[flang] [llvm] [flang] improve compatibility with mingw headers (PR #172041)

[Jameson Nash via llvm-commits]

https://github.com/vtjnash created https://github.com/llvm/llvm-project/pull/172041

The mingw headers declare `__environ` already, leading to warnings due
to missing dllimport here. Similarly with _WIN32_WINNT may be already
defined from a header leading to nuisance warnings. And the getpid is
not defined in the current header set (it is in process.h), so that
needs to be defined, just like MSVC (this replaces
https://github.com/msys2/MINGW-packages/blob/576fc4bbfa9bff4d5ab81779a706723b5214fd7d/mingw-w64-flang/0103-fix-build-on-mingw.patch).


>From e9cfe16fc5cfdd4c53016429f74dfeb2a4c2911c Mon Sep 17 00:00:00 2001
From: Jameson Nash <vtjnash at gmail.com>
Date: Fri, 12 Dec 2025 11:17:22 -0500
Subject: [PATCH] [flang] improve compatibility with mingw headers

The mingw headers declare `__environ` already, leading to warnings due
to missing dllimport here. Similarly with _WIN32_WINNT may be already
defined from a header leading to nuisance warnings. And the getpid is
not defined in the current header set (it is in process.h), so that
needs to be defined, just like MSVC (this replaces
https://github.com/msys2/MINGW-packages/blob/576fc4bbfa9bff4d5ab81779a706723b5214fd7d/mingw-w64-flang/0103-fix-build-on-mingw.patch).
---
 flang-rt/lib/runtime/command.cpp             | 2 --
 flang-rt/lib/runtime/environment.cpp         | 2 ++
 flang/include/flang/Common/windows-include.h | 2 ++
 3 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/flang-rt/lib/runtime/command.cpp b/flang-rt/lib/runtime/command.cpp
index 6b5d7722d9ebf..f0f4e8bf64e99 100644
--- a/flang-rt/lib/runtime/command.cpp
+++ b/flang-rt/lib/runtime/command.cpp
@@ -23,11 +23,9 @@
 #define unlink _unlink
 #define PATH_MAX MAX_PATH
 
-#ifdef _MSC_VER
 // On Windows GetCurrentProcessId returns a DWORD aka uint32_t
 #include <processthreadsapi.h>
 inline pid_t getpid() { return GetCurrentProcessId(); }
-#endif
 #else
 #include <unistd.h> //getpid() unlink()
 
diff --git a/flang-rt/lib/runtime/environment.cpp b/flang-rt/lib/runtime/environment.cpp
index be4f7308ab027..aa3daf8baf894 100644
--- a/flang-rt/lib/runtime/environment.cpp
+++ b/flang-rt/lib/runtime/environment.cpp
@@ -16,7 +16,9 @@
 #include <limits>
 
 #ifdef _WIN32
+#ifdef _MSC_VER
 extern char **_environ;
+#endif
 #elif defined(__FreeBSD__)
 // FreeBSD has environ in crt rather than libc. Using "extern char** environ"
 // in the code of a shared library makes it fail to link with -Wl,--no-undefined
diff --git a/flang/include/flang/Common/windows-include.h b/flang/include/flang/Common/windows-include.h
index 01bc6fc9eb94f..1263768e97339 100644
--- a/flang/include/flang/Common/windows-include.h
+++ b/flang/include/flang/Common/windows-include.h
@@ -20,7 +20,9 @@
 
 // Target Windows 2000 and above. This is needed for newer Windows API
 // functions, e.g. GetComputerNameExA()
+#ifndef _WIN32_WINNT
 #define _WIN32_WINNT 0x0500
+#endif
 
 #include <windows.h>