[llvm-dev] RFC: LTO should use -disable-llvm-verifier

[Teresa Johnson via llvm-dev]

On Tue, Sep 15, 2015 at 3:31 PM, Duncan P. N. Exon Smith via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>
>> On 2015-Sep-02, at 19:31, Peter Collingbourne <peter at pcc.me.uk> wrote:
>>
>> On Thu, Sep 03, 2015 at 01:10:42AM +0000, Eric Christopher wrote:
>>> On Tue, Sep 1, 2015 at 10:43 AM Duncan P. N. Exon Smith <
>>> dexonsmith at apple.com> wrote:
>>>
>>>>
>>>>> On 2015-Aug-31, at 18:09, Eric Christopher <echristo at gmail.com> wrote:
>>>>>
>>>>>
>>>>>
>>>>> On Mon, Aug 31, 2015 at 5:50 PM Duncan P. N. Exon Smith <
>>>> dexonsmith at apple.com> wrote:
>>>>>
>>>>>> On 2015-Aug-31, at 12:21, Eric Christopher <echristo at gmail.com> wrote:
>>>>>> Yep. This is where I was going :)
>>>>>
>>>>> Glad I found consensus, but I want to double-check that this makes
>>>>> sense to add to the driver.  I didn't quite think through the
>>>>> implications myself.
>>>>>
>>>>> Since the driver doesn't know if there's any bitcode, or if LTO is
>>>>> going to be invoked, it seems like I'll have to change the noasserts
>>>>> driver to *always* pass the option to the linker just in case we are
>>>>> doing LTO.  Is this reasonable?
>>>>>
>>>>> Also, I realized that passing `-mllvm -disable-llvm-verifier` to ld64
>>>>> is redundant... so I'm thinking `-mllvm -disable-verify`.  Make
>>>>> sense?
>>>>>
>>>>> *sigh* Reasons to hate the driver interface again...
>>>>>
>>>>> I guess this is ok. Could possibly add it to the existing terrible
>>>> "enable this pass" interface on liblto as well.
>>>>
>>>> The linker doesn't know whether clang was built with asserts, though.
>>>>
>>>> We could just make it implicit: move the decision to libLTO itself.  Given
>>>> that clang and libLTO.dylib are different executables anyway -- and you
>>>> might be interposing an asserts libLTO.dylib to use with an installed clang
>>>> -- maybe it's even better?
>>>>
>>>>
>>> *nod* We could do that. Seems better than the alternative.
>>
>> +1
>
> Finally got back to this.  Done in r247729.
>
> I didn't modify gold-plugin.cpp, as I don't have a good way to test it,
> but someone else should be able to do it pretty easily.

I can do this for gold (presumably also controlled via an option, but
set default based on NDEBUG).

Couple questions:
- For your patch the default is set based on NDEBUG for lto.cpp, but
in llvm-lto it always defaults to false. Is that intentional?
- You mentioned that the verifier was currently being run 3 times: (1)
after parsing, (2) at the beginning of the optimization pipeline, and
(3) at the end of it. It looks to me like (1) is done via the
createVerifierPass() added in
LTOCodeGenerator::applyScopeRestrictions(). However, gold does not use
LTOCodeGenerator, and I don't see it explicitly adding an initial
createVerifierPass. So it looks like for gold it is only being called
twice (beginning of optimization pipeline and at the end). So I think
for gold I need to leave VerifyInput on the pass manager builder set
to true unconditionally in order to get an initial round of input
verification. Does that sound right?

Teresa

> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev



-- 
Teresa Johnson | Software Engineer | tejohnson at google.com | 408-460-2413