RFR(S): 8210389: C2: assert(n->outcnt() != 0 || C->top() == n || n->is_Proj()) failed: No dead instructions after post-alloc

[Vladimir Kozlov]

Why you are not using subsume_by()?

Thanks,
Vladimir

On 9/18/18 12:47 PM, Roland Westrelin wrote:
> 
> http://cr.openjdk.java.net/~roland/8210389/webrev.00/
> 
> With volatile loads, the trailing membar has an edge to the load. After
> optimizations, that edge can point to a chain of Phis and the membar can
> be the one use that keeps the phis alive. After matching, that required
> edge is converted to a precedence edge. Liveness analysis ignores
> precedence edges, the chain of phis is killed and register allocation
> finds a node with no use.
> 
> As a fix, I propose that, at the end of optimizations, the edge between
> the volatile load's membar and the phis be removed and all dead phis be
> killed. As I understand, that edge is not required for correctness
> because anti dependencies detection code adds a precedence edge between
> a volatile load and its membar if needed. I ran full jcstress on x86 and
> aarch64 with this patch successfully.
> 
> Roland.
>