[Support] Improve readNativeFile(Slice) interface

Summary:
There was a subtle, but pretty important difference between the Slice
and regular versions of this function. The Slice function was
zero-initializing the rest of the buffer when the read syscall returned
less bytes than expected, while the regular function did not.

This patch removes the inconsistency by making both functions *not*
zero-initialize the buffer. The zeroing code is moved to the
MemoryBuffer class, which is currently the only user of this code. This
makes the API more consistent, and the code shorter.

While in there, I also refactor the functions to return the number of
bytes through the regular return value (via Expected<size_t>) instead of
a separate by-ref argument.

Reviewers: aganea, rnk

Subscribers: kristina, Bigcheese, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66471

llvm-svn: 369627

1 files changed, 23 insertions, 10 deletions

diff --git a/llvm/lib/Support/MemoryBuffer.cpp b/llvm/lib/Support/MemoryBuffer.cpp
index 2f31f05..e4027ca 100644
--- a/llvm/lib/Support/MemoryBuffer.cpp
+++ b/llvm/lib/Support/MemoryBuffer.cpp
@@ -211,15 +211,17 @@ static ErrorOr<std::unique_ptr<WritableMemoryBuffer>>
 getMemoryBufferForStream(sys::fs::file_t FD, const Twine &BufferName) {
   const ssize_t ChunkSize = 4096*4;
   SmallString<ChunkSize> Buffer;
-  size_t ReadBytes;
   // Read into Buffer until we hit EOF.
-  do {
+  for (;;) {
     Buffer.reserve(Buffer.size() + ChunkSize);
-    if (auto EC = sys::fs::readNativeFile(
-            FD, makeMutableArrayRef(Buffer.end(), ChunkSize), &ReadBytes))
-      return EC;
-    Buffer.set_size(Buffer.size() + ReadBytes);
-  } while (ReadBytes != 0);
+    Expected<size_t> ReadBytes = sys::fs::readNativeFile(
+        FD, makeMutableArrayRef(Buffer.end(), ChunkSize));
+    if (!ReadBytes)
+      return errorToErrorCode(ReadBytes.takeError());
+    if (*ReadBytes == 0)
+      break;
+    Buffer.set_size(Buffer.size() + *ReadBytes);
+  }
 
   return getMemBufferCopyImpl(Buffer, BufferName);
 }
@@ -458,9 +460,20 @@ getOpenFileImpl(sys::fs::file_t FD, const Twine &Filename, uint64_t FileSize,
     return make_error_code(errc::not_enough_memory);
   }
 
-  if (std::error_code EC =
-          sys::fs::readNativeFileSlice(FD, Buf->getBuffer(), Offset))
-    return EC;
+  // Read until EOF, zero-initialize the rest.
+  MutableArrayRef<char> ToRead = Buf->getBuffer();
+  while (!ToRead.empty()) {
+    Expected<size_t> ReadBytes =
+        sys::fs::readNativeFileSlice(FD, ToRead, Offset);
+    if (!ReadBytes)
+      return errorToErrorCode(ReadBytes.takeError());
+    if (*ReadBytes == 0) {
+      std::memset(ToRead.data(), 0, ToRead.size());
+      break;
+    }
+    ToRead = ToRead.drop_front(*ReadBytes);
+    Offset += *ReadBytes;
+  }
 
   return std::move(Buf);
 }