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8368953: Document the reason why Serial/Parallel/G1 use zero as dirty card value #27575

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8368953: Document the reason why Serial/Parallel/G1 use zero as dirty card value #27575

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8368953
tschatzl Sep 30, 2025
9ce7f81
Update src/hotspot/share/gc/shared/cardTable.hpp
tschatzl Sep 30, 2025
cef0183
Update src/hotspot/share/gc/shared/cardTable.hpp
tschatzl Sep 30, 2025
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14 changes: 14 additions & 0 deletions src/hotspot/share/gc/shared/cardTable.hpp
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Expand Up @@ -61,6 +61,20 @@ class CardTable: public CHeapObj<mtGC> {

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,

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