disas/libvixl: Fix fall-through annotation for GCC >= 7

For compiling with -Wimplicit-fallthrough we need to fix the
fallthrough annotations in the libvixl code. This is based on
the following upstream vixl commit by Martyn Capewell:

 https://git.linaro.org/arm/vixl.git/commit/?id=de326f850f736c3a337

 "GCC 7 enables switch/case fallthrough checking, but this fails in
  VIXL, because the annotation we use is Clang specific.

  Also, fix a missing annotation in the disassembler."

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20201211152426.350966-2-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

2 files changed, 8 insertions, 2 deletions

diff --git a/disas/libvixl/vixl/a64/disasm-a64.cc b/disas/libvixl/vixl/a64/disasm-a64.cc
index 7a58a5c087..f34d1d68da 100644
--- a/disas/libvixl/vixl/a64/disasm-a64.cc
+++ b/disas/libvixl/vixl/a64/disasm-a64.cc
@@ -2985,6 +2985,10 @@ int Disassembler::SubstituteImmediateField(const Instruction* instr,
           }
           return 3;
         }
+        default: {
+          VIXL_UNIMPLEMENTED();
+          return 0;
+        }
       }
     }
     case 'C': {  // ICondB - Immediate Conditional Branch.
diff --git a/disas/libvixl/vixl/globals.h b/disas/libvixl/vixl/globals.h
index 61dc9f7f7e..7099aa599f 100644
--- a/disas/libvixl/vixl/globals.h
+++ b/disas/libvixl/vixl/globals.h
@@ -108,10 +108,12 @@ inline void USE(T1, T2, T3, T4) {}
   #define __has_warning(x)  0
 #endif
 
-// Note: This option is only available for Clang. And will only be enabled for
-// C++11(201103L).
+// Fallthrough annotation for Clang and C++11(201103L).
 #if __has_warning("-Wimplicit-fallthrough") && __cplusplus >= 201103L
   #define VIXL_FALLTHROUGH() [[clang::fallthrough]] //NOLINT
+// Fallthrough annotation for GCC >= 7.
+#elif __GNUC__ >= 7
+  #define VIXL_FALLTHROUGH() __attribute__((fallthrough))
 #else
   #define VIXL_FALLTHROUGH() do {} while (0)
 #endif