[llvm-dev] [LLVMdev] Ideas for making llvm-config --cxxflags more useful

Pete Cooper via llvm-dev
2015-08-07T17:54:41Z

Sorry about this. I think if I'd finished the work to remove vtables from
Value then it wouldn't be an issue, but I put that on hold due to
performance concerns.

I can add back in a bunch of anchor functions where needed. Will just need to go
through and find all the classes which need them.

Pete

Sent from my iPhone

> On Aug 7, 2015, at 10:32 AM, David Blaikie <dblaikie at gmail.com>
wrote:
>
>
>
>> On Fri, Aug 7, 2015 at 10:22 AM, Hans Wennborg via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>> On Thu, Aug 6, 2015 at 12:04 PM, David Chisnall via llvm-dev
>> <llvm-dev at lists.llvm.org> wrote:
>> > [Ooops, sent to the old list address by mistake]
>> >
>> > On 30 Jul 2015, at 21:04, tom at stellard.net wrote:
>> >>
>> >> For flags like -fno-rtti (are there others?) that are required
in some cases
>> >> (I think -fno-rtti is required only if you sub-class LLVM
objects), I would propose
>> >> adding a separate flag like --uses-rtti.  This would give
users more fine-grained
>> >> control over which flags they use, and also would let them
choose the correct
>> >> flag since, for example, -fno-rtti is not understood by MSVC.
>> >
>> > There appears to be a regression in LLVM 3.7, which means that you
must compile with -fno-rtti if you include llvm’s Instructions.h.  The issue is
that a few of the classes (ICmpInst, GetElementPtrInst and PHINode) are now
defined entirely in the header, so every compilation unit that includes the
header will emit them.  These classes all inherit from Instruction (indirectly
via CmpInst in the case of ICmpInst) and so fail to link if compiled with
-fno-rtti, because they can’t find the vtable for ICmpInst.
>>
>> I looked at the file, and this didn't seem true (e.g.
>> GetElementPtrInst::init is still out-of-line). But then I realized you
>> mean virtual functions, so these classes no longer have a key
>> function.
>>
>> This is probably Pete's r240588. I suppose we could add key
functions
>> to these classes (even if they're not used for anything). I'm
not sure
>> how we'd prevent this from regressing though :-/
>
> In theory the LLVM style guide mandates key functions for all dynamic
classes (under the claim of build performance - avoiding duplicate vtable
emission, etc). We've never strongly enforced it though - if we really
wanted to, we could do so as Clang has a warning that triggers whenever a vtable
is emitted weakly (which is what happens when there isn't a key function).
>
> - David
>
>>
>>  - Hans
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org         http://llvm.cs.uiuc.edu
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20150807/857d9d4c/attachment.html>