diff --git a/clang/lib/Basic/FileManager.cpp b/clang/lib/Basic/FileManager.cpp
--- a/clang/lib/Basic/FileManager.cpp
+++ b/clang/lib/Basic/FileManager.cpp
@@ -489,7 +489,7 @@
   uint64_t FileSize = Entry->getSize();
   // If there's a high enough chance that the file have changed since we
   // got its size, force a stat before opening it.
-  if (isVolatile)
+  if (isVolatile || Entry->isNamedPipe())
     FileSize = -1;
 
   StringRef Filename = Entry->getName();
diff --git a/clang/lib/Basic/SourceManager.cpp b/clang/lib/Basic/SourceManager.cpp
--- a/clang/lib/Basic/SourceManager.cpp
+++ b/clang/lib/Basic/SourceManager.cpp
@@ -115,23 +115,6 @@
   // return paths.
   IsBufferInvalid = true;
 
-  // Check that the file's size fits in an 'unsigned' (with room for a
-  // past-the-end value). This is deeply regrettable, but various parts of
-  // Clang (including elsewhere in this file!) use 'unsigned' to represent file
-  // offsets, line numbers, string literal lengths, and so on, and fail
-  // miserably on large source files.
-  if ((uint64_t)ContentsEntry->getSize() >=
-      std::numeric_limits<unsigned>::max()) {
-    if (Diag.isDiagnosticInFlight())
-      Diag.SetDelayedDiagnostic(diag::err_file_too_large,
-                                ContentsEntry->getName());
-    else
-      Diag.Report(Loc, diag::err_file_too_large)
-        << ContentsEntry->getName();
-
-    return None;
-  }
-
   auto BufferOrError = FM.getBufferForFile(ContentsEntry, IsFileVolatile);
 
   // If we were unable to open the file, then we are in an inconsistent
@@ -153,9 +136,31 @@
 
   Buffer = std::move(*BufferOrError);
 
-  // Check that the file's size is the same as in the file entry (which may
+  // Check that the file's size fits in an 'unsigned' (with room for a
+  // past-the-end value). This is deeply regrettable, but various parts of
+  // Clang (including elsewhere in this file!) use 'unsigned' to represent file
+  // offsets, line numbers, string literal lengths, and so on, and fail
+  // miserably on large source files.
+  //
+  // Note: ContentsEntry could be a named pipe, in which case
+  // ContentsEntry::getSize() could have the wrong size. Use
+  // MemoryBuffer::getBufferSize() instead.
+  if (Buffer->getBufferSize() >= std::numeric_limits<unsigned>::max()) {
+    if (Diag.isDiagnosticInFlight())
+      Diag.SetDelayedDiagnostic(diag::err_file_too_large,
+                                ContentsEntry->getName());
+    else
+      Diag.Report(Loc, diag::err_file_too_large)
+        << ContentsEntry->getName();
+
+    return None;
+  }
+
+  // Unless this is a named pipe (in which case we can handle a mismatch),
+  // check that the file's size is the same as in the file entry (which may
   // have come from a stat cache).
-  if (Buffer->getBufferSize() != (size_t)ContentsEntry->getSize()) {
+  if (!ContentsEntry->isNamedPipe() &&
+      Buffer->getBufferSize() != (size_t)ContentsEntry->getSize()) {
     if (Diag.isDiagnosticInFlight())
       Diag.SetDelayedDiagnostic(diag::err_file_modified,
                                 ContentsEntry->getName());
@@ -541,6 +546,12 @@
                                    int LoadedID, unsigned LoadedOffset) {
   SrcMgr::ContentCache &IR = getOrCreateContentCache(&SourceFile.getFileEntry(),
                                                      isSystem(FileCharacter));
+
+  // If this is a named pipe, immediately load the buffer to ensure subsequent
+  // calls to ContentCache::getSize() are accurate.
+  if (IR.ContentsEntry->isNamedPipe())
+    (void)IR.getBufferOrNone(Diag, getFileManager(), SourceLocation());
+
   return createFileIDImpl(IR, SourceFile.getName(), IncludePos, FileCharacter,
                           LoadedID, LoadedOffset);
 }
diff --git a/clang/lib/Frontend/CompilerInstance.cpp b/clang/lib/Frontend/CompilerInstance.cpp
--- a/clang/lib/Frontend/CompilerInstance.cpp
+++ b/clang/lib/Frontend/CompilerInstance.cpp
@@ -856,32 +856,9 @@
       Diags.Report(diag::err_fe_error_reading) << InputFile;
       return false;
     }
-    FileEntryRef File = *FileOrErr;
-
-    // The natural SourceManager infrastructure can't currently handle named
-    // pipes, but we would at least like to accept them for the main
-    // file. Detect them here, read them with the volatile flag so FileMgr will
-    // pick up the correct size, and simply override their contents as we do for
-    // STDIN.
-    if (File.getFileEntry().isNamedPipe()) {
-      auto MB =
-          FileMgr.getBufferForFile(&File.getFileEntry(), /*isVolatile=*/true);
-      if (MB) {
-        // Create a new virtual file that will have the correct size.
-        FileEntryRef FE =
-            FileMgr.getVirtualFileRef(InputFile, (*MB)->getBufferSize(), 0);
-        SourceMgr.overrideFileContents(FE, std::move(*MB));
-        SourceMgr.setMainFileID(
-            SourceMgr.createFileID(FE, SourceLocation(), Kind));
-      } else {
-        Diags.Report(diag::err_cannot_open_file) << InputFile
-                                                 << MB.getError().message();
-        return false;
-      }
-    } else {
-      SourceMgr.setMainFileID(
-          SourceMgr.createFileID(File, SourceLocation(), Kind));
-    }
+
+    SourceMgr.setMainFileID(
+        SourceMgr.createFileID(*FileOrErr, SourceLocation(), Kind));
   } else {
     llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>> SBOrErr =
         llvm::MemoryBuffer::getSTDIN();
diff --git a/clang/test/Misc/dev-fd-fs.c b/clang/test/Misc/dev-fd-fs.c
--- a/clang/test/Misc/dev-fd-fs.c
+++ b/clang/test/Misc/dev-fd-fs.c
@@ -8,7 +8,13 @@
 // RUN: cat %s | %clang -x c /dev/fd/0 -E > %t
 // RUN: FileCheck --check-prefix DEV-FD-INPUT < %t %s
 //
+// RUN: cat %s | %clang -x c %s -E -DINCLUDE_FROM_STDIN > %t
+// RUN: FileCheck --check-prefix DEV-FD-INPUT \
+// RUN:           --check-prefix DEV-FD-INPUT-INCLUDE < %t %s
+//
+// DEV-FD-INPUT-INCLUDE: int w;
 // DEV-FD-INPUT: int x;
+// DEV-FD-INPUT-INCLUDE: int y;
 
 
 // Check writing to /dev/fd named pipes. We use cat here as before to ensure we
@@ -27,4 +33,11 @@
 //
 // DEV-FD-REG-OUTPUT: int x;
 
+#ifdef INCLUDE_FROM_STDIN
+#undef INCLUDE_FROM_STDIN
+int w;
+#include "/dev/fd/0"
+int y;
+#else
 int x;
+#endif