[SystemZ][z/OS] Update autoconversion functions to improve support for UTF-8 (#98652)

This fixes the following error when reading source and header files on
z/OS: error: source file is not valid UTF-8

1 files changed, 14 insertions, 2 deletions

diff --git a/llvm/lib/Support/MemoryBuffer.cpp b/llvm/lib/Support/MemoryBuffer.cpp
index 7ea68ee..e2044bc 100644
--- a/llvm/lib/Support/MemoryBuffer.cpp
+++ b/llvm/lib/Support/MemoryBuffer.cpp
@@ -361,6 +361,11 @@ static bool shouldUseMmap(sys::fs::file_t FD,
                           bool RequiresNullTerminator,
                           int PageSize,
                           bool IsVolatile) {
+#if defined(__MVS__)
+  // zOS Enhanced ASCII auto convert does not support mmap.
+  return false;
+#endif
+
   // mmap may leave the buffer without null terminator if the file size changed
   // by the time the last page is mapped in, so avoid it if the file size is
   // likely to change.
@@ -503,9 +508,16 @@ getOpenFileImpl(sys::fs::file_t FD, const Twine &Filename, uint64_t FileSize,
   }
 
 #ifdef __MVS__
-  // Set codepage auto-conversion for z/OS.
-  if (auto EC = llvm::enablezOSAutoConversion(FD))
+  ErrorOr<bool> NeedConversion = needzOSConversion(Filename.str().c_str(), FD);
+  if (std::error_code EC = NeedConversion.getError())
     return EC;
+  // File size may increase due to EBCDIC -> UTF-8 conversion, therefore we
+  // cannot trust the file size and we create the memory buffer by copying
+  // off the stream.
+  // Note: This only works with the assumption of reading a full file (i.e,
+  // Offset == 0 and MapSize == FileSize). Reading a file slice does not work.
+  if (Offset == 0 && MapSize == FileSize && *NeedConversion)
+    return getMemoryBufferForStream(FD, Filename);
 #endif
 
   auto Buf =