[llvm-dev] LLD/Link find out what triggers 80mb exe

[Carlo Kok via llvm-dev]

Oke so this turned out to be caused by the presence of .debug_info, which pulled in the methods, and those pulled in the types. It seems just referencing a .lib with .obj that have (dwarf) debug info makes it pull in a lot of stuff. But at the very least, I now know what to avoid. Thanks!

On Fri, Jun 7, 2019, at 21:37, Zachary Turner wrote:
> You could try analyzing the PDB file using llvm-pdbutil. We won't add any symbols to the PDB that were eliminated by linker gc, so this should at least allow you to get a good idea of what symbols are actually contributing to the output. It won't give you a full dependency graph like Rui suggests, but it might be a good starting point.
> 
> On Fri, Jun 7, 2019 at 6:42 AM Carlo Kok via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>> __
>> I did try that yes. Didn't seem to make any difference (still 80ishmb)
>> 
>> On Fri, Jun 7, 2019, at 15:29, David Major wrote:
>>> Just to double check: are you calling lld-link with `-opt:ref`? It may or may not be enabled by default, depending on other flags.
>>> 
>>> 
>>> On Fri, Jun 7, 2019 at 8:26 AM Carlo Kok via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>>>> __
>>>> Unfortunately, lld link doesn't do -map yet (and link.exe /map is kinda useless; it shows what get linked in, but only refers to an object file, not function name when it does so, like a single reference in that object file triggers all it's dependencies)
>>>> 
>>>> Yeah I'm fairly sure it's caused by something I'm doing wrong, I just have no idea how to find out. I thought -function-section would have pretty much covered it.
>>>> 
>>>> On Fri, Jun 7, 2019, at 14:22, Rui Ueyama wrote:
>>>>> If you use only one function from a static library, and that adds 80 MiB to the executable, it is clear that that undefined symbol causes a lot of other files to be loaded, so the question is why that function has such a large (transitive) dependency and how to find the cause.
>>>>> 
>>>>> Unfortunately there's no exact command line option or a feature to find it out. I have an idea to write a post-link analysis tool, which processes a linker's debug output, to find out a "weak link" in a dependency graph. If we can split a graph into two by removing a small number of symbols (or vertices) from the graph, we can reduce a program size by removing these symbols from the program. But that kind of analysis tool is not available yet, and I don't know how powerful it would actually be.
>>>>> 
>>>>> There are a few options that may be useful when debugging the issue.
>>>>> 
>>>>> `-v` makes lld to print out a file name as the linker loads a new file.
>>>>> 
>>>>> `-Map=<filename>` makes lld to write an output file layout information to a given file.
>>>>> 
>>>>> On Fri, Jun 7, 2019 at 8:59 PM Carlo Kok via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>>>>>> Is there any way to find out what causes a 80mb executable with lld/link?
>>>>>> 
>>>>>> 
>>>>>> The object files are compiled with -function-sections and -data-sections, the originating .lib is 270mb, but I'm calling 1 function in it, which might call some stuff recursively, but I don't know how to find out what exactly triggers the use. Is there a commandline option that shows this?
>>>>>> 
>>>>>> (repro is 286 mb) : http://gofile.me/4Gflq/OOr8WKU12
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