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_mips.cc b/compiler/optimizing/code_generator_mips.cc
index 0ed5756..476e8ab 100644
--- a/compiler/optimizing/code_generator_mips.cc
+++ b/compiler/optimizing/code_generator_mips.cc
@@ -1868,12 +1868,27 @@
if (value_can_be_null) {
__ Beqz(value, &done);
}
+ // Load the address of the card table into `card`.
__ LoadFromOffset(kLoadWord,
card,
TR,
Thread::CardTableOffset<kMipsPointerSize>().Int32Value());
+ // Calculate the address of the card corresponding to `object`.
__ Srl(temp, object, gc::accounting::CardTable::kCardShift);
__ Addu(temp, card, temp);
+ // 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 SB 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).
__ Sb(card, temp, 0);
if (value_can_be_null) {
__ Bind(&done);