[PATCH] Prevent MDNode's RAUW from introducing a reference to a void Value.

[David Blaikie]

On Thu, Oct 16, 2014 at 7:26 PM, Frédéric Riss <friss at apple.com> wrote:

>
> On 16 Oct 2014, at 18:58, David Blaikie <dblaikie at gmail.com> wrote:
>
>
>
> On Thu, Oct 16, 2014 at 6:20 PM, Duncan P. N. Exon Smith <
> dexonsmith at apple.com> wrote:
>
>>
>> > On Oct 16, 2014, at 6:08 PM, Eric Christopher <echristo at gmail.com>
>> wrote:
>> >
>> > On Thu, Oct 16, 2014 at 6:06 PM, Duncan P. N. Exon Smith
>> > <dexonsmith at apple.com> wrote:
>> >>
>> >>> On Oct 16, 2014, at 3:54 PM, Frederic Riss <friss at apple.com> wrote:
>> >>>
>> >>> If you look at the added testcase, it cast a void (*func)() to a int
>> (*func)(). The int typed result is referenced in a debug info metadata node
>> (an argument to the dbg.value intrinsic). Then instcombine would remove the
>> bitcast and turn the int returning function into a void returning function
>> that gets RAUWd over the value in the MDNode. And at this point in the
>> current code you have invalid IR.
>> >>>
>> >>> The code in the testcase is reduced from a huge LTO link that
>> encountered this situation in a much more complicated setup.
>> >>>
>> >>> http://reviews.llvm.org/D5828
>> >>
>> >> Weird.  This LGTM (after fixing a few super minor whitespace changes);
>> >> not sure if Eric is still looking.
>> >
>> > Enh, I was trying to figure out if there was some way to avoid the
>> > void metadata node being create rather than avoiding RAUW'ing it. It's
>> > probably not a huge distinction at the moment.
>>
>> I don't think it is a void metadata node; it's a void `CallInst`.
>>
>> We have:
>>
>>     call void @llvm.dbg.declare(metadata !{i32* %deadvar}, ...)
>>     %call = ...
>>     store i32 %call, i32* %deadvar, align 4, !dbg !21
>>
>> Which converts to:
>>
>>     call void @llvm.dbg.value(metadata !{i32* %call}, ...)
>>     %call = ...
>>
>> Then, when %call gets RAUW'ed, `metadata !{i32* %call}` gets updated
>> to `void`.
>>
>
> My only thought is that this code has UB, right? I don't know whether we
> should do something else with it, which might involve killing the dbg.value
> metadata along with all the other instructions that are fruit of the UB
> tree. (the whole basic block, anything else reachable from it, etc)
>
>
> Are you advocating against the patch, or merely pointing out that the
> compiler should handle the testcase code differently?
>

A little of both.


> In the former case, here’s my answer :-) :
>
> Yes, it involves UB (at least I think, the bogus return value is never
> used, but I suppose the call through the wrong pointer type in itself is
> UB). And thus we would be in our right to just drop the call and the
> associated dbg.value, but the fix in itself has nothing to do with UB. It
> prevents any call of RAUW with a void type replacement Value to generate
> invalid MDNodes. We could audit all RAUV callers to never do that, but
> handling it directly in MDNode seems safer (and what would the caller do
> anyway? the end-result would certainly be the same).
>
> The counter argument could be that only input code with UB could trigger
> such RAUW calls, but I wouldn’t bet on it.
>

Generally my interest is in not conceding conditionals like this until we
know they're necessary. It's an easy example, but consider my recent commit
219702 - a fairly innocuous conditional allowed a slew of bugs to be
introduced. Where possible I'd like to understand the root cause of bugs
and continue to fix those until we find cases where a certain thing
actually needs to be supported. We can add assertions to make sure
violations of those invariants fail early and are easier to debug, but
silently accepting them because its expedient now may cost us not just the
bugs we have today but many more we may introduce in the future by giving
up that invariant.

So it's not that I bet there wouldn't be other violations of this rule - I
just wouldn't bet that the next one we find will be for unquestionably
valid code (maybe the 5th one we find is, and we have to remove this
invariant then - but if so, we've just found/fixed 5 bugs in the mean time
- granted, we still lose the ability to enforce the invariant, but we have
a solid example as to why its not a reasonable invariant).

Not a "over my dead body" argument against this patch, just a
thought/perspective.

- David


>
> Fred
>
>
>
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