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Re: Issue with LTO/-fwhole-program
On 11 June 2010 20:48, Cary Coutant <ccoutant@google.com> wrote:
>> But if I understand correctly, mixed LTO/non-LTO + whole-program is
>> almost never correct. So we should really emit a warning for this
>> specific combination. I think making this mistake would be quite easy
>> but hard to debug.
>
> It's not only correct, it's essential. "Whole Program" doesn't mean
> that the compiler has to see all the IR for the whole program.
> Instead, the compiler has visibility over the whole program in the
> sense that it knows what variables and functions are referenced and
> defined by the non-LTO code. Linking your program with non-LTO code is
> inescapable unless we start shipping language and system libraries as
> archives of objects compiled with -flto (and remove all assembly code
> from such libraries).
>
> The plugin interface was designed to provide this essential
> information to the compiler about all the non-LTO code; until Gnu ld
> implements this or the collect2 interface provides something similar,
> you're simply working with an incomplete implementation, and you'll
> have to live with the limitations.
This also means that linking your program with non-LTO+whole-program
code may lead to miscompilations without any warning, which is really
bad. I don't think it is a reasonable limitation and we will get bad
press when programs start breaking for users. They won't care (and
they won't even listen) about the reasons. The conclusion will be: LTO
is broken in GCC, and just use another compiler.
Manuel.