[Driver] -fno-plt: warn for unsupported targets

[MaskRay]

-fno-plt is an ELF specific option that is only implemented for x86 (for
a long time) and AArch64 (#78890). GCC doesn't bother to give a
diagnostic on Windows. -fno-plt is somewhat popular and we've been
ignoring it for unsupported targets for a while, so just report a
warning for unsupported targets.

[llvmbot]

@llvm/pr-subscribers-clang-driver

@llvm/pr-subscribers-clang

Author: Fangrui Song (MaskRay)

Changes

-fno-plt is an ELF specific option that GCC doesn't bother to give a
diagnostic on Windows. This option is somewhat popular and we've been
ignoring it for unsupported targets for a while (x86 support for a
while, #78890 for AArch64 support), so just report a warning for
unsupported object file formats.

---

Full diff: https://github.com/llvm/llvm-project/pull/124081.diff

2 Files Affected:

• (modified) clang/lib/Driver/ToolChains/Clang.cpp (+2-3)
• (added) clang/test/Driver/fno-plt.c (+7)

diff --git a/clang/lib/Driver/ToolChains/Clang.cpp b/clang/lib/Driver/ToolChains/Clang.cpp
index 33f08cf28feca1..5df58005a52373 100644
--- a/clang/lib/Driver/ToolChains/Clang.cpp
+++ b/clang/lib/Driver/ToolChains/Clang.cpp
@@ -6141,9 +6141,8 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
     CmdArgs.push_back("-fno-direct-access-external-data");
   }
 
-  if (Args.hasFlag(options::OPT_fno_plt, options::OPT_fplt, false)) {
-    CmdArgs.push_back("-fno-plt");
-  }
+  if (Triple.isOSBinFormatELF())
+    Args.addOptOutFlag(CmdArgs, options::OPT_fplt, options::OPT_fno_plt);
 
   // -fhosted is default.
   // TODO: Audit uses of KernelOrKext and see where it'd be more appropriate to
diff --git a/clang/test/Driver/fno-plt.c b/clang/test/Driver/fno-plt.c
new file mode 100644
index 00000000000000..d5189e2b508b2a
--- /dev/null
+++ b/clang/test/Driver/fno-plt.c
@@ -0,0 +1,7 @@
+// RUN: %clang -### -c --target=aarch64 -fno-plt -Werror %s 2>&1 | FileCheck %s
+// RUN: %clang -### -c --target=aarch64 -fno-plt -fplt -Werror %s 2>&1 | FileCheck %s --check-prefix=NO
+// RUN: %clang -### -c --target=aarch64-windows -fno-plt %s 2>&1 | FileCheck %s --check-prefixes=WARN,NO
+
+// WARN: warning: argument unused during compilation: '-fno-plt' [-Wunused-command-line-argument]
+// CHECK: "-fno-plt"
+// NO-NOT: "-fno-plt"

[mstorsjo]

LGTM, thanks! This seems to nicely fix the issue and make the situation clearer.

CC @Martchus.

[mstorsjo]

There's quite many negations involved in this check line, can you rename the NO prefix to e.g. PLT or something like that, to signify the case when -fplt is in effect? Then again, using --check-prefix=PLT for the aarch64-windows target feels a bit misleading as well. Perhaps if we'd rename CHECK into NOPLT and NO into DEFAULT?