[LLVMdev] External names for LTO in gold plugin

[Duncan P. N. Exon Smith]

> On 2014-Nov-10, at 14:53, Rafael Espíndola <rafael.espindola at gmail.com> wrote:
> 
> On 10 November 2014 17:33, Tom Roeder <tmroeder at google.com> wrote:
>> Hi,
>> 
>> In my work applying -flto to Chrome, I need to set some names to be skipped
>> by the InternalizePass; otherwise, the linking stage fails (i.e., when
>> building the chrome binary). In the past, I had a tiny patch that I hadn't
>> submitted to LLVM: it was something like:
>> 
>> Index: Internalize.cpp
>> ===================================================================
>> --- Internalize.cpp     (revision 221403)
>> +++ Internalize.cpp     (working copy)
>> @@ -87,6 +87,7 @@
>>         itr != ExportList.end(); itr++) {
>>     ExternalNames.insert(*itr);
>>   }
>> +  ExternalNames.insert(APIList.begin(), APIList.end());
>> }
>> 
>> 
>> This adds the contents of the -internalize-public-api-list flag to the set
>> of external names for the internalize pass, even when there are already some
>> names being passed in the constructor.
>> 
>> Then I would pass, e.g., "-Wl,-internalize-public-api-list=__xstat64" to the
>> linking phase, and all would be well. However, since the gold plugin has
>> been changed significantly, this no longer works, since the flags from
>> Internalize.cpp don't get up to the gold plugin, and I don't know the new
>> gold plugin bits well enough to see immediately how to replace it. I could
>> go spelunking for the answer, but I wanted to ask here first.
>> 
>> Is there currently a way for me to give the gold plugin a list of names that
>> shouldn't be internalized?
> 
> The current plugin doesn't even use internalize. It depends on the
> linker's decision of what should be kept. Given that, your best option
> is convincing the linker that the symbol is needed. One way to do it
> is with the -u option:
> 
> --------------------------------------------------------------
> $ llvm-dis  test.o -o -
> ; ModuleID = 'test.o'
> 
> define i32 @f() {
>  ret i32 42
> }
> 
> define i32 @main() {
>  ret i32 42
> }
> $ ./build/bin/clang  test.o -flto -o test -Wl,-plugin-opt=save-temps
> $ llvm-dis test.bc -o -
> ; ModuleID = 'test.bc'
> target triple = "x86_64-unknown-linux-gnu"
> 
> define internal i32 @f() {
>  ret i32 42
> }
> 
> define i32 @main() {
>  ret i32 42
> }
> $ ./build/bin/clang  test.o -flto -o test -Wl,-plugin-opt=save-temps
> -Wl,-u -Wl,f
> $ llvm-dis test.bc -o -
> ; ModuleID = 'test.bc'
> target triple = "x86_64-unknown-linux-gnu"
> 
> define i32 @f() {
>  ret i32 42
> }
> 
> define i32 @main() {
>  ret i32 42
> }
> ------------------------------------------------------
> 
> Note how on the second run @f is not internalized.
> 
> Cheers,
> Rafael

If you're able to modify IR, you should also be able to keep `@f` around
by adding it to `@llvm.used`.