[LLVMdev] [RFC] New ToolsSupport library for stuff that only tools need

[Chris Bieneman]

I’ve been playing around with trying to accomplish my goal without a new library and I think that while it isn’t impossible, it is nasty and not ideal.

To recap, the goal I’m trying to accomplish is being able to build (from a single configure) LLVM libraries that exclude crash handler support (specifically the Darwin-specific abort and raise overrides), and LLVM tools which include that functionality.

The simple naive solution to this would be to make those hooks implemented in the headers, and conditionally compile it based on whether you’re building for a tool or a library. Unfortunately this doesn’t really work in this case. Header implemented functions need to be weak so that the linker can properly resolve the duplicates. These symbols need to be strong so that they override the versions present in the C library. That means that the only way to do this in a header gets unwieldy because it would need to be a header that is included once and only once in a tool, otherwise you’ll fail linking.

This just comes back to the ideal solution for this (to me) being separating out this functionality into a new library.

Thoughts anyone?
-Chris

> On Nov 7, 2014, at 1:22 PM, Chris Bieneman <beanz at apple.com> wrote:
> 
> To further this point, it was recently brought to my attention that the CMake libLLVM.so on Linux is empty because the GNU linker on linux doesn’t treat symbols mentioned in an export list as being used, so the linker dead strips it all.
> 
> -Chris
> 
>> On Nov 7, 2014, at 12:28 PM, Jim Grosbach <grosbach at apple.com <mailto:grosbach at apple.com>> wrote:
>> 
>>> 
>>> On Nov 7, 2014, at 11:05 AM, Chandler Carruth <chandlerc at google.com <mailto:chandlerc at google.com>> wrote:
>>> 
>>> 
>>> On Fri, Nov 7, 2014 at 12:20 PM, Owen Anderson <resistor at mac.com <mailto:resistor at mac.com>> wrote:
>>>> On Nov 7, 2014, at 8:57 AM, Daniel Dunbar <daniel at zuster.org <mailto:daniel at zuster.org>> wrote:
>>>> 
>>>> 2. I agree with Chandler in that I don't see a good need to try hard to factor out code from the Support library that can just be conditional disabled or would be unused by normal .a link semantics. For example, whether or not the regex or YAML code belongs in Support doesn't seem worth worrying too much about, because they are very isolated, don't introduce extra dependencies, and won't be linked by projects that don't use them.
>>> 
>>> This assumes that the client is statically linking against Support.  Chris has been pretty explicit about his goals of building and using LLVM as a monolithic dynamic library, in which case things like regex or YAML support will *not* be automatically removed.
>>> 
>>> I see...
>>> 
>>> Maybe what we really need is (lacking a complete export list for the monolithic dynamic library) to use visibility so that these kinds of things can be dropped from the final dynamic library if they aren't used within it? I suspect would could mark essentially all of the support library, as unless it is used in the interface of some other library, we probably don't want to export it... (Having just started thinking about this, I've no idea whether folks using the LLVM dynamic library are independently using bits of the ADT for example….)
>> 
>> To the best of my understanding, many linkers don’t have sufficiently granular dead code stripping to rely on it for this. 
>> 
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