llvm dev - Jun 2019 - LLD/Link find out what triggers 80mb exe

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

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
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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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
>>  _______________________________________________
>>  LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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