[llvm-dev] Dropping COMDAT with LTO

[Teresa Johnson via llvm-dev]

+rafael for comments on lld, which is the other linker using the LTO API
and +pcc for LTO handling

Any change we make wouldn't be gold-plugin specific - that's all in the
underlying LLVM IR linking and LTO handling. Currently, the LTO API which
is guided by linker symbol resolution does strip non-prevailing symbols
from COMDATs. However, as you note, prevailing COMDAT are left in place. Is
your concern just for gcc LTO compatibility?

There are some other places in LLVM where we check for COMDAT membership,
but it is mostly around ensuring that DCE and similar optimizations don't
yield incomplete COMDAT. I suppose for LTO since the linker has already
decided to drop other COMDAT copies we could presumably remove these
prevailing symbols from their the COMDAT group and optimize the symbols
normally?

Teresa

On Wed, Dec 20, 2017 at 2:59 PM, Stephen Crane via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> I've been digging into COMDAT with regular LTO, specifically in the
> context of the LLVM gold plugin. The GCC WHOPR documentation specifies
> that the linker will resolve all COMDAT groups to the IR-provided
> definitions, if available. Additionally it specifies that "When the
> WPA phase produces the definition of the COMDAT symbol in a new object
> file, that definition should not be in a COMDAT group."
> (https://gcc.gnu.org/wiki/whopr/driver#COMDAT_Sections)
>
> The gold LTO plugin does not currently remove COMDAT groups from
> symbols emitted to the new object file. This doesn't actually seem to
> matter, as the linker ignores the COMDAT groups. However, it might be
> nice to strip the COMDAT group early in module linking since we know
> we won't need them.
>
> Do other linkers that use the LTO api also resolve COMDAT symbols to
> the LTO definition? Would it be possible to drop COMDAT groups for
> prevailing symbols, at least when linking with the gold plugin?
>
> - stephen
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-- 
Teresa Johnson |  Software Engineer |  tejohnson at google.com |  408-460-2413
<(408)%20460-2413>
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