perf(lsp): clear table by table.clear() #39222

benchmark: https://gist.github.com/ofseed/6224529d77c016c36f7ab2f977059848

    local rounds = tonumber(arg[1]) or 1000
    local count = tonumber(arg[2]) or 1000

    -- Load the table.clear function.
    local clear = require("table.clear")

    local function fill(t, n)
      for i = 1, n do
        t[i] = i
      end
    end

    local function bench_reassign(n_rounds, n_items)
      local t = {}
      local start = os.clock()

      for _ = 1, n_rounds do
        t = {}
        collectgarbage("collect")
        fill(t, n_items)
      end

      return os.clock() - start
    end

    local function bench_reassign_no_gc(n_rounds, n_items)
      local t = {}
      local start = os.clock()

      for _ = 1, n_rounds do
        t = {}
        fill(t, n_items)
      end

      return os.clock() - start
    end

    local function bench_clear(n_rounds, n_items)
      local t = {}
      local start = os.clock()

      for _ = 1, n_rounds do
        clear(t)
        fill(t, n_items)
      end

      return os.clock() - start
    end

    -- Warm up LuaJIT before the real benchmark.
    do
      local t = {}
      for _ = 1, 2000 do
        clear(t)
        fill(t, count)
      end
    end

    collectgarbage("collect")

    local reassign_time = bench_reassign(rounds, count)
    collectgarbage("collect")

    local reassign_no_gc_time = bench_reassign_no_gc(rounds, count)
    collectgarbage("collect")

    local clear_time = bench_clear(rounds, count)

    print(string.format("rounds=%d count=%d", rounds, count))
    print(string.format("t = {} + GC   : %.6f s", reassign_time))
    print(string.format("t = {}        : %.6f s", reassign_no_gc_time))
    print(string.format("table.clear   : %.6f s", clear_time))
    print(string.format("vs + GC       : %.2fx", reassign_time / clear_time))
    print(string.format("vs no GC      : %.2fx", reassign_no_gc_time / clear_time))

benchmark result:

    rounds=1000 count=1000
    t = {} + GC   : 0.022469 s
    t = {}        : 0.002570 s
    table.clear   : 0.000387 s
    vs + GC       : 58.06x
    vs no GC      : 6.64x

`count` is how many items the table has, and `round` is how many rounds we fill
the table, clear, and then refill it. `table = {}` is clear the table by
resigning a new empty one, because this script does not run persistently like
nvim so GC is not triggered, so I added another extreme control group that
manually triggers GC.

5 files changed, 53 insertions, 23 deletions

diff --git a/runtime/doc/news.txt b/runtime/doc/news.txt
index 9f78179c61..ef6adb92d2 100644
--- a/runtime/doc/news.txt
+++ b/runtime/doc/news.txt
@@ -168,6 +168,8 @@ PERFORMANCE
   functions, which skips the Vimscript <=> Lua "bridge" (no data
   conversion/marshalling) entirely, if the `vim.fn` function is called from
   Lua.
+• The table holding LSP data is now cleared using `table.clear`,
+  thus reducing GC and memory reallocation during each data reset.
 
 PLUGINS
 
diff --git a/runtime/lua/vim/_core/table.lua b/runtime/lua/vim/_core/table.lua
new file mode 100644
index 0000000000..58833b1c8d
--- /dev/null
+++ b/runtime/lua/vim/_core/table.lua
@@ -0,0 +1,27 @@
+-- Basic shim for LuaJIT's table.new and table.clear.
+local has_new, new = pcall(require, 'table.new')
+local has_clear, clear = pcall(require, 'table.clear')
+
+local M = {}
+
+if not has_new then
+  ---@diagnostic disable-next-line: unused-local
+  new = function(narr, nrec)
+    return {}
+  end
+end
+
+if not has_clear then
+  clear = function(tab)
+    ---@diagnostic disable-next-line: no-unknown
+    for k in pairs(tab) do
+      ---@diagnostic disable-next-line: no-unknown
+      tab[k] = nil
+    end
+  end
+end
+
+M.new = new
+M.clear = clear
+
+return M
diff --git a/runtime/lua/vim/lsp/_folding_range.lua b/runtime/lua/vim/lsp/_folding_range.lua
index e7ceee95db..b35fdfb1ea 100644
--- a/runtime/lua/vim/lsp/_folding_range.lua
+++ b/runtime/lua/vim/lsp/_folding_range.lua
@@ -1,5 +1,6 @@
 local util = require('vim.lsp.util')
 local log = require('vim.lsp.log')
+local tableclear = require('vim._core.table').clear
 local api = vim.api
 
 ---@type table<lsp.FoldingRangeKind, true>
@@ -51,12 +52,13 @@ Capability.all[State.name] = State
 
 --- Re-evaluate the cached foldinfo in the buffer.
 function State:evaluate()
-  ---@type table<integer, [integer, ">" | "<"?]?>
-  local row_level = {}
-  ---@type table<integer, table<lsp.FoldingRangeKind, true?>?>>
-  local row_kinds = {}
-  ---@type table<integer, string?>
-  local row_text = {}
+  local row_level, row_kinds, row_text, row_virt_text =
+    self.row_level, self.row_kinds, self.row_text, self.row_virt_text
+
+  tableclear(row_level)
+  tableclear(row_kinds)
+  tableclear(row_text)
+  tableclear(row_virt_text)
 
   for client_id, ranges in pairs(self.client_state) do
     for _, range in ipairs(ranges) do
@@ -88,11 +90,6 @@ function State:evaluate()
       end
     end
   end
-
-  self.row_level = row_level
-  self.row_kinds = row_kinds
-  self.row_text = row_text
-  self.row_virt_text = {}
 end
 
 --- Force `foldexpr()` to be re-evaluated, without opening folds.
@@ -190,10 +187,10 @@ end
 
 function State:reset()
   self.lang = vim.treesitter.language.get_lang(vim.bo[self.bufnr].filetype)
-  self.row_level = {}
-  self.row_kinds = {}
-  self.row_text = {}
-  self.row_virt_text = {}
+  tableclear(self.row_level)
+  tableclear(self.row_kinds)
+  tableclear(self.row_text)
+  tableclear(self.row_virt_text)
 end
 
 --- Initialize `state` and event hooks, then request folding ranges.
@@ -201,7 +198,11 @@ end
 ---@return vim.lsp.folding_range.State
 function State:new(bufnr)
   self = Capability.new(self, bufnr)
-  self:reset()
+  self.lang = vim.treesitter.language.get_lang(vim.bo[self.bufnr].filetype)
+  self.row_level = {}
+  self.row_kinds = {}
+  self.row_text = {}
+  self.row_virt_text = {}
 
   api.nvim_buf_attach(bufnr, false, {
     -- Reset `bufstate` and request folding ranges.
diff --git a/runtime/lua/vim/lsp/codelens.lua b/runtime/lua/vim/lsp/codelens.lua
index 07212e6554..1846d0f752 100644
--- a/runtime/lua/vim/lsp/codelens.lua
+++ b/runtime/lua/vim/lsp/codelens.lua
@@ -1,5 +1,6 @@
 local util = require('vim.lsp.util')
 local log = require('vim.lsp.log')
+local tableclear = require('vim._core.table').clear
 local api = vim.api
 local M = {}
 
@@ -99,10 +100,10 @@ end
 function Provider:clear()
   self:reset_timer()
   self.version = nil
-  self.row_version = {}
+  tableclear(self.row_version)
 
   for _, state in pairs(self.client_state) do
-    state.row_lenses = {}
+    tableclear(state.row_lenses)
     api.nvim_buf_clear_namespace(self.bufnr, state.namespace, 0, -1)
   end
 
@@ -130,8 +131,8 @@ function Provider:handler(err, result, ctx)
     return
   end
 
-  ---@type table<integer, lsp.CodeLens[]>
-  local row_lenses = {}
+  local row_lenses = state.row_lenses
+  tableclear(row_lenses)
 
   -- Code lenses should only span a single line.
   for _, lens in ipairs(result or {}) do
@@ -140,8 +141,6 @@ function Provider:handler(err, result, ctx)
     table.insert(lenses, lens)
     row_lenses[row] = lenses
   end
-
-  state.row_lenses = row_lenses
   self.version = ctx.version
 
   api.nvim__redraw({ buf = self.bufnr, valid = true, flush = false })
@@ -516,7 +515,7 @@ function M.on_refresh(err, _, ctx)
         if not provider.timer then
           provider:request(client_id, function()
             if api.nvim_buf_is_valid(bufnr) then
-              provider.row_version = {}
+              tableclear(provider.row_version)
               vim.api.nvim__redraw({ buf = bufnr, valid = true, flush = false })
             end
           end)
diff --git a/test/functional/core/main_spec.lua b/test/functional/core/main_spec.lua
index e95fe80343..b800aa5bf8 100644
--- a/test/functional/core/main_spec.lua
+++ b/test/functional/core/main_spec.lua
@@ -233,6 +233,7 @@ describe('vim._core', function()
       'vim._core.shared',
       'vim._core.stringbuffer',
       'vim._core.system',
+      'vim._core.table',
       'vim._core.ui2',
       'vim._core.util',
       'vim._core.vimfn',