From d37d1c8deaa34216248ad51e549a701852e29fa3 Mon Sep 17 00:00:00 2001
From: Tom Eccles <tom.eccles@arm.com>
Date: Mon, 22 Jan 2024 23:14:09 +0000
Subject: [PATCH] [flang][driver] deprecate manual usage of -lFortran_main
 (#79016)

Intended to warn users of the 18.x release not to do this.

A better solution should be found for the 19.x release. See discussion
in https://github.com/llvm/llvm-project/pull/78152.

Unfortunately there is no warning on Windows currently. I am rushing to
get this landed before 18.x branches.
---
 clang/include/clang/Basic/DiagnosticDriverKinds.td |  2 ++
 clang/lib/Driver/ToolChains/CommonArgs.cpp         | 16 ++++++++++++++--
 flang/test/Driver/linker-flags.f90                 |  4 ++++
 3 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/clang/include/clang/Basic/DiagnosticDriverKinds.td b/clang/include/clang/Basic/DiagnosticDriverKinds.td
index 090b169a0e72..094fe1950941 100644
--- a/clang/include/clang/Basic/DiagnosticDriverKinds.td
+++ b/clang/include/clang/Basic/DiagnosticDriverKinds.td
@@ -427,6 +427,8 @@ def warn_drv_clang_unsupported : Warning<
   "the clang compiler does not support '%0'">;
 def warn_drv_deprecated_arg : Warning<
   "argument '%0' is deprecated, use '%1' instead">, InGroup<Deprecated>;
+def warn_drv_deprecated_custom : Warning<
+  "argument '%0' is deprecated, %1">, InGroup<Deprecated>;
 def warn_drv_assuming_mfloat_abi_is : Warning<
   "unknown platform, assuming -mfloat-abi=%0">;
 def warn_drv_unsupported_float_abi_by_lib : Warning<
diff --git a/clang/lib/Driver/ToolChains/CommonArgs.cpp b/clang/lib/Driver/ToolChains/CommonArgs.cpp
index 385f66f3782b..fadaf3e60c66 100644
--- a/clang/lib/Driver/ToolChains/CommonArgs.cpp
+++ b/clang/lib/Driver/ToolChains/CommonArgs.cpp
@@ -1194,6 +1194,18 @@ static void addFortranMain(const ToolChain &TC, const ArgList &Args,
   }
 
   // 2. GNU and similar
+  const Driver &D = TC.getDriver();
+  const char *FortranMainLinkFlag = "-lFortran_main";
+
+  // Warn if the user added `-lFortran_main` - this library is an implementation
+  // detail of Flang and should be handled automaticaly by the driver.
+  for (const char *arg : CmdArgs) {
+    if (strncmp(arg, FortranMainLinkFlag, strlen(FortranMainLinkFlag)) == 0)
+      D.Diag(diag::warn_drv_deprecated_custom)
+          << FortranMainLinkFlag
+          << "see the Flang driver documentation for correct usage";
+  }
+
   // The --whole-archive option needs to be part of the link line to make
   // sure that the main() function from Fortran_main.a is pulled in by the
   // linker. However, it shouldn't be used if it's already active.
@@ -1201,12 +1213,12 @@ static void addFortranMain(const ToolChain &TC, const ArgList &Args,
   if (!isWholeArchivePresent(Args) && !TC.getTriple().isMacOSX() &&
       !TC.getTriple().isOSAIX()) {
     CmdArgs.push_back("--whole-archive");
-    CmdArgs.push_back("-lFortran_main");
+    CmdArgs.push_back(FortranMainLinkFlag);
     CmdArgs.push_back("--no-whole-archive");
     return;
   }
 
-  CmdArgs.push_back("-lFortran_main");
+  CmdArgs.push_back(FortranMainLinkFlag);
 }
 
 /// Add Fortran runtime libs
diff --git a/flang/test/Driver/linker-flags.f90 b/flang/test/Driver/linker-flags.f90
index ea91946316cf..5e00520fcc09 100644
--- a/flang/test/Driver/linker-flags.f90
+++ b/flang/test/Driver/linker-flags.f90
@@ -11,6 +11,7 @@
 ! RUN: %flang -### --target=x86_64-unknown-dragonfly %S/Inputs/hello.f90 2>&1 | FileCheck %s --check-prefixes=CHECK,UNIX
 ! RUN: %flang -### --target=x86_64-unknown-haiku %S/Inputs/hello.f90 2>&1 | FileCheck %s --check-prefixes=CHECK,HAIKU
 ! RUN: %flang -### --target=x86_64-windows-gnu %S/Inputs/hello.f90 2>&1 | FileCheck %s --check-prefixes=CHECK,MINGW
+! RUN: %flang -### --target=aarch64-unknown-linux-gnu %S/Inputs/hello.f90 -lFortran_main 2>&1 | FileCheck %s --check-prefixes=DEPRECATED
 
 ! NOTE: Clang's driver library, clangDriver, usually adds 'oldnames' on Windows,
 !       but it is not needed when compiling Fortran code and they might bring in
@@ -53,3 +54,6 @@
 ! MSVC-LABEL: link
 ! MSVC-SAME: /subsystem:console
 ! MSVC-SAME: "[[object_file]]"
+
+! Check that we warn when using -lFortran_main
+! DEPRECATED: warning: argument '-lFortran_main' is deprecated, see the Flang driver documentation for correct usage [-Wdeprecated]
-- 
2.11.4.GIT