8005522: use fast-string instructions on x86 for zeroing

Summary: use 'rep stosb' instead of 'rep stosq' when fast-string operations are available.
Reviewed-by: twisti, roland

1 files changed, 16 insertions, 0 deletions

diff --git a/src/cpu/x86/vm/macroAssembler_x86.cpp b/src/cpu/x86/vm/macroAssembler_x86.cpp
index b9d85636c..d2549472c 100644
--- a/src/cpu/x86/vm/macroAssembler_x86.cpp
+++ b/src/cpu/x86/vm/macroAssembler_x86.cpp
@@ -5224,6 +5224,22 @@ void MacroAssembler::verified_entry(int framesize, bool stack_bang, bool fp_mode
 
 }
 
+void MacroAssembler::clear_mem(Register base, Register cnt, Register tmp) {
+  // cnt - number of qwords (8-byte words).
+  // base - start address, qword aligned.
+  assert(base==rdi, "base register must be edi for rep stos");
+  assert(tmp==rax,   "tmp register must be eax for rep stos");
+  assert(cnt==rcx,   "cnt register must be ecx for rep stos");
+
+  xorptr(tmp, tmp);
+  if (UseFastStosb) {
+    shlptr(cnt,3); // convert to number of bytes
+    rep_stosb();
+  } else {
+    NOT_LP64(shlptr(cnt,1);) // convert to number of dwords for 32-bit VM
+    rep_stos();
+  }
+}
 
 // IndexOf for constant substrings with size >= 8 chars
 // which don't need to be loaded through stack.