RFR: Improve C2 no-fwdptr codegen: assume object is forwarded, avoid long immediate

[Aleksey Shipilev]

Continuing to meditate over no-fwdptr generated code. Two wrinkles are there: a) we better assume
that the object is forwarded, this is likely because obj is evacuated once; b) avoid long immediate
0xfff...fc, instead use the already available bits. Both these things seem to cut out about 5..7
bytes from LRB midpath.

diff -r 6041f00b663f src/hotspot/share/gc/shenandoah/c2/shenandoahSupport.cpp
--- a/src/hotspot/share/gc/shenandoah/c2/shenandoahSupport.cpp  Fri May 10 21:43:07 2019 +0200
+++ b/src/hotspot/share/gc/shenandoah/c2/shenandoahSupport.cpp  Mon May 13 16:30:38 2019 +0200
@@ -1521,19 +1521,20 @@

     // Only branch to LRB stub if object is not forwarded; otherwise reply with fwd ptr
     Node* bol = new BoolNode(cmp, BoolTest::eq); // Equals 3 means it's forwarded
     phase->register_new_node(bol, ctrl);

-    IfNode* iff = new IfNode(ctrl, bol, PROB_UNLIKELY(0.999), COUNT_UNKNOWN);
+    IfNode* iff = new IfNode(ctrl, bol, PROB_LIKELY(0.999), COUNT_UNKNOWN);
     phase->register_control(iff, loop, ctrl);
     Node* if_fwd = new IfTrueNode(iff);
     phase->register_control(if_fwd, loop, iff);
     Node* if_not_fwd = new IfFalseNode(iff);
     phase->register_control(if_not_fwd, loop, iff);

-    // Decode forward pointer.
-    Node* masked2 = new AndXNode(markword, phase->igvn().MakeConX(~markOopDesc::lock_mask_in_place));
+    // Decode forward pointer: since we already have the lowest bits, we can just subtract them
+    // from the mark word without the need for large immediate mask.
+    Node* masked2 = new SubXNode(markword, masked);
     phase->register_new_node(masked2, if_fwd);
     Node* fwdraw = new CastX2PNode(masked2);
     fwdraw->init_req(0, if_fwd);
     phase->register_new_node(fwdraw, if_fwd);
     Node* fwd = new CheckCastPPNode(NULL, fwdraw, val->bottom_type());

Testing: hotspot_gc_shenandoah x86_64

--
Thanks,
-Aleksey