[LLVMdev] MCStreamer interface

[Nathan Jeffords]

On Wed, May 5, 2010 at 11:15 AM, Chris Lattner <clattner at apple.com> wrote:

> On May 4, 2010, at 11:03 AM, Nathan Jeffords wrote:
> ...  We basically want one MCStreamer callback to correspond to one
> statement in the .s file.  This makes it easier to handle from the compiler
> standpoint, but is also very important for the llvm-mc assembly parser
> itself.
>
> This is an assumption I question. From an evolutionary perspective I agree;
Given the existing code base I do see this as a logical transformation. As
far as the assembly parser/streamer is concerned it certainly
simplifies their implementations. But I also think that this interface could
evolve in a direction that simplifies the common case (compiler -> object
file) at a small expense to handling assembly language files.

> All fragments should be associated with a symbol. For assembler
> components, a
> > unnammed "virtual" symbol can be used when there is no explicit label
> defined.
>
> What do you mean by fragment?  Can you give me an analogy with what the
> syntax looks like in a .s file, I'm not sure exactly what you mean here.


> I use the term fragment to refer to the MCFragment class and
its derivatives. I understand that to mean any entity representing data in
the final linked and loaded form. (something with an address)

> Section assignment should be the responsiblity of the object imlementing
> the
> > MCStreamer interface, with the caller givin the ability to give hints as
> to
> > what section to place the symbol into.
>
> Section assignment really needs to happen at a higher level.  The
> TargetLoweringObjectFile interfaces are the ones responsible for mapping a
> global/function -> section.  This interface (not mcstreamer) should handle
> this.
>
> The important point here is that the COFF MCSection needs to have the right
> level of semantic information.  In fact, MCSection is the place that I'd
> start for COFF bringup.
>
> OK, I see that now. The current isolation between TargetLoweringObjectFile
-> MCStreamer -> MCObjectWriter has proven somewhat problematic, mostly due
to my lack of understanding. I guess MCSectionXXX was meant to provide
communication between them. Should the same be true of MCSymbol, and their
data counterparts?


> > instead of SwitchSection, there would be BeginSymbol, and SymbolSymbol,
> it would
> > be illegal to call any EmitXXX function outside of these two calls
> >
> > BeginSymbol(Symbol, SectionHint)
> >   EmitAttribute(...)
> >   EmitAttribute(...)
> >   ...
> > StartFragmentEmission()
> >   EmitFragment(...)
> >   EmitFragment(...)
> >   ...
> > EndSymbol()
> >
> > Object file writers would typically start recording fragments and
> attributes for
> > a symbol on the BeginSymbol, then at EndSymbol they would evaluate what
> was
> > streamed, and decide what section the symbol should be placed in.
>
> Why do you need this?  This concept doesn't exist in the .s file, so I
> don't think that MCStreamer is the right level for this.



I realize that my expectations of the MCStreamer interface are not quite the
same as the intentions behind its design. It seemed to be that having these
calls with the restrictions on when they would be allowed would simplify an
object file writers job sifting though the incoming data and organizing into
what will become the output file.

I had a problem with MCStreamer::EmitCommonSymbol
& MCStreamer::EmitLocalCommonSymbol. When I implemented them I assumed this
meant to put those symbols into the .bss segment. This required me to get a
hold of the TLOF from the streamer. I now realize this is wrong after
re-reading the description of the '.comm' directive a few times.  I am not
sure why an uninitialized global variable was being emitted using this, that
seems wrong since global variables in different compilation units with the
same name would get merged together at link time. (this is using clang on a
C source file)

Thanks for taking the time to read and respond to my post. I think I need to
get better acquainted with these components.

- Nathan
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