60 lines
2.0 KiB
C
60 lines
2.0 KiB
C
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// tiny_nextptr.h - Safe load/store for header-aware next pointers
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//
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// Context:
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// - Tiny classes 0–6 place a 1-byte header immediately before the user pointer
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// - Freelist "next" is stored inside the block at an offset that depends on class
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// - Many hot paths currently cast to void** at base+1, which is unaligned and UB in C
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//
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// This header centralizes the offset calculation and uses memcpy-based loads/stores
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// to avoid undefined behavior from unaligned pointer access. Compilers will optimize
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// these to efficient byte moves on x86_64 while remaining standards-compliant.
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#ifndef TINY_NEXTPTR_H
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#define TINY_NEXTPTR_H
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#include <stdint.h>
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#include <string.h>
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#include "hakmem_build_flags.h"
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// Compute freelist next-pointer offset within a block for the given class.
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// - Class 7 (1024B) is headerless → next at offset 0 (block base)
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// - Classes 0–6 have 1-byte header → next at offset 1
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static inline __attribute__((always_inline)) size_t tiny_next_off(int class_idx) {
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#if HAKMEM_TINY_HEADER_CLASSIDX
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return (class_idx == 7) ? 0 : 1;
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#else
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(void)class_idx;
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return 0;
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#endif
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}
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// Safe load of next pointer from a block base
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static inline __attribute__((always_inline)) void* tiny_next_load(const void* base, int class_idx) {
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size_t off = tiny_next_off(class_idx);
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#if HAKMEM_TINY_HEADER_CLASSIDX
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if (__builtin_expect(off != 0, 0)) {
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void* next = NULL;
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const uint8_t* p = (const uint8_t*)base + off;
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memcpy(&next, p, sizeof(void*));
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return next;
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}
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#endif
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// Either headers are disabled, or this class uses offset 0 (aligned)
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return *(void* const*)base;
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}
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// Safe store of next pointer into a block base
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static inline __attribute__((always_inline)) void tiny_next_store(void* base, int class_idx, void* next) {
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size_t off = tiny_next_off(class_idx);
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#if HAKMEM_TINY_HEADER_CLASSIDX
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if (__builtin_expect(off != 0, 0)) {
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uint8_t* p = (uint8_t*)base + off;
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memcpy(p, &next, sizeof(void*));
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return;
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}
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#endif
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*(void**)base = next;
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}
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#endif // TINY_NEXTPTR_H
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