[clang] `clang -mpopcnt hello.c <link flag>` fails to generate warning when file precedes link flag

[paparodeo]

on aarch64 darwin using the -mpopcnt flag will result in a warning on clang 16 and error on clang 19.1.3. however, if a linker flag trails the file name no warning / error is generated. eg:

$ clang -mpopcnt hello.c -lc
$ ./a.out
hello world

however, re-ordering the command line so the file name comes after -lc will generate the expected error

$ clang -mpopcnt -lc hello.c
clang: error: unsupported option '-mpopcnt' for target 'aarch64-apple-darwin'

this behavior is not seen with all link flags, eg -flat_namespace always will generate an error, but when used in conjunction with -Wl will not generate an error (as do other flags, like -v)

$ clang -mpopcnt -flat_namespace hello.c
clang: error: unsupported option '-mpopcnt' for target 'aarch64-apple-darwin'
$ clang -mpopcnt hello.c -flat_namespace
clang: error: unsupported option '-mpopcnt' for target 'aarch64-apple-darwin'
$ clang -mpopcnt -Wl,-flat_namespace hello.c
clang: error: unsupported option '-mpopcnt' for target 'aarch64-apple-darwin'
$ clang -mpopcnt hello.c -Wl,-flat_namespace
$ ./a.out
hello world

$ clang -v
clang version 19.1.3
Target: aarch64-apple-darwin24.1.0
Thread model: posix
InstalledDir: /nix/store/6b4vxqc16xq1nq8jhdsylfml244vah22-clang-19.1.3/bin

[llvmbot]

@llvm/issue-subscribers-clang-driver

Author: Reno Dakota (paparodeo)

on aarch64 darwin using the `-mpopcnt` flag will result in a warning on clang 16 and error on clang 19.1.3. however, if a linker flag trails the file name no warning / error is generated. eg:

$ clang -mpopcnt hello.c -lc
$ ./a.out
hello world

however, re-ordering the command line so the file name comes after -lc will generate the expected error

$ clang -mpopcnt -lc hello.c
clang: error: unsupported option '-mpopcnt' for target 'aarch64-apple-darwin'

this behavior is not seen with all link flags, eg -flat_namespace always will generate an error, but when used in conjunction with -Wl will generate an error (as do other flags, like -v)

$ clang -mpopcnt -flat_namespace hello.c
clang: error: unsupported option '-mpopcnt' for target 'aarch64-apple-darwin'
$ clang -mpopcnt hello.c -flat_namespace
clang: error: unsupported option '-mpopcnt' for target 'aarch64-apple-darwin'
$ clang -mpopcnt -Wl,-flat_namespace hello.c
clang: error: unsupported option '-mpopcnt' for target 'aarch64-apple-darwin'
$ clang -mpopcnt hello.c -Wl,-flat_namespace
$ ./a.out
hello world

$ clang -v
clang version 19.1.3
Target: aarch64-apple-darwin24.1.0
Thread model: posix
InstalledDir: /nix/store/6b4vxqc16xq1nq8jhdsylfml244vah22-clang-19.1.3/bin

[paparodeo]

this can be reproduce on x64 linux by finding some other unsupported option: eg

$ clang -mno-mma -mhtm hello.c  
clang: error: unsupported option '-mno-mma' for target 'x86_64-unknown-linux-gnu'
clang: error: unsupported option '-mhtm' for target 'x86_64-unknown-linux-gnu'
$ clang -mno-mma -mhtm hello.c -lc
$ ./a.out
hello world

[paparodeo]

seems to be this block which is swallowing the errors:

llvm-project/clang/lib/Driver/Driver.cpp

Lines 4132 to 4139 in 47928ab

	// If we are linking, claim any options which are obviously only used for
	// compilation.
	// FIXME: Understand why the last Phase List length is used here.
	if (FinalPhase == phases::Link && LastPLSize == 1) {
	Args.ClaimAllArgs(options::OPT_CompileOnly_Group);
	Args.ClaimAllArgs(options::OPT_cl_compile_Group);
	}
	}

[paparodeo]

this patch seems to fix the issue for me. if files are only getting linked it will still throw away compile related flags, but if there are files which are not getting linked + something else it will not throw the flags away.

diff --git a/clang/lib/Driver/Driver.cpp b/clang/lib/Driver/Driver.cpp
index 93e85f7df..ce2b21299 100644
--- a/clang/lib/Driver/Driver.cpp
+++ b/clang/lib/Driver/Driver.cpp
@@ -4064,13 +4064,15 @@ void Driver::handleArguments(Compilation &C, DerivedArgList &Args,
     YcArg = YuArg = nullptr;
   }
 
-  unsigned LastPLSize = 0;
+  unsigned MaxPLSize = 0;
   for (auto &I : Inputs) {
     types::ID InputType = I.first;
     const Arg *InputArg = I.second;
 
     auto PL = types::getCompilationPhases(InputType);
-    LastPLSize = PL.size();
+    if (MaxPLSize < PL.size()) {
+        MaxPLSize = PL.size();
+    }
 
     // If the first step comes after the final phase we are doing as part of
     // this compilation, warn the user about it.
@@ -4132,7 +4134,7 @@ void Driver::handleArguments(Compilation &C, DerivedArgList &Args,
   // If we are linking, claim any options which are obviously only used for
   // compilation.
   // FIXME: Understand why the last Phase List length is used here.
-  if (FinalPhase == phases::Link && LastPLSize == 1) {
+  if (FinalPhase == phases::Link && MaxPLSize == 1) {
     Args.ClaimAllArgs(options::OPT_CompileOnly_Group);
     Args.ClaimAllArgs(options::OPT_cl_compile_Group);
   }

[MaskRay]

Thanks for reporting and fixing this issue:)

I noticed the longstanding FIXME but did not realize it would cause this issue.
https://maskray.me/blog/2023-08-25-clang-wunused-command-line-argument