[JIT] Replace LoadLoad membar with load-acquire

Summary: In nmethod entry barrier, use lighter instruction. Testing: CICD Reviewers: mmyxym, kuaiwei Issue: #174
dragonwell-project · Feb 11, 2025 · e8087ef · e8087ef
1 parent cb3eb86
commit e8087ef
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Showing 2 changed files with 13 additions and 2 deletions.
diff --git a/src/hotspot/cpu/aarch64/gc/shared/barrierSetAssembler_aarch64.cpp b/src/hotspot/cpu/aarch64/gc/shared/barrierSetAssembler_aarch64.cpp
@@ -313,7 +313,9 @@ void BarrierSetAssembler::nmethod_entry_barrier(MacroAssembler* masm, Label* slo
   Assembler::Condition condition = slow_path == nullptr ? Assembler::EQ : Assembler::NE;
   Label& barrier_target = slow_path == nullptr ? skip_barrier : *slow_path;
 
-  __ ldrw(rscratch1, *guard);
+  if (ReplaceLLMemBarWithLoadAcquire == false) {
+    __ ldrw(rscratch1, *guard);
+  }
 
   if (patching_type == NMethodPatchingType::stw_instruction_and_data_patch) {
     // With STW patching, no data or instructions are updated concurrently,
@@ -350,7 +352,13 @@ void BarrierSetAssembler::nmethod_entry_barrier(MacroAssembler* masm, Label* slo
     assert(patching_type == NMethodPatchingType::conc_data_patch, "must be");
     // Subsequent loads of oops must occur after load of guard value.
     // BarrierSetNMethod::disarm sets guard with release semantics.
-    __ membar(__ LoadLoad);
+    if (ReplaceLLMemBarWithLoadAcquire) {
+      __ adr(rscratch1, *guard);
+      // ldapr is better on paper, but not significant enough to justify a change.
+      __ ldarw(rscratch1, rscratch1);
+    } else {
+      __ membar(__ LoadLoad);
+    }
     Address thread_disarmed_addr(rthread, in_bytes(bs_nm->thread_disarmed_guard_value_offset()));
     __ ldrw(rscratch2, thread_disarmed_addr);
     __ cmpw(rscratch1, rscratch2);

diff --git a/src/hotspot/share/runtime/globals_ext.hpp b/src/hotspot/share/runtime/globals_ext.hpp
@@ -35,6 +35,9 @@
                          constraint)                                           \
   product(bool, UseNewCode4, false, DIAGNOSTIC,                                \
           "Testing Only: Use the new version while testing")                   \
+                                                                               \
+  product(bool, ReplaceLLMemBarWithLoadAcquire, false,                         \
+          "Replace LoadLoad membar with load-acquire")                         \
 
 #endif // SHARE_RUNTIME_GLOBALS_EXT_HPP