[XRay][compiler-rt] FDRLogWriter Abstraction

Summary:
This change introduces an `FDRLogWriter` type which is responsible for
serialising metadata and function records to character buffers. This is
the first step in a refactoring of the implementation of the FDR runtime
to allow for more granular testing of the individual components of the
implementation.

The main contribution of this change is a means of hiding the details of
how specific records are written to a buffer, and for managing the
extents of these buffers. We make use of C++ features (templates and
some metaprogramming) to reduce repetition in the act of writing out
specific kinds of records to the buffer.

In this process, we make a number of changes across both LLVM and
compiler-rt to allow us to use the `Trace` abstraction defined in the
LLVM project in the testing of the runtime implementation. This gives us
a closer end-to-end test which version-locks the runtime implementation
with the loading implementation in LLVM.

We also allow using gmock in compiler-rt unit tests, by adding the
requisite definitions in the `AddCompilerRT.cmake` module.

Finally, we've gone ahead and updated the FDR logging implementation to
use the FDRLogWriter for the lowest-level record-writing details.

Following patches will isolate the state machine transitions which
manage the set-up and tear-down of the buffers we're using in multiple
threads.

Reviewers: mboerger, eizan

Subscribers: mgorny, jfb, llvm-commits

Differential Revision: https://reviews.llvm.org/D52220

llvm-svn: 342518

1 files changed, 9 insertions, 4 deletions

diff --git a/llvm/include/llvm/XRay/Trace.h b/llvm/include/llvm/XRay/Trace.h
index 262ff96bc23..924addd1560 100644
--- a/llvm/include/llvm/XRay/Trace.h
+++ b/llvm/include/llvm/XRay/Trace.h
@@ -46,20 +46,25 @@ namespace xray {
 ///
 class Trace {
   XRayFileHeader FileHeader;
-  std::vector<XRayRecord> Records;
+  using RecordVector = std::vector<XRayRecord>;
+  RecordVector Records;
 
   typedef std::vector<XRayRecord>::const_iterator citerator;
 
   friend Expected<Trace> loadTrace(const DataExtractor &, bool);
 
 public:
+  using size_type = RecordVector::size_type;
+  using value_type = RecordVector::value_type;
+  using const_iterator = RecordVector::const_iterator;
+
   /// Provides access to the loaded XRay trace file header.
   const XRayFileHeader &getFileHeader() const { return FileHeader; }
 
-  citerator begin() const { return Records.begin(); }
-  citerator end() const { return Records.end(); }
+  const_iterator begin() const { return Records.begin(); }
+  const_iterator end() const { return Records.end(); }
   bool empty() const { return Records.empty(); }
-  size_t size() const { return Records.size(); }
+  size_type size() const { return Records.size(); }
 };
 
 /// This function will attempt to load XRay trace records from the provided