocated that is owned by the LTO InputFile, which gets destructed when doing the compilation.

In the case of LTO objects from an older version of LLVM, which require being upgraded when loaded, the directives string gets destructed, while when using LTO objects of a matching version (the common case), the directives string points into memory that doesn't get destructed on LTO compilation.

Test this by linking a bundled binary LTO object file, from an older version of LLVM.

This fixes issue #<!-- -->78591, and downstream issue https://github.com/mstorsjo/llvm-mingw/issues/392.

---
Full diff: https://github.com/llvm/llvm-project/pull/78802.diff


3 Files Affected:

- (modified) lld/COFF/InputFiles.cpp (+1-1) 
- (added) lld/test/COFF/Inputs/lto-directives.obj () 
- (added) lld/test/COFF/lto-directives.test (+15) 


``````````diff
diff --git a/lld/COFF/InputFiles.cpp b/lld/COFF/InputFiles.cpp
index cd744800cb0dec..22cc0e3e5dbaf7 100644
--- a/lld/COFF/InputFiles.cpp
+++ b/lld/COFF/InputFiles.cpp
@@ -1081,7 +1081,7 @@ void BitcodeFile::parse() {
     if (objSym.isUsed())
       ctx.config.gcroot.push_back(sym);
   }
-  directives = obj->getCOFFLinkerOpts();
+  directives = saver.save(obj->getCOFFLinkerOpts());
 }
 
 void BitcodeFile::parseLazy() {
diff --git a/lld/test/COFF/Inputs/lto-directives.obj b/lld/test/COFF/Inputs/lto-directives.obj
new file mode 100644
index 00000000000000..a413b0ab92f4e4
Binary files /dev/null and b/lld/test/COFF/Inputs/lto-directives.obj differ
diff --git a/lld/test/COFF/lto-directives.test b/lld/test/COFF/lto-directives.test
new file mode 100644
index 00000000000000..5890d426764c4e
--- /dev/null
+++ b/lld/test/COFF/lto-directives.test
@@ -0,0 +1,15 @@
+; REQUIRES: x86
+
+;; Test linking an LTO object file that contains directives. The
+;; LTO object file is built with an older toolchain, to force it
+;; to be upgraded when loaded.
+
+;; The input file is compiled from source that looks like this:
+;;   void __declspec(dllexport) entry(void) { }
+;; with this command:
+;;   clang -target x86_64-windows-msvc -c main.c -flto
+
+; RUN: lld-link /entry:entry /subsystem:console %p/Inputs/lto-directives.obj /dll /out:%t.dll
+; RUN: llvm-readobj --coff-exports %t.dll | FileCheck %s
+
+; CHECK: Name: entry

``````````

</details>


https://github.com/llvm/llvm-project/pull/78802