[PATCH] [mc-coff] Forward Linker Option flags into the .drectve section

[Reid Kleckner]

On Wed, Apr 24, 2013 at 6:58 PM, Rafael Espíndola
<rafael.espindola at gmail.com> wrote:
>> +
>> +  // Emit the linker options if present.
>> +  if (LinkerOptions) {
>
> Please use an early return
>
> if (!LinkerOptions)
>  return;

Sure.

>> +    const MCSection *Sec = getDrectveSection();
>> +    Streamer.SwitchSection(Sec);
>> +    for (unsigned i = 0, e = LinkerOptions->getNumOperands(); i != e; ++i) {
>> +      MDNode *MDOptions = cast<MDNode>(LinkerOptions->getOperand(i));
>> +      for (unsigned ii = 0, ie = MDOptions->getNumOperands(); ii != ie; ++ii) {
>> +        MDString *MDOption = cast<MDString>(MDOptions->getOperand(ii));
>> +        Streamer.EmitBytes(" ");
>> +        Streamer.EmitBytes(MDOption->getString());
>>
>
> Is the format documented somewhere we could link to?

The .drectve section format, or the module flags metadata format?

The .drectve section, which we already use for dllexported symbols, is
part of the PE-COFF spec:
"""
A .drectve section consists of a string of text that can be encoded as ANSI or
UTF8. If the UTF8 byte order marker (BOM, a three-byte prefix that consists of
0xEF, 0xBB, and 0xBF) is not present, the directive string is
interpreted as ANSI.
The directive string is a series of linker options that are separated
by spaces. Each
option contains a hyphen, the option name, and any appropriate attribute. If
an option contains spaces, the option must be enclosed in quotes. The .drectve
section must not have relocations or line numbers.
"""

One thing that's a little vague is how quotes and backslashes should
be escaped.  I'd guess it's the same escaping algorithm used for
CreateProcess, since these are flags.

Worth linking the doc here, or is the spec kind of an implicit reference?

So far as I know, the metadata flags are only documented by
implementation and mailing list discussion.  See Daniel's RFC about
auto-linking:
http://lists.cs.uiuc.edu/pipermail/llvmdev/2013-January/058204.html

Is it common to link the mailing list from the source code?

>  Index: test/MC/COFF/linker-options.ll
>> ===================================================================
>> --- /dev/null
>> +++ test/MC/COFF/linker-options.ll
>> @@ -0,0 +1,28 @@
>> +; RUN: llc -O0 -mtriple=i386-pc-win32 -filetype=obj -o - %s | llvm-readobj -s -sd | FileCheck %s
>> +
>
> Please test the output of the assembly file. No need to use -filetype=obj.

Sounds good.  For some reason I thought that wouldn't work with COFF,
but it works fine.