FileOutputBuffer: support non-mmap'able file.

[ruiu]

Summary

Previously, FileOutputBuffer didn't work for non-mmap'able files, so
you cannot use that class for /dev/null for example. This patch fixes
the issue.

Now, before mmap'ing a file, it checks whether a given file is a regular
file or not. If it is a regular file, it uses mmap as before. If not, it
creates an internal buffer and write that buffer contents on commit().

This patch enables "-o /dev/null" for LLD. Previouly, it failed.

[grimar]

llvm/lib/Support/FileOutputBuffer.cpp

159 ↗

(On Diff #82187)

llvm::make_unique may be ?

[ruiu]

llvm/lib/Support/FileOutputBuffer.cpp

159 ↗

(On Diff #82187)

Why?

[grimar]

llvm/lib/Support/FileOutputBuffer.cpp

159 ↗

(On Diff #82187)

That should be shorter and avoids using "new".

It is generally also consistent with recomendations to use std::make_shared instead of std::shared_ptr(new X) which is preffered because of different reasons usually (single allocation of T and control block in most of implementations for example).

[ruiu]

llvm/lib/Support/FileOutputBuffer.cpp

159 ↗

(On Diff #82187)

Ah, there's a reason why we can't use make_unique. The FileOutputBuffer ctor is private, so we need to instantiate it from this function. This function is a member of that class, so it can call the ctor. However, make_unique is not, so it can't.

lld/trunk/ELF/Writer.cpp

static void unlinkAsync(StringRef Path) {
   if (!Config->Threads || !sys::fs::exists(Config->OutputFile))
     return;
 
   // First, rename Path to avoid race condition. We cannot remove
   // Path from a different thread because we are now going to create
   // Path as a new file. If we do that in a different thread, the new
   // thread can remove the new file.
   SmallString<128> TempPath;
-  if (auto EC = sys::fs::createUniqueFile(Path + "tmp%%%%%%%%", TempPath))
-    fatal(EC, "createUniqueFile failed");
-  if (auto EC = sys::fs::rename(Path, TempPath))
-    fatal(EC, "rename failed");
+  if (sys::fs::createUniqueFile(Path + "tmp%%%%%%%%", TempPath))
+    return;
+  if (sys::fs::rename(Path, TempPath)) {
+    sys::fs::remove(TempPath);
+    return;
+  }
 
   // Remove TempPath in background.
   std::thread([=] { ::remove(TempPath.str().str().c_str()); }).detach();
 }
 
 // Open a result file.
 template <class ELFT> void Writer<ELFT>::openFile() {
   unlinkAsync(Config->OutputFile);

lld/trunk/test/ELF/basic.s

 # REQUIRES: x86
 
 # RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
 # RUN: ld.lld %t -o %t2
 # RUN: llvm-readobj -file-headers -sections -program-headers -symbols %t2 \
 # RUN:   | FileCheck %s
+# RUN: ld.lld %t -o /dev/null
 
 # exits with return code 42 on linux
 .globl _start
 _start:
   mov $60, %rax
   mov $42, %rdi
   syscall