[lld] lld/ELF/Writer.cpp: write directly to outs when output is "-"

[keithw]

This PR lets the LLD ELF driver write directly to lld::outs() (the stdoutOS argument to lld::elf::link()), when the output filename is "-".
This lets the LLD ELF driver be used as a library with its output written to an in-memory buffer.

[llvmbot]

@llvm/pr-subscribers-lld-elf

Author: Keith Winstein (keithw)

Changes

This PR lets the LLD ELF driver write directly to outs (skipping the filesystem) when the output filename is "-".
This is a step towards being able to use LLD as a library without using an open syscall.

---

Full diff: https://github.com/llvm/llvm-project/pull/72061.diff

1 Files Affected:

• (modified) lld/ELF/Writer.cpp (+7)

diff --git a/lld/ELF/Writer.cpp b/lld/ELF/Writer.cpp
index a84e4864ab0e5a5..b5d0c0cafd230dd 100644
--- a/lld/ELF/Writer.cpp
+++ b/lld/ELF/Writer.cpp
@@ -652,6 +652,13 @@ template <class ELFT> void Writer<ELFT>::run() {
     if (errorCount())
       return;
 
+    if (config->outputFile == "-") {
+      lld::outs() << StringRef((const char *)buffer->getBufferStart(),
+                               buffer->getBufferSize());
+      lld::outs().flush();
+      return;
+    }
+
     if (auto e = buffer->commit())
       fatal("failed to write output '" + buffer->getPath() +
             "': " + toString(std::move(e)));

[llvmbot]

@llvm/pr-subscribers-lld

Author: Keith Winstein (keithw)

Changes

This PR lets the LLD ELF driver write directly to outs (skipping the filesystem) when the output filename is "-".
This is a step towards being able to use LLD as a library without using an open syscall.

---

Full diff: https://github.com/llvm/llvm-project/pull/72061.diff

1 Files Affected:

• (modified) lld/ELF/Writer.cpp (+7)

diff --git a/lld/ELF/Writer.cpp b/lld/ELF/Writer.cpp
index a84e4864ab0e5a5..b5d0c0cafd230dd 100644
--- a/lld/ELF/Writer.cpp
+++ b/lld/ELF/Writer.cpp
@@ -652,6 +652,13 @@ template <class ELFT> void Writer<ELFT>::run() {
     if (errorCount())
       return;
 
+    if (config->outputFile == "-") {
+      lld::outs() << StringRef((const char *)buffer->getBufferStart(),
+                               buffer->getBufferSize());
+      lld::outs().flush();
+      return;
+    }
+
     if (auto e = buffer->commit())
       fatal("failed to write output '" + buffer->getPath() +
             "': " + toString(std::move(e)));

[keithw]

@smithp35 @MaskRay

[MaskRay]

ld.lld a.o -o - already writes to stdout. See https://reviews.llvm.org/D56940

[keithw]

Yeah -- unfortunately that writes to llvm::outs(), i.e. the STDOUT_FILENO file descriptor. This PR makes it write to lld::outs(), the stdoutOS argument to the lld::elf::link() call. We're using this so we can use the LLD ELF driver as a library where the object gets written to an in-memory raw_string_ostream that we pass as stdoutOS.

Maybe there's a cleaner way to do this? I couldn't think of any that didn't involve a much bigger change (e.g. I guess we could think about changing the FileOutputBuffer in lld/ELF/Writer.cpp and changing the signature to lld::elf::link() to pass in an explicit output buffer, but that would require changing all the drivers' link function signatures which seems like overkill...).

[MaskRay]

Ah, thanks for the explanation. The description can mention raw_ostream:

This lets the LLD ELF driver be used as a library with its output written to a raw_ostream.

message calls write to stdoutOS, so you'll need to set disableOutput if you use any feature that calls message.

I think this is still fine but we need a unittest in lld/unittest/ (ninja check-lld-unit)