[15] RFR(S): 8240905: assert(mem == (Node*)1 || mem == mem2) failed: multiple Memories being matched at once?

[Tobias Hartmann]

Thanks Vladimir!

Best regards,
Tobias

On 23.03.20 14:24, Vladimir Ivanov wrote:
> 
>> http://cr.openjdk.java.net/~thartmann/8240905/webrev.00/
> 
> Looks good.
> 
> Best regards,
> Vladimir Ivanov
> 
>> Since JDK-8031321 which added support for Intel's bit manipulation instructions to JDK 8u20, we have
>> rules in the .ad file that are matching two LoadNodes and therefore two memory inputs to a single
>> instruction (for example, blsiI_rReg_mem [1]). As of today, these BMI1 instructions are still the
>> only ones that match multiple memory.
>>
>> Since the matcher does not match memory edges, it can happen that the memory inputs of the LoadNodes
>> differ and we hit the reported assert. For example, with volatile field accesses, a MemBarAcquire is
>> added between the first and the second LoadNode (see test_blsiI_rReg_mem_2). With a product build,
>> we would simply ignore the inconsistent memory inputs and match to a 'blsi'. I think this would
>> break the happens-before guarantee of volatile memory accesses because the first load is effectively
>> re-ordered with the volatile access.
>>
>> We already have checks in Matcher::Label_Root() that stop recursion when the memory differs (see [2]
>> and [3]) but these only work if the LoadNodes have the same depth in the tree.
>>
>> I propose to strengthen these checks and stop recursion if there are multiple loads with different
>> memory inputs in the tree. Since that code is using a breadth-first-search, the depth of the first
>> load (which will be part of the tree) is always <= the depth of the offending load (which will not
>> be part of the tree and is matched into a register). Now I think this could still lead to different
>> match results but I couldn't come up with an example where an existing match rule is affected.
>>
>> Thoughts?
>>
>> Thanks,
>> Tobias
>>
>> [1] http://hg.openjdk.java.net/jdk/jdk/file/89ec93d09e7e/src/hotspot/cpu/x86/x86_64.ad#l9480
>> [2] http://hg.openjdk.java.net/jdk/jdk/file/89ec93d09e7e/src/hotspot/share/opto/matcher.cpp#l1489
>> [3] http://hg.openjdk.java.net/jdk/jdk/file/89ec93d09e7e/src/hotspot/share/opto/matcher.cpp#l1525
>>