[llvm] [offload] Fix duplicate __llvm_write_custom_profile on Windows PGO builds (PR #207366)

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<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-offload

Author: Wenju He (wenju-he)

<details>
<summary>Changes</summary>

MSVC/clang-cl lack __attribute__((weak)), so GlobalHandler.cpp provided a strong stub for __llvm_write_custom_profile. When clang_rt.profile is linked (e.g. instrumented/PGO builds) lld-link rejects the duplicate strong symbol.

Replace the stub + CMake workaround with the /alternatename linker pragma, which is the standard Windows weak-symbol equivalent used by compiler-rt itself. The linker picks compiler-rt's strong definition when present.

Fix the guard: use _WIN32 && !__MINGW32__ (not _MSC_VER) so the stub and /alternatename pragma apply to plain clang-on-Windows too, while MinGW (which supports __attribute__((weak)) natively) continues to use the real weak declaration. Also fix the stub signature from 8 to 10 parameters to match the header.

Assisted-by: Claude

---
Full diff: https://github.com/llvm/llvm-project/pull/207366.diff


1 Files Affected:

- (modified) offload/plugins-nextgen/common/src/GlobalHandler.cpp (+14-12) 


``````````diff
diff --git a/offload/plugins-nextgen/common/src/GlobalHandler.cpp b/offload/plugins-nextgen/common/src/GlobalHandler.cpp
index 0298e238a469b..1611a7a6b1ce2 100644
--- a/offload/plugins-nextgen/common/src/GlobalHandler.cpp
+++ b/offload/plugins-nextgen/common/src/GlobalHandler.cpp
@@ -30,17 +30,20 @@ using namespace plugin;
 using namespace error;
 using namespace llvm::offload::debug;
 
-// Windows/MSVC doesn't support weak symbols properly, so provide a stub
-// implementation that will be used if compiler-rt is not linked.
-#ifdef _WIN32
-extern "C" int __llvm_write_custom_profile(
-    const char *Target, const __llvm_profile_data *DataBegin,
-    const __llvm_profile_data *DataEnd, const char *CountersBegin,
-    const char *CountersEnd, const char *NamesBegin, const char *NamesEnd,
-    const uint64_t *VersionOverride) {
-  // Return error code indicating profiling is not available
+// Windows (MSVC/clang, but not MinGW) does not support __attribute__((weak)),
+// so provide a fallback stub via /alternatename. The linker picks compiler-rt's
+// strong definition when clang_rt.profile is linked; otherwise the stub is
+// used.
+#if defined(_WIN32) && !defined(__MINGW32__)
+extern "C" int __llvm_write_custom_profile_default(
+    const char *, const __llvm_profile_data *, const __llvm_profile_data *,
+    const char *, const char *, const char *, const char *, const char *,
+    const char *, const uint64_t *) {
   return -1;
 }
+#pragma comment(                                                               \
+    linker,                                                                    \
+    "/alternatename:__llvm_write_custom_profile=__llvm_write_custom_profile_default")
 #endif
 
 Expected<std::unique_ptr<ObjectFile>>
@@ -275,9 +278,8 @@ void GPUProfGlobals::dump() const {
 }
 
 Error GPUProfGlobals::write() const {
-#ifndef _WIN32
-  // On Windows __llvm_write_custom_profile is a strong stub (see above), so its
-  // address is never null and this check triggers -Wpointer-bool-conversion.
+#if !defined(_WIN32) || defined(__MINGW32__)
+  // On Windows (non-MinGW) the /alternatename stub above is never null.
   if (!__llvm_write_custom_profile)
     return Plugin::error(ErrorCode::INVALID_BINARY,
                          "could not find symbol __llvm_write_custom_profile. "

``````````

</details>


https://github.com/llvm/llvm-project/pull/207366