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8368953: Document the reason why Serial/Parallel/G1 use zero as dirty card value

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Co-authored-by: Albert Mingkun Yang <ayang@openjdk.org>
Reviewed-by: ayang, iwalulya
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Thomas Schatzl 2025-10-01 08:08:19 +00:00
parent 1188ca55f5
commit 6c2d383492
1 changed files with 14 additions and 0 deletions
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14
src/hotspot/share/gc/shared/cardTable.hpp
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@ -61,6 +61,20 @@ protected:
inline size_t compute_byte_map_size(size_t num_bytes);
// We use 0x00 (zero) to represent Dirty and 0xFF to represent Clean because
// this choice reduces the barrier code by one instruction on architectures with
// a constant-zero register. On such architectures, the Dirty value (0x00) is
// directly accessible through the zero register, eliminating the need to load
// the value explicitly and thereby saving one instruction
//
// E.g. see
// Urs Hölzle. A fast write barrier for generational garbage collectors.
// In Eliot Moss, Paul R. Wilson, and Benjamin Zorn, editors, OOPSLA/ECOOP '93
// Workshop on Garbage Collection in Object-Oriented Systems, October 1993
//
// that shows this for SPARC (but aarch32/aarch64/RISC-V are similar in this
// respect).
//
enum CardValues {
clean_card = (CardValue)-1,