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

Wed Sep 16 10:51:02 PDT 2015

> On Sep 16, 2015, at 10:46 AM, Duncan P. N. Exon Smith <dexonsmith at apple.com> wrote:
> 
> 
>> 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.

It was a suggestion to be able to test gold, not for llvm-lto.

— 
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
>>>> 
>>>>> _______________________________________________
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>>>>> llvm-dev at lists.llvm.org
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>>>> 
>>>> 
>>>> 
>>>> --
>>>> Teresa Johnson | Software Engineer | tejohnson at google.com | 408-460-2413
>>> 
>>> 
>>> 
>>> -- 
>>> Teresa Johnson | Software Engineer | tejohnson at google.com | 408-460-2413
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