Alternative solution to performance regression.

Author: johnno1962

lld/MachO/InputFiles.cpp

@@ -217,8 +217,7 @@ std::optional<MemoryBufferRef> macho::readFile(StringRef path) {
   if (entry != cachedReads.end())
     return entry->second;
 
-  ErrorOr<std::unique_ptr<MemoryBuffer>> mbOrErr =
-      MemoryBuffer::getFile(path, false, /*RequiresNullTerminator*/ false);
+  ErrorOr<std::unique_ptr<MemoryBuffer>> mbOrErr = MemoryBuffer::getFile(path);
   if (std::error_code ec = mbOrErr.getError()) {
     error("cannot open " + path + ": " + ec.message());
     return std::nullopt;

llvm/lib/Object/Archive.cpp

@@ -584,8 +584,7 @@ Expected<StringRef> Archive::Child::getBuffer() const {
   if (!FullNameOrErr)
     return FullNameOrErr.takeError();
   const std::string &FullName = *FullNameOrErr;
-  ErrorOr<std::unique_ptr<MemoryBuffer>> Buf =
-      MemoryBuffer::getFile(FullName, false, /*RequiresNullTerminator*/ false);
+  ErrorOr<std::unique_ptr<MemoryBuffer>> Buf = MemoryBuffer::getFile(FullName);
   if (std::error_code EC = Buf.getError())
     return errorCodeToError(EC);
   Parent->ThinBuffers.push_back(std::move(*Buf));

llvm/lib/Support/MemoryBuffer.cpp

@@ -505,8 +505,9 @@ getOpenFileImpl(sys::fs::file_t FD, const Twine &Filename, uint64_t FileSize,
       // On at least Linux, and possibly on other systems, mmap may return pages
       // from the page cache that are not properly filled with trailing zeroes,
       // if some prior user of the page wrote non-zero bytes. Detect this and
-      // don't use mmap in that case.
-      if (!RequiresNullTerminator || *Result->getBufferEnd() == '\0')
+      // don't use mmap in that case (unless it is an object file).
+      if (!RequiresNullTerminator || *Result->getBufferEnd() == '\0' ||
+          StringRef(Filename.str()).ends_with(".o"))
         return std::move(Result);
     }
   }