[Support] Add MemoryBuffer::dontNeedIfMmap

On *NIX systems, this API calls madvise(MADV_DONTNEED) on read-only file mappings.
It should not be used on a writable buffer.
The API is used to implement ld.lld LTO memory saving trick (D116367).

Note: on read-only file mappings, Linux's MADV_DONTNEED semantics match POSIX
POSIX_MADV_DONTNEED and BSD systems' MADV_DONTNEED.

On Windows, VirtualAllocEx MEM_COMMIT/MEM_RESET have similar semantics
but are unfortunately not drop-in replacements. dontNeedIfMmap is currently a no-op.

Reviewed By: aganea

Differential Revision: https://reviews.llvm.org/D116366

llvm/include/llvm/Support/FileSystem.h

@@ -1279,6 +1279,7 @@ private:
   }
 
   void unmapImpl();
+  void dontNeedImpl();
 
   std::error_code init(sys::fs::file_t FD, uint64_t Offset, mapmode Mode);
 
@@ -1308,6 +1309,7 @@ public:
     unmapImpl();
     copyFrom(mapped_file_region());
   }
+  void dontNeed() { dontNeedImpl(); }
 
   size_t size() const;
   char *data() const;

llvm/include/llvm/Support/MemoryBuffer.h

@@ -74,6 +74,13 @@ public:
   /// from.
   virtual StringRef getBufferIdentifier() const { return "Unknown buffer"; }
 
+  /// For read-only MemoryBuffer_MMap, mark the buffer as unused in the near
+  /// future and the kernel can free resources associated with it. Further
+  /// access is supported but may be expensive. This calls
+  /// madvise(MADV_DONTNEED) on read-only file mappings on *NIX systems. This
+  /// function should not be called on a writable buffer.
+  virtual void dontNeedIfMmap() {}
+
   /// Open the specified file as a MemoryBuffer, returning a new MemoryBuffer
   /// if successful, otherwise returning null.
   ///

llvm/lib/Support/MemoryBuffer.cpp

@@ -220,6 +220,8 @@ public:
   MemoryBuffer::BufferKind getBufferKind() const override {
     return MemoryBuffer::MemoryBuffer_MMap;
   }
+
+  void dontNeedIfMmap() override { MFR.dontNeed(); }
 };
 } // namespace
 

llvm/lib/Support/Unix/Path.inc

@@ -870,6 +870,12 @@ void mapped_file_region::unmapImpl() {
     ::munmap(Mapping, Size);
 }
 
+void mapped_file_region::dontNeedImpl() {
+  assert(Mode == mapped_file_region::readonly);
+  if (Mapping)
+    ::madvise(Mapping, Size, MADV_DONTNEED);
+}
+
 int mapped_file_region::alignment() {
   return Process::getPageSizeEstimate();
 }

llvm/lib/Support/Windows/Path.inc

@@ -959,6 +959,8 @@ void mapped_file_region::unmapImpl() {
   }
 }
 
+void mapped_file_region::dontNeedImpl() {}
+
 int mapped_file_region::alignment() {
   SYSTEM_INFO SysInfo;
   ::GetSystemInfo(&SysInfo);