2024-09-08 12:43:16 +00:00
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/* SPDX-License-Identifier: GPL-2.0-only */
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#ifndef HASH_H
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#define HASH_H
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static inline unsigned int hash_str(const char *s)
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{
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/* fnv32 hash */
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unsigned int hash = 2166136261U;
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for (; *s; s++)
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hash = (hash ^ *s) * 0x01000193;
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return hash;
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}
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kconfig: use hash table to reuse expressions
Currently, every expression in Kconfig files produces a new abstract
syntax tree (AST), even if it is identical to a previously encountered
one.
Consider the following code:
config FOO
bool "FOO"
depends on (A || B) && C
config BAR
bool "BAR"
depends on (A || B) && C
config BAZ
bool "BAZ"
depends on A || B
The "depends on" lines are similar, but currently a separate AST is
allocated for each one.
The current data structure looks like this:
FOO->dep ==> AND BAR->dep ==> AND BAZ->dep ==> OR
/ \ / \ / \
OR C OR C A B
/ \ / \
A B A B
This is redundant; FOO->dep and BAR->dep have identical ASTs but
different memory instances.
We can optimize this; FOO->dep and BAR->dep can share the same AST, and
BAZ->dep can reference its sub tree.
The optimized data structure looks like this:
FOO->dep, BAR->dep ==> AND
/ \
BAZ->dep ==> OR C
/ \
A B
This commit introduces a hash table to keep track of allocated
expressions. If an identical expression is found, it is reused.
This does not necessarily result in memory savings, as menu_finalize()
transforms expressions without freeing up stale ones. This will be
addressed later.
One optimization that can be easily implemented is caching the
expression's value. Once FOO's dependency, (A || B) && C, is calculated,
it can be cached, eliminating the need to recalculate it for BAR.
This commit also reverts commit e983b7b17ad1 ("kconfig/menu.c: fix
multiple references to expressions in menu_add_prop()").
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
2024-09-08 12:43:20 +00:00
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/* simplified version of functions from include/linux/hash.h */
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#define GOLDEN_RATIO_32 0x61C88647
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static inline unsigned int hash_32(unsigned int val)
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{
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return 0x61C88647 * val;
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}
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static inline unsigned int hash_ptr(const void *ptr)
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{
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return hash_32((unsigned int)(unsigned long)ptr);
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}
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2024-09-08 12:43:16 +00:00
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#endif /* HASH_H */
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