Use entrypoint switching to reduce code size of GcRoot read barrier
Set the read barrier mark register entrypoints to null when the GC
is not marking. The compiler uses this to avoid needing to load the
is_gc_marking boolean.
Code size results on ritzperf CC:
arm32: 13439400 -> 13242792 (-1.5%)
arm64: 16380544 -> 16208512 (-1.05%)
Implemented for arm32 and arm64. TODO: Consider implementing on x86.
Bug: 32638713
Bug: 29516974
Test: test-art-host + run ritzperf
Change-Id: I527ca5dc4cd43950ba43b872d0ac81e1eb5791eb
diff --git a/compiler/optimizing/code_generator_arm64.cc b/compiler/optimizing/code_generator_arm64.cc
index 6f55b42..7860138 100644
--- a/compiler/optimizing/code_generator_arm64.cc
+++ b/compiler/optimizing/code_generator_arm64.cc
@@ -607,10 +607,16 @@
// probably still be a from-space reference (unless it gets updated by
// another thread, or if another thread installed another object
// reference (different from `ref`) in `obj.field`).
+// If entrypoint is a valid location it is assumed to already be holding the entrypoint. The case
+// where the entrypoint is passed in is for the GcRoot read barrier.
class ReadBarrierMarkSlowPathARM64 : public SlowPathCodeARM64 {
public:
- ReadBarrierMarkSlowPathARM64(HInstruction* instruction, Location ref)
- : SlowPathCodeARM64(instruction), ref_(ref) {
+ ReadBarrierMarkSlowPathARM64(HInstruction* instruction,
+ Location ref,
+ Location entrypoint = Location::NoLocation())
+ : SlowPathCodeARM64(instruction),
+ ref_(ref),
+ entrypoint_(entrypoint) {
DCHECK(kEmitCompilerReadBarrier);
}
@@ -665,10 +671,16 @@
//
// rX <- ReadBarrierMarkRegX(rX)
//
- int32_t entry_point_offset =
- CodeGenerator::GetReadBarrierMarkEntryPointsOffset<kArm64PointerSize>(ref_.reg());
- // This runtime call does not require a stack map.
- arm64_codegen->InvokeRuntimeWithoutRecordingPcInfo(entry_point_offset, instruction_, this);
+ if (entrypoint_.IsValid()) {
+ arm64_codegen->ValidateInvokeRuntimeWithoutRecordingPcInfo(instruction_, this);
+ __ Blr(XRegisterFrom(entrypoint_));
+ } else {
+ // Entrypoint is not already loaded, load from the thread.
+ int32_t entry_point_offset =
+ CodeGenerator::GetReadBarrierMarkEntryPointsOffset<kArm64PointerSize>(ref_.reg());
+ // This runtime call does not require a stack map.
+ arm64_codegen->InvokeRuntimeWithoutRecordingPcInfo(entry_point_offset, instruction_, this);
+ }
__ B(GetExitLabel());
}
@@ -676,6 +688,9 @@
// The location (register) of the marked object reference.
const Location ref_;
+ // The location of the entrypoint if it is already loaded.
+ const Location entrypoint_;
+
DISALLOW_COPY_AND_ASSIGN(ReadBarrierMarkSlowPathARM64);
};
@@ -5371,8 +5386,9 @@
// Baker's read barrier are used:
//
// root = obj.field;
- // if (Thread::Current()->GetIsGcMarking()) {
- // root = ReadBarrier::Mark(root)
+ // temp = Thread::Current()->pReadBarrierMarkReg ## root.reg()
+ // if (temp != null) {
+ // root = temp(root)
// }
// /* GcRoot<mirror::Object> */ root = *(obj + offset)
@@ -5389,16 +5405,22 @@
"art::mirror::CompressedReference<mirror::Object> and int32_t "
"have different sizes.");
- // Slow path marking the GC root `root`.
- SlowPathCodeARM64* slow_path =
- new (GetGraph()->GetArena()) ReadBarrierMarkSlowPathARM64(instruction, root);
- codegen_->AddSlowPath(slow_path);
+ Register temp = lr;
- MacroAssembler* masm = GetVIXLAssembler();
- UseScratchRegisterScope temps(masm);
- Register temp = temps.AcquireW();
- // temp = Thread::Current()->GetIsGcMarking()
- __ Ldr(temp, MemOperand(tr, Thread::IsGcMarkingOffset<kArm64PointerSize>().Int32Value()));
+ // Slow path marking the GC root `root`. The entrypoint will alrady be loaded in temp.
+ SlowPathCodeARM64* slow_path =
+ new (GetGraph()->GetArena()) ReadBarrierMarkSlowPathARM64(instruction,
+ root,
+ LocationFrom(temp));
+ codegen_->AddSlowPath(slow_path);
+ const int32_t entry_point_offset =
+ CodeGenerator::GetReadBarrierMarkEntryPointsOffset<kArm64PointerSize>(root.reg());
+ // temp = Thread::Current()->pReadBarrierMarkReg ## root.reg()
+ // Loading the entrypoint does not require a load acquire since it is only changed when
+ // threads are suspended or running a checkpoint.
+ __ Ldr(temp, MemOperand(tr, entry_point_offset));
+ // The entrypoint is null when the GC is not marking, this prevents one load compared to
+ // checking GetIsGcMarking.
__ Cbnz(temp, slow_path->GetEntryLabel());
__ Bind(slow_path->GetExitLabel());
} else {