[webkit-dev] Why does RELEASE_ASSERT not have an error message?

[Geoffrey Garen]

> On Feb 22, 2017, at 12:16 PM, Filip Pizlo <fpizlo at apple.com> wrote:
> 
> 
>> On Feb 22, 2017, at 11:58 AM, Geoffrey Garen <ggaren at apple.com> wrote:
>> 
>> I’ve lost countless hours to investigating CrashTracers that would have been easy to solve if I had access to register state.
> 
> The current RELEASE_ASSERT means that every assertion in what the compiler thinks is a function (i.e. some function and everything inlined into it) is coalesced into a single trap site.  I’d like to understand how you use the register state if you don’t even know which assertion you are at.

Not all functions suffer from this problem. Few enough functions suffer from this problem that I haven’t felt an urgent need to address it.

> I believe that if you do want to analyze register state, then switching back to calling some function that prints out diagnostic information is strictly better.  Sure, you get less register state, but at least you know where you crashed.  Knowing where you crashed is much more important than knowing the register state, since the register state is not useful if you don’t know where you crashed.

See above.

Also, printed diagnostic information is not available in CrashTracer reports, while register state is.

> 
>> 
>> I also want the freedom to add RELEASE_ASSERT without ruining performance due to bad register allocation or making the code too large to inline. For example, hot paths in WTF::Vector use RELEASE_ASSERT.
> 
> Do we have data about the performance benefits of the current RELEASE_ASSERT implementation?

Oliver needed it in order to turn on assertions in side Vector.h without regression. You can follow the svn / bugzilla history to find out more.

> 
>> 
>> Is some compromise solution possible?
>> 
>> Some options:
>> 
>> (1) Add a variant of RELEASE_ASSERT that takes a string and logs.
> 
> The point of C++ assert macros is that I don’t have to add a custom string.  I want a RELEASE_ASSERT macro that automatically stringifies the expression and uses that as the string.

FWIW, all of the existing cases of dataLog followed by RELEASE_ASSERT that I could find wanted to log something other than just the expression — for example, a pointer value.

> If I had a choice between a RELEASE_ASSERT that can accurate report where it crashed but sometimes trashes the register state, and a RELEASE_ASSERT that always gives me the register state but cannot tell me which assert in the function it’s coming from, then I would always choose the one that can tell me where it crashed.  That’s much more important, and the register state is not useful without that information.

I don’t think it’s helpful to conflate (a) I want a string message with (b) I want to solve a coalescing problem.

> 
>> 
>> (2) Change RELEASE_ASSERT to do the normal debug ASSERT thing in Debug builds. (There’s not much need to preserve register state in debug builds.)
> 
> That would be nice, but doesn’t make RELEASE_ASSERT useful for debugging issues where timing is important.  I no longer use RELEASE_ASSERTS for those kinds of assertions, because if I do it then I will never know where I crashed.  So, I use the explicit:
> 
> if (!thing) {
>   dataLog(“…”);
>   RELEASE_ASSERT_NOT_REACHED();
> }

If asserts are generally set up to provide the extra data we want, and we just need a temporary change in assertion policy to satisfy a use case like this, that’s easy enough to achieve with a local configuration change to Assertions.h.

Geoff

> 
> -Filip
> 
> 
>> 
>> Geoff
>> 
>>> On Feb 22, 2017, at 11:09 AM, Filip Pizlo <fpizlo at apple.com> wrote:
>>> 
>>> I disagree actually.  I've lost countless hours to converting this:
>>> 
>>> RELEASE_ASSERT(blah)
>>> 
>>> into this:
>>> 
>>> if (!blah) {
>>> dataLog("Reason why I crashed");
>>> RELEASE_ASSERT_NOT_REACHED();
>>> }
>>> 
>>> Look in the code - you'll find lots of stuff like this.
>>> 
>>> I don't think analyzing register state at crashes is more important than keeping our code sane.
>>> 
>>> -Filip
>>> 
>>> 
>>>> On Feb 21, 2017, at 5:56 PM, Mark Lam <mark.lam at apple.com> wrote:
>>>> 
>>>> Oh yeah, I forgot about that.  I think the register state is more important for crash analysis, especially if we can make sure that the compiler does not aggregate the int3s.  I’ll explore alternatives.
>>>> 
>>>>> On Feb 21, 2017, at 5:54 PM, Saam barati <sbarati at apple.com> wrote:
>>>>> 
>>>>> I thought the main point of moving to SIGTRAP was to preserve register state?
>>>>> 
>>>>> That said, there are probably places where we care more about the message than the registers.
>>>>> 
>>>>> - Saam
>>>>> 
>>>>>> On Feb 21, 2017, at 5:43 PM, Mark Lam <mark.lam at apple.com> wrote:
>>>>>> 
>>>>>> Is there a reason why RELEASE_ASSERT (and friends) does not call WTFReportAssertionFailure() to report where the assertion occur?  Is this purely to save memory?  svn blame tells me that it has been this way since the introduction of RELEASE_ASSERT in r140577 many years ago.
>>>>>> 
>>>>>> Would anyone object to adding a call to WTFReportAssertionFailure() in RELEASE_ASSERT() like we do for ASSERT()?  One of the upside (side-effect) of adding this call is that it appears to stop the compiler from aggregating all the RELEASE_ASSERTS into a single code location, and this will help with post-mortem crash debugging.
>>>>>> 
>>>>>> Any thoughts?
>>>>>> 
>>>>>> Mark
>>>>>> 
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>>>>> 
>>>> 
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