[clang] [Clang][Interpreter] Respect default setting on Windows (PR #210507)

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<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: aokblast

<details>
<summary>Changes</summary>

MSCV does not support the -fPIC option. When it is passed, Clang falls back to PIC level 2 on x86_64 and to a static binary on AArch64. Without -fPIC, however, PIC is already enabled by default on both targets.

As a result, the PCH (built with the default PIC level by toolchain hint) and the clang-repl instance (using the fallback PIC level) end up with different PIC settings, consuming a mismatch.

Avoid passing -fPIC when targeting MSVC on Windows. This preserve the existing semantics, since PIC is already enabled by default, while ensuring that both the PCH and clang-repl use the same PIC level.

---
Full diff: https://github.com/llvm/llvm-project/pull/210507.diff


1 Files Affected:

- (modified) clang/lib/Interpreter/Interpreter.cpp (+8-2) 


``````````diff
diff --git a/clang/lib/Interpreter/Interpreter.cpp b/clang/lib/Interpreter/Interpreter.cpp
index 933a68b50db41..0536fdcd548a2 100644
--- a/clang/lib/Interpreter/Interpreter.cpp
+++ b/clang/lib/Interpreter/Interpreter.cpp
@@ -53,6 +53,7 @@
 #include "llvm/Support/VirtualFileSystem.h"
 #include "llvm/Support/raw_ostream.h"
 #include "llvm/TargetParser/Host.h"
+#include "llvm/TargetParser/Triple.h"
 #include "llvm/Transforms/Utils/Cloning.h" // for CloneModule
 
 #define DEBUG_TYPE "clang-repl"
@@ -228,8 +229,13 @@ IncrementalCompilerBuilder::create(std::string TT,
   // host symbol may be out of range when the JIT memory is mapped more than
   // 2GB away (as on FreeBSD), breaking tests such as
   // Interpreter/simple-exception.cpp. Insert before user arguments so it can
-  // still be overridden.
-  ClangArgv.insert(ClangArgv.begin() + 1, "-fPIC");
+  // still be overridden. On Windows (excluding Cygwin/MinGW) an explicit
+  // -fPIC is an unsupported driver option that would drop non-x86_64 targets
+  // to PIC level 0; PIC is already the forced default there where relevant,
+  // so don't inject it.
+  llvm::Triple TargetTriple(TT);
+  if (!TargetTriple.isOSWindows() || TargetTriple.isOSCygMing())
+    ClangArgv.insert(ClangArgv.begin() + 1, "-fPIC");
 
   // Prepending -c to force the driver to do something if no action was
   // specified. By prepending we allow users to override the default

``````````

</details>


https://github.com/llvm/llvm-project/pull/210507