Support,lld: Rename misnamed F_no_mmap to F_mmap

[MaskRay]

F_no_mmap introduced by https://reviews.llvm.org/D69294 is misnamed.
It oughts to be F_mmap

When the output is a regular file or do not exist,
--no-mmap-output-file is the default. Relands #134787 by fixing the
lld option default. Note: changing the default to --map-output-file
would likely fail on llvm-clang-x86_64-sie-win
(https://lab.llvm.org/buildbot/#/builders/46/builds/14847)

[llvmbot]

@llvm/pr-subscribers-lld-elf

@llvm/pr-subscribers-llvm-support

Author: Fangrui Song (MaskRay)

Changes

F_no_mmap introduced by https://reviews.llvm.org/D69294 is misnamed.
It oughts to be F_mmap

When the output is a regular file or do not exist,
--no-mmap-output-file is the default. Relands #134787 by fixing the
lld option default. Note: changing the default to --map-output-file
would likely fail on llvm-clang-x86_64-sie-win
(https://lab.llvm.org/buildbot/#/builders/46/builds/14847)

---

Full diff: https://github.com/llvm/llvm-project/pull/139836.diff

6 Files Affected:

• (modified) lld/ELF/Arch/ARM.cpp (+1-1)
• (modified) lld/ELF/Driver.cpp (+1-1)
• (modified) lld/ELF/Writer.cpp (+2-2)
• (modified) llvm/include/llvm/Support/FileOutputBuffer.h (+2-3)
• (modified) llvm/lib/Support/FileOutputBuffer.cpp (+1-1)
• (modified) llvm/unittests/Support/FileOutputBufferTest.cpp (+1-1)

diff --git a/lld/ELF/Arch/ARM.cpp b/lld/ELF/Arch/ARM.cpp
index e667fdc0633c5..e45dd4d354afb 100644
--- a/lld/ELF/Arch/ARM.cpp
+++ b/lld/ELF/Arch/ARM.cpp
@@ -1489,7 +1489,7 @@ template <typename ELFT> void elf::writeARMCmseImportLib(Ctx &ctx) {
   const uint64_t fileSize =
       sectionHeaderOff + shnum * sizeof(typename ELFT::Shdr);
   const unsigned flags =
-      ctx.arg.mmapOutputFile ? 0 : (unsigned)FileOutputBuffer::F_no_mmap;
+      ctx.arg.mmapOutputFile ? (unsigned)FileOutputBuffer::F_mmap : 0;
   unlinkAsync(ctx.arg.cmseOutputLib);
   Expected<std::unique_ptr<FileOutputBuffer>> bufferOrErr =
       FileOutputBuffer::create(ctx.arg.cmseOutputLib, fileSize, flags);
diff --git a/lld/ELF/Driver.cpp b/lld/ELF/Driver.cpp
index e8acdbefa32bb..76a37b706c5fa 100644
--- a/lld/ELF/Driver.cpp
+++ b/lld/ELF/Driver.cpp
@@ -1421,7 +1421,7 @@ static void readConfigs(Ctx &ctx, opt::InputArgList &args) {
   ctx.arg.mergeArmExidx =
       args.hasFlag(OPT_merge_exidx_entries, OPT_no_merge_exidx_entries, true);
   ctx.arg.mmapOutputFile =
-      args.hasFlag(OPT_mmap_output_file, OPT_no_mmap_output_file, true);
+      args.hasFlag(OPT_mmap_output_file, OPT_no_mmap_output_file, false);
   ctx.arg.nmagic = args.hasFlag(OPT_nmagic, OPT_no_nmagic, false);
   ctx.arg.noinhibitExec = args.hasArg(OPT_noinhibit_exec);
   ctx.arg.nostdlib = args.hasArg(OPT_nostdlib);
diff --git a/lld/ELF/Writer.cpp b/lld/ELF/Writer.cpp
index e2aebff20e174..6a0552e808c7b 100644
--- a/lld/ELF/Writer.cpp
+++ b/lld/ELF/Writer.cpp
@@ -2908,8 +2908,8 @@ template <class ELFT> void Writer<ELFT>::openFile() {
   unsigned flags = 0;
   if (!ctx.arg.relocatable)
     flags |= FileOutputBuffer::F_executable;
-  if (!ctx.arg.mmapOutputFile)
-    flags |= FileOutputBuffer::F_no_mmap;
+  if (ctx.arg.mmapOutputFile)
+    flags |= FileOutputBuffer::F_mmap;
   Expected<std::unique_ptr<FileOutputBuffer>> bufferOrErr =
       FileOutputBuffer::create(ctx.arg.outputFile, fileSize, flags);
 
diff --git a/llvm/include/llvm/Support/FileOutputBuffer.h b/llvm/include/llvm/Support/FileOutputBuffer.h
index d4b73522115db..f98e7a5470b55 100644
--- a/llvm/include/llvm/Support/FileOutputBuffer.h
+++ b/llvm/include/llvm/Support/FileOutputBuffer.h
@@ -31,9 +31,8 @@ class FileOutputBuffer {
     /// Set the 'x' bit on the resulting file.
     F_executable = 1,
 
-    /// Don't use mmap and instead write an in-memory buffer to a file when this
-    /// buffer is closed.
-    F_no_mmap = 2,
+    /// Use mmap for in-memory file buffer.
+    F_mmap = 2,
   };
 
   /// Factory method to create an OutputBuffer object which manages a read/write
diff --git a/llvm/lib/Support/FileOutputBuffer.cpp b/llvm/lib/Support/FileOutputBuffer.cpp
index 58a06a34e8cf3..a2396d7629488 100644
--- a/llvm/lib/Support/FileOutputBuffer.cpp
+++ b/llvm/lib/Support/FileOutputBuffer.cpp
@@ -186,7 +186,7 @@ FileOutputBuffer::create(StringRef Path, size_t Size, unsigned Flags) {
   case fs::file_type::regular_file:
   case fs::file_type::file_not_found:
   case fs::file_type::status_error:
-    if (Flags & F_no_mmap)
+    if (Flags & F_mmap)
       return createInMemoryBuffer(Path, Size, Mode);
     else
       return createOnDiskBuffer(Path, Size, Mode);
diff --git a/llvm/unittests/Support/FileOutputBufferTest.cpp b/llvm/unittests/Support/FileOutputBufferTest.cpp
index f7bb0833e5a0e..423a6e12240c0 100644
--- a/llvm/unittests/Support/FileOutputBufferTest.cpp
+++ b/llvm/unittests/Support/FileOutputBufferTest.cpp
@@ -123,7 +123,7 @@ TEST(FileOutputBuffer, Test) {
   File5.append("/file5");
   {
     Expected<std::unique_ptr<FileOutputBuffer>> BufferOrErr =
-        FileOutputBuffer::create(File5, 8000, FileOutputBuffer::F_no_mmap);
+        FileOutputBuffer::create(File5, 8000, FileOutputBuffer::F_mmap);
     ASSERT_NO_ERROR(errorToErrorCode(BufferOrErr.takeError()));
     std::unique_ptr<FileOutputBuffer> &Buffer = *BufferOrErr;
     // Start buffer with special header.

[MaskRay]

@terrelln

We've observed 3-5x faster builds with -no-mmap-output-file when we hit this scenario.

https://reviews.llvm.org/D69294 misnames F_no_mmap, which confuses me. Can you clarify whether the performance improvement is due to createInMemoryBuffer or createOnDiskBuffer on BtrFS with compression?