RFR: 8373697: AArch64: Remove deoptimization stub code for nmethods with small stack frames

[Andrew Haley]

On Tue, 16 Dec 2025 23:15:12 GMT, Ruben <duke at openjdk.org> wrote:

> Removal of deoptimization stub codes improves density of compiled code.
> It is possible at least for methods with relatively small stack frames.

Try this:


diff --git a/src/hotspot/share/runtime/frame.cpp b/src/hotspot/share/runtime/frame.cpp
index 9fdffa053c4..3c816ee8aad 100644
--- a/src/hotspot/share/runtime/frame.cpp
+++ b/src/hotspot/share/runtime/frame.cpp
@@ -359,6 +359,25 @@ void frame::deoptimize(JavaThread* thread) {
 
   NativePostCallNop* inst = nativePostCallNop_at(pc());
 
+  const ImmutableOopMap* blob_map = thread->last_frame().get_oop_map();
+  blob_map->print();
+
+  VMReg r7_VMReg = r7->as_VMReg();
+  intptr_t* r8_location = nullptr;
+  for (OopMapStream oms(blob_map); !oms.is_done(); oms.next()) {
+    OopMapValue omv = oms.current();
+    if (omv.type() == OopMapValue::callee_saved_value) {
+      VMReg reg = omv.content_reg();
+      if (reg == r7_VMReg) {
+        r8_location = thread->last_frame().sp()
+          + omv.stack_offset() / VMRegImpl::slots_per_word
+          + 1;
+        tty->print_cr("offset = %d\n", omv.stack_offset() * VMRegImpl::stack_slot_size);
+        break;
+      }
+    }
+  }
+  *r8_location = 0xcafebeefcafebabe;
   // Save the original pc before we patch in the new one
   nm->set_original_pc(this, pc());
   patch_pc(thread, deopt);



You will find that when you arrive at the deopt handler entry, r8 contains 0xcafebeefcafebabe. You can pass anything you want in the slots for r8 and r9.
 
This is a bit of a kludge in that we don't have an oopmap entry for r8 so I assume it's in the next entry after r7, but all registers are saved on the stack, and they're saved in their natural order. If needs be we can add an oop map entry for r8. That may be a good idea.

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PR Comment: https://git.openjdk.org/jdk/pull/28857#issuecomment-3745612971