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

Wed Sep 16 10:46:04 PDT 2015

> On 2015-Sep-16, at 10:09, Mehdi Amini <mehdi.amini at apple.com> wrote:
> 
>> 
>> On Sep 16, 2015, at 9:45 AM, Teresa Johnson via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>> 
>> On Wed, Sep 16, 2015 at 7:47 AM, Teresa Johnson <tejohnson at google.com> wrote:
>>> 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?
>> 
>> After writing a test for my gold-plugin changes, I think I know the
>> answer to the above question. It is presumably because you don't want
>> the behavior of new test ./test/LTO/X86/disable-verify.ll which uses
>> llvm-lto to change based on whether the compiler is built NDEBUG or
>> not, since you are also testing the default behavior without
>> -disable-verify in this test.
>> 
>> Since gold-plugin isn't a testing tool, I think we do want the default
>> controlled by NDEBUG. So presumably my new gold-plugin-based test
>> cannot test the default behavior, just the behavior with the new
>> disable-verify plugin option.
> 
> Note sure if it would help but you can add “REQUIRE: assert” in a test to only run it when NDEBUG is enabled.
> 
> There is no “REQUIRE: noassert” though.

It would be possible, but I like that llvm-lto (as a testing tool) behaves
consistently regardless of assertions.

> — 
> Mehdi
> 
> 
> 
>> 
>> Teresa
>> 
>>> - 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
>> 
>> 
>> 
>> -- 
>> Teresa Johnson | Software Engineer | tejohnson at google.com | 408-460-2413
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev