llvm-isel-fuzzer: Make buildable and testable without libFuzzer

This adds a dummy main so we can build and run the llvm-isel-fuzzer
functionality when we aren't building LLVM with coverage. The approach
here should serve as a template to stop in-tree fuzzers from
bitrotting (See llvm.org/pr34314).

Note that I'll probably move most of the logic in DummyISelFuzzer's
`main` to a library so it's easy to reuse it in other fuzz targets,
but I'm planning on doing that in a follow up that also consolidates
argument handling in our LLVMFuzzerInitialize implementations.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@312338 91177308-0d34-0410-b5e6-96231b3b80d8

cmake/modules/AddLLVM.cmake

@@ -893,11 +893,15 @@ macro(add_llvm_utility name)
 endmacro(add_llvm_utility name)
 
 macro(add_llvm_fuzzer name)
+  cmake_parse_arguments(ARG "" "DUMMY_MAIN" "" ${ARGN})
   if( LLVM_USE_SANITIZE_COVERAGE )
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fsanitize=fuzzer")
-    add_llvm_executable(${name} ${ARGN})
+    add_llvm_executable(${name} ${ARG_UNPARSED_ARGUMENTS})
     set_target_properties(${name} PROPERTIES FOLDER "Fuzzers")
-  endif()
+  elseif( ARG_DUMMY_MAIN )
+    add_llvm_executable(${name} ${ARG_DUMMY_MAIN} ${ARG_UNPARSED_ARGUMENTS})
+    set_target_properties(${name} PROPERTIES FOLDER "Fuzzers")
+endif()
 endmacro()
 
 macro(add_llvm_target target_name)

tools/llvm-isel-fuzzer/CMakeLists.txt

@@ -12,4 +12,5 @@ set(LLVM_LINK_COMPONENTS
     Support
     Target
 )
-add_llvm_fuzzer(llvm-isel-fuzzer llvm-isel-fuzzer.cpp)
+add_llvm_fuzzer(llvm-isel-fuzzer llvm-isel-fuzzer.cpp
+  DUMMY_MAIN DummyISelFuzzer.cpp)

tools/llvm-isel-fuzzer/DummyISelFuzzer.cpp

@@ -0,0 +1,56 @@
+//===--- DummyFuzzerMain.cpp - Entry point to sanity check the fuzzer -----===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// Implementation of main so we can build and test without linking libFuzzer.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/ADT/StringRef.h"
+#include "llvm/Support/Compiler.h"
+#include "llvm/Support/Error.h"
+#include "llvm/Support/MemoryBuffer.h"
+#include "llvm/Support/raw_ostream.h"
+
+using namespace llvm;
+
+extern "C" int LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size);
+extern "C" LLVM_ATTRIBUTE_WEAK int LLVMFuzzerInitialize(int *argc,
+                                                        char ***argv) {
+  return 0;
+}
+
+int main(int argc, char *argv[]) {
+  errs() << "*** This tool was not linked to libFuzzer.\n"
+         << "*** No fuzzing will be performed.\n";
+  if (int RC = LLVMFuzzerInitialize(&argc, &argv)) {
+    errs() << "Initialization failed\n";
+    return RC;
+  }
+
+  for (int I = 1; I < argc; ++I) {
+    StringRef Arg(argv[I]);
+    if (Arg.startswith("-")) {
+      if (Arg.equals("-ignore_remaining_args=1"))
+        break;
+      continue;
+    }
+
+    auto BufOrErr = MemoryBuffer::getFile(Arg, /*FileSize-*/ -1,
+                                          /*RequiresNullTerminator=*/false);
+    if (std::error_code EC = BufOrErr.getError()) {
+      errs() << "Error reading file: " << Arg << ": " << EC.message() << "\n";
+      return 1;
+    }
+    std::unique_ptr<MemoryBuffer> Buf = std::move(BufOrErr.get());
+    errs() << "Running: " << Arg << " (" << Buf->getBufferSize() << " bytes)\n";
+    LLVMFuzzerTestOneInput(
+        reinterpret_cast<const uint8_t *>(Buf->getBufferStart()),
+        Buf->getBufferSize());
+  }
+}