Document the use of the biased card table in ART's code generators.
Test: n/a
Change-Id: Ie03a6f6dc87fd0766fc2b685ec39a0a0ebe3fb57
diff --git a/compiler/optimizing/code_generator_arm64.cc b/compiler/optimizing/code_generator_arm64.cc
index 415c6bf..a43ab0b 100644
--- a/compiler/optimizing/code_generator_arm64.cc
+++ b/compiler/optimizing/code_generator_arm64.cc
@@ -1463,8 +1463,24 @@
if (value_can_be_null) {
__ Cbz(value, &done);
}
+ // Load the address of the card table into `card`.
__ Ldr(card, MemOperand(tr, Thread::CardTableOffset<kArm64PointerSize>().Int32Value()));
+ // Calculate the offset (in the card table) of the card corresponding to
+ // `object`.
__ Lsr(temp, object, gc::accounting::CardTable::kCardShift);
+ // Write the `art::gc::accounting::CardTable::kCardDirty` value into the
+ // `object`'s card.
+ //
+ // Register `card` contains the address of the card table. Note that the card
+ // table's base is biased during its creation so that it always starts at an
+ // address whose least-significant byte is equal to `kCardDirty` (see
+ // art::gc::accounting::CardTable::Create). Therefore the STRB instruction
+ // below writes the `kCardDirty` (byte) value into the `object`'s card
+ // (located at `card + object >> kCardShift`).
+ //
+ // This dual use of the value in register `card` (1. to calculate the location
+ // of the card to mark; and 2. to load the `kCardDirty` value) saves a load
+ // (no need to explicitly load `kCardDirty` as an immediate value).
__ Strb(card, MemOperand(card, temp.X()));
if (value_can_be_null) {
__ Bind(&done);