https://bugs.llvm.org/show_bug.cgi?id=38105
Bug ID: 38105
Summary: LTO builds fail when dllimport functions are called
locally
Product: new-bugs
Version: unspecified
Hardware: PC
OS: Windows NT
Status: NEW
Severity: enhancement
Priority: P
Component: new bugs
Assignee: unassignedb...@nondot.org
Reporter: dma...@mozilla.com
CC: llvm-bugs@lists.llvm.org, l...@inglorion.net,
r...@google.com, tejohn...@google.com
When a function is declared dllimport but called directly (i.e. in the same
binary), a non-LTO build will issue a warning but still succeed (same as MSVC).
LTO and ThinLTO builds fail with linkage complaints.
This is preventing Firefox's use of LTO on Windows:
https://bugzilla.mozilla.org/show_bug.cgi?id=1448976
clang version 7.0.0 (trunk 336407)
$ cat a.cpp
__declspec(dllimport) int foo();
int main() { return foo(); }
$ cat b.cpp
int foo() { return 42; }
$ cat test.sh
#!/bin/bash
rm -rf *.obj *.exe
clang-cl $FLAG -O2 -c a.cpp b.cpp
lld-link -nodefaultlib -entry:main a.obj b.obj
$ ./test.sh
lld-link.exe: warning: a.obj: locally defined symbol imported: ?foo@@YAHXZ
(defined in b.obj) [LNK4217]
$ FLAG=-flto ./test.sh
Global is external, but doesn't have external or weak linkage!
i32 ()* @"?foo@@YAHXZ"
LLVM ERROR: Broken module found, compilation aborted!
$ FLAG=-flto=thin ./test.sh
lld-link.exe: error: undefined symbol: ?foo@@YAHXZ
>>> referenced by lto.tmp:(main)
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