aarch64: RFR: Block zeroing by 'DC ZVA'

[Andrew Haley]

+  void dc(cache_maintenance cm, Register Rt) {
+    sys(0b011, 0b0111, cm, 0b001, Rt);
+  }
+
+  void ic(cache_maintenance cm, Register Rt) {
+    sys(0b011, 0b0111, cm, 0b001, Rt);
   }

Are DC and IC really synonyms?

+typedef void (*_zero_Fn)(HeapWord* to, size_t count);
+
 static void pd_fill_to_aligned_words(HeapWord* tohw, size_t count, juint value) {
-  pd_fill_to_words(tohw, count, value);
+  if (UseBlockZeroing
+      && value == 0
+      && count >= (size_t)(BlockZeroingLowLimit >> LogHeapWordSize)) {
+    ((_zero_Fn)StubRoutines::zero_aligned_words())(tohw, count);
+  }
+  else {
+    pd_fill_to_words(tohw, count, value);
+  }
 }

I'm not convinced of the value of this.  We already know that a simple

  while (count-- > 0) {
    *to++ = v;
  }

turns into a call to memset() which does DC ZVA.

diff --git a/src/cpu/aarch64/vm/macroAssembler_aarch64.cpp b/src/cpu/aarch64/vm/macroAssembler_aarch64.cpp
--- a/src/cpu/aarch64/vm/macroAssembler_aarch64.cpp
+++ b/src/cpu/aarch64/vm/macroAssembler_aarch64.cpp
@@ -4670,11 +4670,54 @@
   BLOCK_COMMENT(is_string ? "} string_equals" : "} array_equals");
 }

+
+// base:     Address of a buffer to be zeroed, 8 bytes aligned.
+// cnt:      Count in 8-byte unit.
+// is_large: True when 'cnt' is known to be >= BlockZeroingLowLimit.
+void MacroAssembler::zero_words(Register base, Register cnt, bool is_large)
+{
+  if (UseBlockZeroing) {
+    Label non_block_zeroing;
+    block_zeroing(base, cnt, non_block_zeroing, is_large);

Always use the imperative form of a verb for methods: "block_zero",
not, "block_zeroing".

 // base:   Address of a buffer to be zeroed, 8 bytes aligned.
-// cnt:    Count in 8-byte unit.
-void MacroAssembler::zero_words(Register base, Register cnt)
+// cnt:    Immediate count in 8-byte unit.

Please make this
// cnt: count in HeapWords

Thanks,

Andrew.