[Support] mmap when possible in getSTDIN.

When stdin is executed like ./prog < file, we are able to mmap the
buffer so that we can reduce the total memory usage if we try to open a
large file.

Author: aokblast

llvm/lib/Support/MemoryBuffer.cpp

@@ -569,10 +569,30 @@ ErrorOr<std::unique_ptr<MemoryBuffer>> MemoryBuffer::getOpenFileSlice(
 
 ErrorOr<std::unique_ptr<MemoryBuffer>> MemoryBuffer::getSTDIN() {
   // Read in all of the data from stdin, we cannot mmap stdin.
-  //
-  // FIXME: That isn't necessarily true, we should try to mmap stdin and
-  // fallback if it fails.
   sys::ChangeStdinMode(sys::fs::OF_Text);
+  std::error_code EC;
+  sys::fs::file_type Type;
+  sys::fs::file_status Status;
+  EC = sys::fs::status(sys::fs::getStdinHandle(), Status);
+  if (EC)
+    return EC;
+
+  Type = Status.type();
+
+  // If the FD is regular file or block file,
+  // we try to create a mmap buffer first.
+  // If failed, rollback to read and copy.
+  if ((Type == sys::fs::file_type::regular_file ||
+       Type == sys::fs::file_type::block_file) &&
+      shouldUseMmap(sys::fs::getStdinHandle(), Status.getSize(),
+                    Status.getSize(), 0, false,
+                    sys::Process::getPageSizeEstimate(), false)) {
+    std::unique_ptr<MemoryBuffer> Result(
+        new (NamedBufferAlloc("<stdin>")) MemoryBufferMMapFile<MemoryBuffer>(
+            false, sys::fs::getStdinHandle(), Status.getSize(), 0, EC));
+    if (!EC)
+      return std::move(Result);
+  }
 
   return getMemoryBufferForStream(sys::fs::getStdinHandle(), "<stdin>");
 }