Avoid including FileSystem.h from MemoryBuffer.h

Lots of headers pass around MemoryBuffer objects, but very few open
them. Let those that do include FileSystem.h.

Saves ~250 includes of Chrono.h & FileSystem.h:

$ diff -u thedeps-before.txt thedeps-after.txt | grep '^[-+] ' | sort | uniq -c | sort -nr
    254 -    ../llvm/include/llvm/Support/FileSystem.h
    253 -    ../llvm/include/llvm/Support/Chrono.h
    237 -    ../llvm/include/llvm/Support/NativeFormatting.h
    237 -    ../llvm/include/llvm/Support/FormatProviders.h
    192 -    ../llvm/include/llvm/ADT/StringSwitch.h
    190 -    ../llvm/include/llvm/Support/FormatVariadicDetails.h
...

This requires duplicating the file_t typedef, which is unfortunate. I
sunk the choice of mapping mode down into the cpp file using variable
template specializations instead of class members in headers.

1 files changed, 15 insertions, 1 deletions

diff --git a/llvm/lib/Support/MemoryBuffer.cpp b/llvm/lib/Support/MemoryBuffer.cpp
index e4027ca..e467daf 100644
--- a/llvm/lib/Support/MemoryBuffer.cpp
+++ b/llvm/lib/Support/MemoryBuffer.cpp
@@ -162,6 +162,20 @@ MemoryBuffer::getFileSlice(const Twine &FilePath, uint64_t MapSize,
 //===----------------------------------------------------------------------===//
 
 namespace {
+
+template <typename MB>
+constexpr sys::fs::mapped_file_region::mapmode Mapmode =
+    sys::fs::mapped_file_region::readonly;
+template <>
+constexpr sys::fs::mapped_file_region::mapmode Mapmode<MemoryBuffer> =
+    sys::fs::mapped_file_region::readonly;
+template <>
+constexpr sys::fs::mapped_file_region::mapmode Mapmode<WritableMemoryBuffer> =
+    sys::fs::mapped_file_region::priv;
+template <>
+constexpr sys::fs::mapped_file_region::mapmode
+    Mapmode<WriteThroughMemoryBuffer> = sys::fs::mapped_file_region::readwrite;
+
 /// Memory maps a file descriptor using sys::fs::mapped_file_region.
 ///
 /// This handles converting the offset into a legal offset on the platform.
@@ -184,7 +198,7 @@ class MemoryBufferMMapFile : public MB {
 public:
   MemoryBufferMMapFile(bool RequiresNullTerminator, sys::fs::file_t FD, uint64_t Len,
                        uint64_t Offset, std::error_code &EC)
-      : MFR(FD, MB::Mapmode, getLegalMapSize(Len, Offset),
+      : MFR(FD, Mapmode<MB>, getLegalMapSize(Len, Offset),
             getLegalMapOffset(Offset), EC) {
     if (!EC) {
       const char *Start = getStart(Len, Offset);