RFR: 8277850: C2: optimize mask checks in counted loops

[Vladimir Kozlov]

On Fri, 3 Dec 2021 10:04:58 GMT, Roland Westrelin <roland at openjdk.org> wrote:

> This is another fix that addresses a performance issue with panama and that was brought up by Maurizio. The pattern to optimize is:
> if ((base + (offset << 2)) & 3) != 0) {
> }
> 
> where base is loop independent but offset depends on a loop variable. This can be transformed to:
> 
> if ((base & 3) != 0) {
> 
> That check becomes loop independent and be optimized by loop predication (or I suppose loop unswitching but that wasn't the case of the micro benchmark I worked on).
> 
> This change also optimizes the pattern:
> 
> (offset << 2) & 3
> 
> to return 0.

src/hotspot/share/opto/mulnode.cpp line 514:

> 512:   }
> 513: 
> 514:   return MulINode::Value(phase);

There is no `MulINode::value()` or `MulLNode::value()`, only based class `MulNode` have it.
You missing initial check done by `MulNode::value()`.
I suggest to move this code into `AndINode::mul_ring()` and `AndLNode::mul_ring()`.
Also in `mul_ring()` you can pass `r1->get_con()` as `mask` value to `AndIL_shift_and_mask()`.

src/hotspot/share/opto/mulnode.cpp line 1716:

> 1714:   }
> 1715: }
> 1716: 

Add comment here too which pattern it is looking for.

src/hotspot/share/opto/mulnode.cpp line 1717:

> 1715: }
> 1716: 
> 1717: bool MulNode::AndIL_shift_and_mask(PhaseGVN* phase, Node* mask, Node* shift, BasicType bt) const {

Pass `mask` as value here.

src/hotspot/share/opto/mulnode.cpp line 1719:

> 1717: bool MulNode::AndIL_shift_and_mask(PhaseGVN* phase, Node* mask, Node* shift, BasicType bt) const {
> 1718:   if (mask == NULL || shift == NULL) {
> 1719:     return false;

You need to check `shift` for `TOP`.

src/hotspot/share/opto/mulnode.cpp line 1755:

> 1753: Node* MulNode::AndIL_add_shift_and_mask(PhaseGVN* phase, BasicType bt) {
> 1754:   Node* in1 = in(1);
> 1755:   Node* in2 = in(2);

Caller already determine that `in2` is `const int mask = t2->get_con()`. You can pass it here.

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PR: https://git.openjdk.java.net/jdk/pull/6697