[flang-commits] [flang] [Flang] Fix more of CMake Fortran detection probes (PR #204438)

[Michael Kruse via flang-commits]

https://github.com/Meinersbur created https://github.com/llvm/llvm-project/pull/204438

Tested with CMake 3.28 and 4.3, supporting the following variants that CMake may use to introspect the Fortran compiler:
1. `__GNUC__` may be tested before `__flang__` which results in the compiler to be identified as "GNU"
2. Invoke with `-E -cpp` to explicitly use the preprocessor

Should fix the [flang-aarch64-rel-assert](https://lab.llvm.org/buildbot/#/builders/29) buildbot

>From da08d8e8cee6f8f0419c5fdbce93ffbb8c3de885 Mon Sep 17 00:00:00 2001
From: Michael Kruse <llvm-project at meinersbur.de>
Date: Wed, 17 Jun 2026 22:39:24 +0200
Subject: [PATCH 1/2] Fix CMake detection

---
 flang/tools/fakeflang/fakeflang.cpp | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/flang/tools/fakeflang/fakeflang.cpp b/flang/tools/fakeflang/fakeflang.cpp
index 500174b612862..635691be60981 100644
--- a/flang/tools/fakeflang/fakeflang.cpp
+++ b/flang/tools/fakeflang/fakeflang.cpp
@@ -84,9 +84,16 @@ int main(int argc, const char **argv) {
   Args.append({ClangExe, "-E", "-P", "-D__flang__=1",
       "-D__flang_major__=" FLANG_VERSION_MAJOR_STRING,
       "-D__flang_minor__=" FLANG_VERSION_MINOR_STRING,
-      "-D__flang_patchlevel__=" FLANG_VERSION_PATCHLEVEL_STRING, "-x", "c"});
-  for (int I = 1; I < argc; ++I)
-    Args.push_back(argv[I]);
+      "-D__flang_patchlevel__=" FLANG_VERSION_PATCHLEVEL_STRING,
+      "-U__GNUC__", "-x", "c"});
+  for (int I = 1; I < argc; ++I) {
+    llvm::StringRef arg = argv[I];
+
+    // Sometime CMake tries to invoke the preprocessor
+    if (arg == "-cpp" || arg == "-E") continue ;
+
+    Args.push_back(arg);
+  }
   if (!hasDashO)
     Args.append({"-o", "a.out"});
 

>From 47a3925439b4201bfdce2cd7178834c7f9d64bd9 Mon Sep 17 00:00:00 2001
From: Michael Kruse <llvm-project at meinersbur.de>
Date: Wed, 17 Jun 2026 22:44:46 +0200
Subject: [PATCH 2/2] Fix fakeflang

---
 flang/test/Driver/fakeflang.F       | 12 ++++++++++--
 flang/tools/fakeflang/fakeflang.cpp |  7 ++++---
 2 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/flang/test/Driver/fakeflang.F b/flang/test/Driver/fakeflang.F
index bd4ed88409bd7..526f535769049 100644
--- a/flang/test/Driver/fakeflang.F
+++ b/flang/test/Driver/fakeflang.F
@@ -4,11 +4,15 @@
 ! RUN: mkdir -p %t
 ! RUN: cd %t
 
-! RUN: fakeflang -v -c --target=x86_64-unknown-linux-gnu %s
+! RUN: fakeflang -v -c -cpp -E --target=x86_64-unknown-linux-gnu %s
 ! RUN: FileCheck %s --input-file=a.out --check-prefixes=CHECK,GNU
 ! RUN: rm a.out
 
-! RUN: fakeflang -v -c --target=x86_64-pc-windows-msvc %s
+! RUN: fakeflang -v -c -cpp -E --target=aarch64-linux-gnu %s
+! RUN: FileCheck %s --input-file=a.out --check-prefixes=CHECK,GNU
+! RUN: rm a.out
+
+! RUN: fakeflang -v -c -cpp -E --target=x86_64-pc-windows-msvc %s
 ! RUN: FileCheck %s --input-file=a.out --check-prefixes=CHECK,MSVC --match-full-lines
 ! RUN: rm a.out
 
@@ -16,6 +20,10 @@
 ! CHECK: CMAKE_Fortran_COMPILER_ID=1
 CMAKE_Fortran_COMPILER_ID=__flang__
 
+! __GNUC__ is not defined
+! CHECK: __GNUC__
+__GNUC__
+
 ! CHECK: CMAKE_Fortran_COMPILER_VERSION={{[0-9]+}} . {{[0-9]+}} . {{[0-9]+}}
 CMAKE_Fortran_COMPILER_VERSION=__flang_major__.__flang_minor__.__flang_patchlevel__
 
diff --git a/flang/tools/fakeflang/fakeflang.cpp b/flang/tools/fakeflang/fakeflang.cpp
index 635691be60981..f1eb5bf0921d0 100644
--- a/flang/tools/fakeflang/fakeflang.cpp
+++ b/flang/tools/fakeflang/fakeflang.cpp
@@ -84,13 +84,14 @@ int main(int argc, const char **argv) {
   Args.append({ClangExe, "-E", "-P", "-D__flang__=1",
       "-D__flang_major__=" FLANG_VERSION_MAJOR_STRING,
       "-D__flang_minor__=" FLANG_VERSION_MINOR_STRING,
-      "-D__flang_patchlevel__=" FLANG_VERSION_PATCHLEVEL_STRING,
-      "-U__GNUC__", "-x", "c"});
+      "-D__flang_patchlevel__=" FLANG_VERSION_PATCHLEVEL_STRING, "-U__GNUC__",
+      "-x", "c"});
   for (int I = 1; I < argc; ++I) {
     llvm::StringRef arg = argv[I];
 
     // Sometime CMake tries to invoke the preprocessor
-    if (arg == "-cpp" || arg == "-E") continue ;
+    if (arg == "-cpp" || arg == "-E")
+      continue;
 
     Args.push_back(arg);
   }