[LLVMdev] How to tell whether a GlobalValue is user-defined

Rafael Espíndola
Wed, 27 Aug 2014 20:58:38 -0000

>> The literalN sections were developed long ago to support coalescing of
>> unnamed constants like 9.897 in source code for architectures that
could not
>> embed large constants in instructions.  The linker could knew how to
break
>> up the section (e.g. __literal8 is always 8 byte chunks) and coalesce
copies
>> by content.
>>
>> ~6 years ago we discovered that gcc would sometimes put user named
>> constants into the literal sections (e.g. const double foo 9.897).
This was
>> an issue because C language rules say &a != &b, but if ‘a’ and
‘b’ are the
>> contain the same literal value from different translation units, the
linker
>> could merge them to the same address.  For whatever reason, we could
not fix
>> gcc, so we changed to linker to never coalesce items in literal
sections if
>> there was a (non ‘L’ and non ‘l’) symbol on it.
>
>
> Thanks for the info!

+1

>> The current state of LLVM is that is it going out of its way to move
>> “named” constants from __const section to __literalN section. But the
only
>> possible advantage to doing that is that the hopes that the linker
might
>> coalesce it.  But the linker won’t coalesce it because it is named.
So, is
>> there a way to keep the named values in the __const section?
>
>
> Right, LLVM has proven that the address of the data is insignificant, hence
> it is "unnamed", and can be placed in a mergeable section. Is
there any
> reason not to change the linker to merge this stuff, if gcc is no longer
> supported? We won't violate the semantics of C. Is there some immediate
> problem with keeping the data in these sections?


Agreed. If ld64 can drop support for .o produced by the old gcc that
would be awesome. Failing that, what is really needed is

LLVM should only put constants in mergeable sections only if (among
other things) they require only symbols that start with 'l' or
'L'.

Correct?

Cheers,
Rafael