[Driver][ARM] Warn about -mabi= for assembler input

[MaskRay]

Summary

Previously, Clang Driver reported a warning when assembler input was assembled
with the -mabi= option. D152856 added TargetSpecific to -mabi= option and
reported an error for such a case. This change restores the previous behavior by
reporting a warning.

GCC translates -mabi={apcs-gnu,atpcs} to gas -meabi=gnu and other -mabi= values
to -meabi=5. We don't support setting e_flags to any value other than
EF_ARM_EABI_VER5.

Close https://github.com/ClangBuiltLinux/linux/issues/1878

[michaelplatings]

The change seems OK to me in principle, but I'd appreciate the code being more self-descriptive. Thanks.

clang/lib/Driver/ToolChains/Arch/ARM.cpp

528

This comment is quite terse. Could you please expand it? Maybe something like:

// -mabi is not a supported option for the assembler.
// For GCC compatibility accept but warn.

clang/test/Driver/arm-abi.c

65

Other tests in this file begin with a one-line explanation and I think it would be good to do the same here. Maybe something like:

// The combination of -x assember & -mabi is not supported but for GCC compatibility should be accepted with a warning

[MaskRay]

Thanks for the comments. Improved comments.

[michaelplatings]

LGTM

clang/lib/Driver/ToolChains/Arch/ARM.cpp

for (const Arg *A :
           WaCPU = std::make_pair(A, Value.substr(6));
         } else if (Value.startswith("-mhwdiv=")) {
           WaHDiv = std::make_pair(A, Value.substr(8));
         } else if (Value.startswith("-march=")) {
           WaArch = std::make_pair(A, Value.substr(7));
         }
       }
     }
+
+    // The integrated assembler doesn't implement e_flags setting behavior for
+    // -meabi=gnu (gcc -mabi={apcs-gnu,atpcs} passes -meabi=gnu to gas). For
+    // compatibility we accept but warn.
+    if (Arg *A = Args.getLastArgNoClaim(options::OPT_mabi_EQ))
+      A->ignoreTargetSpecific();
   }
 
   if (getReadTPMode(D, Args, Triple, ForAS) == ReadTPMode::TPIDRURW)
     Features.push_back("+read-tp-tpidrurw");
   if (getReadTPMode(D, Args, Triple, ForAS) == ReadTPMode::TPIDRURO)
     Features.push_back("+read-tp-tpidruro");
   if (getReadTPMode(D, Args, Triple, ForAS) == ReadTPMode::TPIDRPRW)
     Features.push_back("+read-tp-tpidrprw");

clang/test/Driver/arm-abi.c

 // RUN: %clang -target arm---gnueabi -mabi=aapcs %s -### -o %t.o 2>&1 \
 // RUN:   | FileCheck -check-prefix=CHECK-AAPCS %s
 // RUN: %clang -target arm---eabi -mabi=aapcs-linux %s -### -o %t.o 2>&1 \
 // RUN:   | FileCheck -check-prefix=CHECK-AAPCS-LINUX %s
 
 // CHECK-APCS-GNU: "-target-abi" "apcs-gnu"
 // CHECK-AAPCS: "-target-abi" "aapcs"
 // CHECK-AAPCS-LINUX: "-target-abi" "aapcs-linux"
+
+// RUN: %clang --target=arm---gnueabi -mabi=aapcs -x assembler %s -### -o /dev/null 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-ASM %s
+
+/// The combination -x assember & -mabi is not implemented, but for GCC compatibility we accept with a warning.
+// CHECK-ASM: warning: argument unused during compilation: '-mabi={{.*}}'