[LLVMdev] MemoryBuffer/raw_ostream hybrid for linker?

[Nick Kledzik]

Existing llvm code tends to use raw_ostream for writing files.  But raw_ostream is not a good match for a linker for a couple of reasons:

1) When the linker creates an executable, the file needs the 'x' bit set.  Currently raw_fd_ostream has no way to set that.

2) The Unix conformance suite actually has some test cases where the linker is run and the output file does exists but is not writable, or is not writable but is in a writable directory, or with funky umask values.   raw_fd_ostream interface has no way to match those semantics.

3) On darwin we have found the linker performs better if it opens the output file, truncates it to the output size, then mmaps in the file, then writes directly into that memory buffer.  This avoids the memory copy from the private buffer to the OS file system buffer in the write() syscall.

4) In the model we are using for lld, a streaming output interface is not optimal.   Currently, lld copies chunks of code from the (read-only) input files, to a temporary buffer, then applies any fixups (relocations), then streams out that temporary buffer.  If instead we had a big output buffer, the linker could copy the code chunks directly to the output buffer and apply the fixups there, avoiding an extra copy. 

Is there an existing solution for these issues in llvm I've overlooked?   I've searched the bug database and did not find any similar requests.

Should I propose a new llvm/Support/ class?

-Nick