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

[Tobias Hartmann]

Hi,

please review the following patch:
https://bugs.openjdk.java.net/browse/JDK-8240905
http://cr.openjdk.java.net/~thartmann/8240905/webrev.00/

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