[AArch64] Avoid crashing on invalid -Wa,-march= values

[dim]

Summary

As reported in https://bugs.freebsd.org/260078, the gnutls Makefiles
pass -Wa,-march=all to compile a number of assembly files. Clang does
not support this -march value, but because of a mistake in handling
the arguments, an unitialized Arg pointer is dereferenced, which can
cause a segfault.

Work around this by adding a check if the local WaMArch variable is
initialized, and if so, using its value in the diagnostic message.

[tschuett]

clang/lib/Driver/ToolChains/Arch/AArch64.cpp

263

Would an if statement be more readable and actually show what the issue is?

Bonus points for a comment.

[tschuett]

There is precedence for checking WaMArch.size() in the file. I would expected !WaMArch.empty().

[dim]

Address review comments.

[tschuett]

LGTM

clang/lib/Driver/ToolChains/Arch/AArch64.cpp

else if (success && (A = Args.getLastArg(options::OPT_mcpu_EQ)))
     success =
         getAArch64MicroArchFeaturesFromMcpu(D, A->getValue(), Args, Features);
   else if (success &&
            (Args.hasArg(options::OPT_arch) || isCPUDeterminedByTriple(Triple)))
     success = getAArch64MicroArchFeaturesFromMcpu(
         D, getAArch64TargetCPU(Args, Triple, A), Args, Features);
 
   if (!success)
-    D.Diag(diag::err_drv_clang_unsupported) << A->getAsString(Args);
+    D.Diag(diag::err_drv_clang_unsupported)
+        << (WaMArch.size() ? "-march=" + WaMArch.str() : A->getAsString(Args));
 
   if (Args.getLastArg(options::OPT_mgeneral_regs_only)) {
     Features.push_back("-fp-armv8");
     Features.push_back("-crypto");
     Features.push_back("-neon");
   }
 
   if (Arg *A = Args.getLastArg(options::OPT_mtp_mode_EQ)) {

clang/test/Driver/aarch64-target-as-march.s

 // RUN: FileCheck --check-prefix=TARGET-FEATURE-3 %s
 // RUN: %clang --target=aarch64-linux-gnueabi -### -c -Wa,-march=armv8.3-a -march=armv8.4-a \
 // RUN: %S/Inputs/wildcard1.c 2>&1 | FileCheck --check-prefix=TARGET-FEATURE-4 %s
 
 // TARGET-FEATURE-3: "-target-feature" "+v8.3a"
 // TARGET-FEATURE-3-NOT: "-target-feature" "+v8.4a"
 // TARGET-FEATURE-4: "-target-feature" "+v8.4a"
 // TARGET-FEATURE-4-NOT: "-target-feature" "+v8.3a"
+
+// Invalid -march settings
+// RUN: %clang --target=aarch64-linux-gnueabi -### -c -Wa,-march=all %s 2>&1 | \
+// RUN: FileCheck --check-prefix=INVALID-ARCH-1 %s
+// RUN: %clang --target=aarch64-linux-gnueabi -### -c -Wa,-march=foobar %s 2>&1 | \
+// RUN: FileCheck --check-prefix=INVALID-ARCH-2 %s
+
+// INVALID-ARCH-1: error: the clang compiler does not support '-march=all'
+// INVALID-ARCH-2: error: the clang compiler does not support '-march=foobar'