[LLD][ELF][Windows] small improvement to D82567

[bd1976llvm]

Summary

Bail early if there is no existing output file to be overwritten.

[grimar]

lld/Common/Filesystem.cpp

69

Should it be at the begining of the method to be shared with !_WIN32?

I see it has this in the condition below:

if (parallel::strategy.ThreadsRequested == 1 || !sys::fs::exists(path) ||
    !sys::fs::is_regular_file(path))
  return;

[bd1976llvm]

Share logic between the two code paths.

[bd1976llvm]

BTW - I realised that this function needs formatting correctly :(

*Update*: I was wrong, the formatting is fine - sorry for the noise!

lld/Common/Filesystem.cpp

69

Yes. We can share these conditions between the two paths.

[grimar]

lld/Common/Filesystem.cpp

43

This empty line is probably excessive.

47

I think you can leave this condition at its original place. I am not sure what was the reason
to move it here (it is not a shared piece anyways)?

[bd1976llvm]

lld/Common/Filesystem.cpp

47

Sorry. Premature optimisation.

[bd1976llvm]

Addressed review comments.

[grimar]

LGTM!

lld/Common/Filesystem.cpp

 // you repeatedly link a 1 GB program in a regular compile-link-debug
 // cycle, every cycle wastes 250 milliseconds only to remove a file.
 // Since LLD can link a 1 GB binary in about 5 seconds, that waste
 // actually counts.
 //
 // This function spawns a background thread to remove the file.
 // The calling thread returns almost immediately.
 void lld::unlinkAsync(StringRef path) {
+  if (!sys::fs::exists(path) || !sys::fs::is_regular_file(path))
+    return;
+
 // Removing a file is async on windows.
 #if defined(_WIN32)
   // On Windows co-operative programs can be expected to open LLD's
   // output in FILE_SHARE_DELETE mode. This allows us to delete the
   // file (by moving it to a temporary filename and then deleting
   // it) so that we can link another output file that overwrites
   // the existing file, even if the current file is in use.
   //
   // This is done on a best effort basis - we do not error if the
   // operation fails. The consequence is merely that the user
   // experiences an inconvenient work-flow.
   //
   // The code here allows LLD to work on all versions of Windows.
   // However, at Windows 10 1903 it seems that the behavior of
   // Windows has changed, so that we could simply delete the output 
   // file. This code should be simplified once support for older
   // versions of Windows is dropped.
   //
   // Warning: It seems that the WINVER and _WIN32_WINNT preprocessor
   // defines affect the behavior of the Windows versions of the calls
   // we are using here. If this code stops working this is worth
   // bearing in mind.
   SmallString<128> tmpName;
   if (!sys::fs::createUniqueFile(path + "%%%%%%%%.tmp", tmpName)) {
     if (!sys::fs::rename(path, tmpName))
       path = tmpName;
     else
       sys::fs::remove(tmpName);
   }
   sys::fs::remove(path);
 #else
-  if (parallel::strategy.ThreadsRequested == 1 || !sys::fs::exists(path) ||
-      !sys::fs::is_regular_file(path))
+  if (parallel::strategy.ThreadsRequested == 1)
     return;
 
   // We cannot just remove path from a different thread because we are now going
   // to create path as a new file.
   // Instead we open the file and unlink it on this thread. The unlink is fast
   // since the open fd guarantees that it is not removing the last reference.
   int fd;
   std::error_code ec = sys::fs::openFileForRead(path, fd);