Redundant barrier elimination

[Vladimir Kozlov]

Hi, Doug

Thank you for your suggestion. I filed RFE:

https://bugs.openjdk.java.net/browse/JDK-8034810

Thanks,
Vladimir

On 2/12/14 6:40 AM, Doug Lea wrote:
>
> While exploring JMM update options, we noticed that hotspot
> will tend to pile up memory barrier nodes without bothering to
> coalesce them into one barrier instruction. Considering the likelihood
> that JMM updates will accentuate pile-ups, I looked into improvements.
>
> Currently, there is only one case where coalescing is attempted.
> Matcher::post_store_load_barrier does a TSO-specific
> forward pass, that handles only MemBarVolatile. This is a
> harder case than others, because it takes into account that
> other MemBars are no-ops on TSO. It is (or
> should be) called only from dfa on x86 and sparc.
> So it does not apply on processors for which MemBarAcquire and
> MemBarRelease are not no-ops. But for all (known) processors,
> you can always do an easier check for redundancy, buttressed
> by hardware-model-specific ones like post_store_load_barrier
> when applicable. I put together the following, that does a
> basic check, but I don't offhand know of a cpu-independent
> place to call it from. Needing to invoke this from each barrier
> case in each .ad file seems suboptimal. Any advice would be
> welcome. Or perhaps suggestions about placing similar functionality
> somewhere other than Matcher?
>
> Thanks!
>
> ... diffs from JDK9 (warning: I haven't even tried to compile this)
>
> diff -r 4c8bda53850f src/share/vm/opto/matcher.cpp
> --- a/src/share/vm/opto/matcher.cpp    Thu Feb 06 13:08:44 2014 -0800
> +++ b/src/share/vm/opto/matcher.cpp    Wed Feb 12 09:07:17 2014 -0500
> @@ -2393,6 +2393,54 @@
>     return false;
>   }
>
> +// Detect if current barrier is redundant.  Returns true if there is
> +// another upcoming barrier or atomic operation with at least the same
> +// properties before next store or load. Assumes that MemBarVolatile
> +// and CompareAndSwap* provide "full" fences, and that non-biased
> +// FastLock/Unlock provide acquire/release
> +bool Matcher::is_redundant_barrier(const Node* vmb) {
> +  Compile* C = Compile::current();
> +  assert(vmb->is_MemBar(), "");
> +  const MemBarNode* membar = vmb->as_MemBar();
> +  int vop = vmb->Opcode();
> +
> +  // Get the Ideal Proj node, ctrl, that can be used to iterate forward
> +  Node* ctrl = NULL;
> +  for (DUIterator_Fast imax, i = membar->fast_outs(imax); i < imax; i++) {
> +    Node* p = membar->fast_out(i);
> +    assert(p->is_Proj(), "only projections here");
> +    if ((p->as_Proj()->_con == TypeFunc::Control) &&
> +        !C->node_arena()->contains(p)) { // Unmatched old-space only
> +      ctrl = p;
> +      break;
> +    }
> +  }
> +  assert((ctrl != NULL), "missing control projection");
> +
> +  for (DUIterator_Fast jmax, j = ctrl->fast_outs(jmax); j < jmax; j++) {
> +    Node *x = ctrl->fast_out(j);
> +    int xop = x->Opcode();
> +
> +    if (xop == vop ||
> +        xop == Op_MemBarVolatile ||
> +        xop == Op_CompareAndSwapL ||
> +        xop == Op_CompareAndSwapP ||
> +        xop == Op_CompareAndSwapN ||
> +        xop == Op_CompareAndSwapI ||
> +        (!UseBiasedLocking &&
> +         ((xop == Op_FastLock && vop == Op_MemBarAcquire) ||
> +          (xop == Op_FastUnlock && vop == Op_MemBarRelease)))) {
> +      return true;
> +    }
> +
> +    if (x->is_Load() || x->is_Store() || x->is_LoadStore() ||
> +        x->is_Call() || x->is_SafePoint() || x->is_block_proj()) {
> +      break;
> +    }
> +  }
> +  return false;
> +}
> +
>   //=============================================================================
>   //---------------------------State---------------------------------------------
>   State::State(void) {
>
>
>