[llvm-objdump] avoid crash disassembling unknown instruction

Disassembly of instructions can fail when llvm-objdump is not given the right set of
architecture features, for example when the source is compiled with:

  clang -march=..+ext1+ext2

and disassembly is attempted with:

  llvm-objdump -mattr=+ext1

This patch avoids further analysing unknown instructions (as was happening
before) when disassembly has failed.

Differential Revision: https://reviews.llvm.org/D73531

llvm/test/tools/llvm-objdump/ARM/unknown-instr.test

@@ -0,0 +1,28 @@
+# RUN: yaml2obj %s -o %t
+# RUN: llvm-objdump -D -triple=thumbv8.1m.main-none-eabi %t | FileCheck %s
+
+## This is a test case with "random" data/instructions, checking that
+## llvm-objdump handles such instructions cleanly. Disassembly of instructions
+## can fail when it e.g. is not given the right set of architecture features,
+## for example when the source is compiled with:
+##
+##   clang -march=..+ext1+ext2
+##
+## and disassembly is attempted with:
+##
+##   llvm-objdump -mattr=+ext1
+
+# CHECK:        00000000 .text:
+# CHECK-NEXT:       0: cb              <unknown>
+# CHECK-NEXT:       1: f3 f7 8b be     b.w #-49898
+
+--- !ELF
+FileHeader:
+  Class:   ELFCLASS32
+  Data:    ELFDATA2LSB
+  Type:    ET_REL
+  Machine: EM_ARM
+Sections:
+  - Name:    .text
+    Type:    SHT_PROGBITS
+    Content: "cbf3f78bbe"

llvm/tools/llvm-objdump/llvm-objdump.cpp

@@ -1431,6 +1431,14 @@ static void disassembleObject(const Target *TheTarget, const ObjectFile *Obj,
         outs() << CommentStream.str();
         Comments.clear();
 
+        // If disassembly has failed, continue with the next instruction, to
+        // avoid analysing invalid/incomplete instruction information.
+        if (!Disassembled) {
+          outs() << "\n";
+          Index += Size;
+          continue;
+        }
+
         // Try to resolve the target of a call, tail call, etc. to a specific
         // symbol.
         if (MIA && (MIA->isCall(Inst) || MIA->isUnconditionalBranch(Inst) ||