[vim/vim] Use hashtable for syn_name2id() lookup (PR #19788)
mattn
In modern Vim usage with plugins and colorschemes, syn_name2id() is one of the most called functions during startup. In my environment, it is called roughly 4,800 times before Vim finishes starting up.
However, the implementation of syn_name2id() uses a linear scan over a growarray, making each lookup O(n) in the number of highlight groups. This results in over 3.4 million STRCMP() calls just during startup.
This change replaces the linear search with a hashtab_T-based lookup, reducing the cost to O(1). The hashtable is maintained alongside the existing growarray using the same offsetof pattern as signgroup_T / HI2SG.
Benchmark (User time, vim -c quit):
- Before: 0.189s
- After: 0.171s (-9.5%)
The more highlight groups are defined, the greater the benefit of this change.
You can view, comment on, or merge this pull request online at:
https://github.com/vim/vim/pull/19788
Commit Summary
- 084319d Use hashtable for syn_name2id() lookup
File Changes
(1 file)
- M src/highlight.c (58)
Patch Links:
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mattn
mattn left a comment (vim/vim#19788)For reference, here is the benchmark result when opening a file (mbyte.c) with syntax highlighting:
Benchmark 1: ./vim_old mbyte.c -c quit
Time (mean ± σ): 2.357 s ± 0.020 s [User: 0.248 s, System: 0.067 s]
Benchmark 2: ./vim_new mbyte.c -c quit
Time (mean ± σ): 2.329 s ± 0.018 s [User: 0.220 s, System: 0.066 s]
User time improved by 11.3% (0.248s → 0.220s). The benefit grows when syntax highlighting is involved, since syn_name2id() is called more frequently.
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mattn
mattn left a comment (vim/vim#19788)Here are realistic benchmark results excluding startup time.
The scenario simulates colorscheme re-application + highlight link + hlID() lookups with 541 highlight groups (82 base + 200 treesitter-style + 256 plugin-style), performing 17,640 name lookups.
master (linear scan): 0.084 sec (~210,000 lookups/sec)
hashtable branch: 0.062 sec (~284,000 lookups/sec)
speedup: ~1.35x
Since the benchmark includes the overhead of execute 'highlight ...' command parsing, the actual improvement in syn_name2id() itself is likely higher. The O(n) → O(1) difference also grows with group count — environments with 800–1000+ groups (e.g., treesitter plugins) will see a larger speedup.
For reference, a microbenchmark with 2000 groups using only hlID() showed ~2.5x speedup.
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mattn
@mattn pushed 2 commits.
- 37aca65 Skip redundant highlight_changed() loop when 'highlight' matches default
- f33a2b2 Avoid unnecessary alloc/free in syn_name2id_len()
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Christian Brabandt
@chrisbra commented on this pull request.
In src/highlight.c:
> @@ -207,6 +207,21 @@ static int hl_flags[HLF_COUNT] = HL_FLAGS;
static garray_T highlight_ga;
#define HL_TABLE() ((hl_group_T *)((highlight_ga.ga_data)))
+// Wrapper struct for hashtable entries: stores the highlight group ID alongside
+// the uppercase name used as a hash key. Uses the same offsetof pattern as
+// signgroup_T / HI2SG.
+typedef struct {
+ int hn_id; // highlight group ID (1-based)
+ char_u hn_key[1]; // uppercase name (stretchable array)
+} hlname_T;
+
+#define HLNAME_KEY_OFF offsetof(hlname_T, hn_key)
+#define HI2HLNAME(hi) ((hlname_T *)((hi)->hi_key - HLNAME_KEY_OFF))
+
+// hashtable for quick highlight group name lookup
+static hashtab_T highlight_ht;
+static int highlight_ht_inited = FALSE;
bool?
In src/highlight.c:
> }
ga_clear(&highlight_ga);
+ hash_clear(&highlight_ht);
this frees all memory, but highlight_ht_inited isn't reset to false
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mattn
@mattn pushed 1 commit.
- afc005a Address review feedback: use bool for highlight_ht_inited and reset on free
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Christian Brabandt
chrisbra left a comment (vim/vim#19788)thanks
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Christian Brabandt
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