[patch] Drop an unnecessary use of writev

[Rafael Espíndola via llvm-commits]

I think the code this patch deletes is based on a misunderstanding of
what guarantees writev provides. In particular, writev with 1 iovec is
not "more atomic" than a write.

With the attached test program I get 'a's and 'b's on the same line on
OS X when using both write and writev.

Cheers,
Rafael
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diff --git a/include/llvm/Support/raw_ostream.h b/include/llvm/Support/raw_ostream.h
index e5cc40e..d1e96f8 100644
--- a/include/llvm/Support/raw_ostream.h
+++ b/include/llvm/Support/raw_ostream.h
@@ -349,10 +349,6 @@ class raw_fd_ostream : public raw_pwrite_stream {
   ///
   bool Error;
 
-  /// Controls whether the stream should attempt to use atomic writes, when
-  /// possible.
-  bool UseAtomicWrites;
-
   uint64_t pos;
 
   bool SupportsSeeking;
@@ -402,16 +398,6 @@ public:
   /// to the offset specified from the beginning of the file.
   uint64_t seek(uint64_t off);
 
-  /// Set the stream to attempt to use atomic writes for individual output
-  /// routines where possible.
-  ///
-  /// Note that because raw_ostream's are typically buffered, this flag is only
-  /// sensible when used on unbuffered streams which will flush their output
-  /// immediately.
-  void SetUseAtomicWrites(bool Value) {
-    UseAtomicWrites = Value;
-  }
-
   raw_ostream &changeColor(enum Colors colors, bool bold=false,
                            bool bg=false) override;
   raw_ostream &resetColor() override;
diff --git a/lib/Support/raw_ostream.cpp b/lib/Support/raw_ostream.cpp
index 49ef400..57c7ac3 100644
--- a/lib/Support/raw_ostream.cpp
+++ b/lib/Support/raw_ostream.cpp
@@ -517,7 +517,7 @@ raw_fd_ostream::raw_fd_ostream(StringRef Filename, std::error_code &EC,
 /// closes the file when the stream is destroyed.
 raw_fd_ostream::raw_fd_ostream(int fd, bool shouldClose, bool unbuffered)
     : raw_pwrite_stream(unbuffered), FD(fd), ShouldClose(shouldClose),
-      Error(false), UseAtomicWrites(false) {
+      Error(false) {
   if (FD < 0 ) {
     ShouldClose = false;
     return;
@@ -568,21 +568,7 @@ void raw_fd_ostream::write_impl(const char *Ptr, size_t Size) {
   pos += Size;
 
   do {
-    ssize_t ret;
-
-    // Check whether we should attempt to use atomic writes.
-    if (LLVM_LIKELY(!UseAtomicWrites)) {
-      ret = ::write(FD, Ptr, Size);
-    } else {
-      // Use ::writev() where available.
-#if defined(HAVE_WRITEV)
-      const void *Addr = static_cast<const void *>(Ptr);
-      struct iovec IOV = {const_cast<void *>(Addr), Size };
-      ret = ::writev(FD, &IOV, 1);
-#else
-      ret = ::write(FD, Ptr, Size);
-#endif
-    }
+    ssize_t ret = ::write(FD, Ptr, Size);
 
     if (ret < 0) {
       // If it's a recoverable error, swallow it and retry the write.
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diff --git a/lib/Frontend/CompilerInstance.cpp b/lib/Frontend/CompilerInstance.cpp
index 04f1390..3edcf5d 100644
--- a/lib/Frontend/CompilerInstance.cpp
+++ b/lib/Frontend/CompilerInstance.cpp
@@ -155,7 +155,6 @@ static void SetUpDiagnosticLog(DiagnosticOptions *DiagOpts,
           << DiagOpts->DiagnosticLogFile << EC.message();
     } else {
       FileOS->SetUnbuffered();
-      FileOS->SetUseAtomicWrites(true);
       OS = FileOS.get();
       StreamOwner = std::move(FileOS);
     }
diff --git a/lib/Frontend/HeaderIncludeGen.cpp b/lib/Frontend/HeaderIncludeGen.cpp
index 9123d53..0bc1169 100644
--- a/lib/Frontend/HeaderIncludeGen.cpp
+++ b/lib/Frontend/HeaderIncludeGen.cpp
@@ -92,7 +92,6 @@ void clang::AttachHeaderIncludeGen(Preprocessor &PP,
       delete OS;
     } else {
       OS->SetUnbuffered();
-      OS->SetUseAtomicWrites(true);
       OutputFile = OS;
       OwnsOutputFile = true;
     }
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