[Support] Add in memory flag for FileOutputBuffer

[terrelln]

Summary

Adds a flag F_in_memory to prefer in-memory output buffers over on-disk
output buffers.

We need this to support -no-mmap-output-file in LLD. LLD currently
explicitly ignores this flag for compatibility with the GNU linker.

Test Plan:

ninja check-llvm

[ruiu]

Could you explain a little bit about the motivation to add -no-mmap-output-file option to lld?

[ruiu]

llvm/include/llvm/Support/FileOutputBuffer.h

36–37

Technically mmap'ed files are also in memory (and on disk at the same time), so this comment might be a bit misleading. I'd name this F_no_mmap and explain that option as "Don't use mmap and instead write a in-memory buffer to a file when this buffer is closed".

[ruiu]

Maybe this should be merged with https://reviews.llvm.org/D69294. Since we are moving to a single repository that contains both llvm and lld, we no longer have to split a commit at svn repo boundaries. Since this flag will be used only by lld at least for a while, it feels more natural to commit this and the other one as a single commit.

[terrelln]

Maybe this should be merged with https://reviews.llvm.org/D69294. Since we are moving to a single repository that contains both llvm and lld, we no longer have to split a commit at svn repo boundaries. Since this flag will be used only by lld at least for a while, it feels more natural to commit this and the other one as a single commit.

Personally I like splitting up the diff by component so it is easier to search the git history of a file. But I'm happy to combine them, so I'll go ahead and do that.

[terrelln]

Merged into D69294

llvm/include/llvm/Support/FileOutputBuffer.h

 /// If the FileOutputBuffer is committed, the target file's content will become
 /// the buffer content at the time of the commit.  If the FileOutputBuffer is
 /// not committed, the file will be deleted in the FileOutputBuffer destructor.
 class FileOutputBuffer {
 public:
   enum {
     /// set the 'x' bit on the resulting file
     F_executable = 1,
+
+    /// Always use an in-memory buffer instead of an on-disk buffer.
+    F_in_memory = 2,
   };
 
   /// Factory method to create an OutputBuffer object which manages a read/write
   /// buffer of the specified size. When committed, the buffer will be written
   /// to the file at the specified path.
   ///
   /// When F_modify is specified and \p FilePath refers to an existing on-disk
   /// file \p Size may be set to -1, in which case the entire file is used.

llvm/lib/Support/FileOutputBuffer.cpp

FileOutputBuffer::create(StringRef Path, size_t Size, unsigned Flags) {
   // file.) If that's the case, we create an in-memory buffer, open the
   // destination file and write to it on commit().
   switch (Stat.type()) {
   case fs::file_type::directory_file:
     return errorCodeToError(errc::is_a_directory);
   case fs::file_type::regular_file:
   case fs::file_type::file_not_found:
   case fs::file_type::status_error:
+    if (Flags & F_in_memory)
+      return createInMemoryBuffer(Path, Size, Mode);
+    else
       return createOnDiskBuffer(Path, Size, Mode);
   default:
     return createInMemoryBuffer(Path, Size, Mode);
   }
 }

llvm/unittests/Support/FileOutputBufferTest.cpp

TEST(FileOutputBuffer, Test) {
   }
   // Verify file exists and is executable.
   fs::file_status Status;
   ASSERT_NO_ERROR(fs::status(Twine(File4), Status));
   bool IsExecutable = (Status.permissions() & fs::owner_exe);
   EXPECT_TRUE(IsExecutable);
   ASSERT_NO_ERROR(fs::remove(File4.str()));
 
+  // TEST 5: In-memory buffer works as expected.
+  SmallString<128> File5(TestDirectory);
+  File5.append("/file5");
+  {
+    Expected<std::unique_ptr<FileOutputBuffer>> BufferOrErr =
+        FileOutputBuffer::create(File5, 8000, FileOutputBuffer::F_in_memory);
+    ASSERT_NO_ERROR(errorToErrorCode(BufferOrErr.takeError()));
+    std::unique_ptr<FileOutputBuffer> &Buffer = *BufferOrErr;
+    // Start buffer with special header.
+    memcpy(Buffer->getBufferStart(), "AABBCCDDEEFFGGHHIIJJ", 20);
+    ASSERT_NO_ERROR(errorToErrorCode(Buffer->commit()));
+    // Write to end of buffer to verify it is writable.
+    memcpy(Buffer->getBufferEnd() - 20, "AABBCCDDEEFFGGHHIIJJ", 20);
+    // Commit buffer.
+    ASSERT_NO_ERROR(errorToErrorCode(Buffer->commit()));
+  }
+
+  // Verify file is correct size.
+  uint64_t File5Size;
+  ASSERT_NO_ERROR(fs::file_size(Twine(File5), File5Size));
+  ASSERT_EQ(File5Size, 8000ULL);
+  ASSERT_NO_ERROR(fs::remove(File5.str()));
   // Clean up.
   ASSERT_NO_ERROR(fs::remove(TestDirectory.str()));
 }
 } // anonymous namespace