jit.p: Enhance 'v' mode of profiler

This change prints more information when profiling at trace
granularity. The idea is to make it easier to audit the runtime
behavior of your apps: how many traces do they compile into, how are
those traces connected together, and which execution path does each
trace represent.

More detail with examples:

Previously the 'v' mode would print a terse summary. For example when
profiling the rate_limiter app selftest:

    $ sudo ./snabb snsh -jdump=+rs,x -jp=v -t apps.rate_limiter.rate_limiter
    47%  TRACE 16
    22%  TRACE 15
    11%  TRACE 14
    10%  TRACE 12
     4%  TRACE 18
     4%  TRACE 24

More information is shown with this change:

    $ sudo ./snabb snsh -jdump=+rs,x -jp=v -t apps.rate_limiter.rate_limiter
    47%  TRACE  16 (14/4)   ->14       rate_limiter.lua:82
    22%  TRACE  15          ->loop     basic_apps.lua:82
    11%  TRACE  14          ->loop     rate_limiter.lua:73
    10%  TRACE  12          ->loop     basic_apps.lua:26
     4%  TRACE  18 (12/13)  ->14       basic_apps.lua:25
     4%  TRACE  24 (19/10)  ->12       app.lua:293

This tells us:

- Is this a side-trace? If so then (PARENT/EXIT) is shown.
- What does the end of the trace link to? (loop, other trace, ..?)
- What source line does the trace begin on? (Requires -jdump to be in use)

Note that it is also possible to combine this profile mode with line
based profiling to have more visibility into what is happening inside
traces:

    45%  TRACE  13 (10/9)   ->10       rate_limiter.lua:82
      -- 33%  counter.lua:add
      -- 32%  packet.lua:free
      -- 20%  freelist.lua:freelist_add
      -- 10%  rate_limiter.lua:method
      --  3%  link.lua:full
    21%  TRACE   8          ->loop     basic_apps.lua:26
      -- 44%  packet.lua:clone
      -- 18%  counter.lua:add
      -- 11%  packet.lua:allocate
      --  9%  freelist.lua:allocate
      --  9%  basic_apps.lua:method
      --  7%  link.lua:full
      --  4%  link.lua:transmit
    19%  TRACE  10          ->loop     rate_limiter.lua:73
      -- 58%  rate_limiter.lua:method
      -- 21%  link.lua:receive
      -- 17%  counter.lua:add
     7%  TRACE  11          ->loop     basic_apps.lua:82
      -- 74%  counter.lua:add
      -- 21%  link.lua:receive
      --  5%  packet.lua:free_internal

lib/luajit/src/jit/dump.lua

@@ -75,6 +75,9 @@ local bcline, disass
 -- Active flag, output file handle and dump mode.
 local active, out, dumpmode
 
+-- Information about traces that is remembered for future reference.
+local info = {}
+
 ------------------------------------------------------------------------------
 
 local symtabmt = { __index = false }
@@ -550,6 +553,7 @@ local function dump_trace(what, tr, func, pc, otr, oex)
     if dumpmode.m then dump_mcode(tr) end
   end
   if what == "start" then
+    info[tr] = { func = func, pc = pc, otr = otr, oex = oex }
     if dumpmode.H then out:write('<pre class="ljdump">\n') end
     out:write("---- TRACE ", tr, " ", what)
     if otr then out:write(" ", otr, "/", oex) end
@@ -701,6 +705,7 @@ end
 return {
   on = dumpon,
   off = dumpoff,
-  start = dumpon -- For -j command line option.
+  start = dumpon, -- For -j command line option.
+  info = info
 }
 

lib/luajit/src/jit/p.lua

@@ -44,6 +44,8 @@ local jit = require("jit")
 assert(jit.version_num == 20100, "LuaJIT core/library version mismatch")
 local profile = require("jit.profile")
 local vmdef = require("jit.vmdef")
+local jutil = require("jit.util")
+local dump = require("jit.dump")
 local math = math
 local pairs, ipairs, tonumber, floor = pairs, ipairs, tonumber, math.floor
 local sort, format = table.sort, string.format
@@ -74,7 +76,38 @@ local function prof_cb(th, samples, vmmode)
   -- Collect keys for sample.
   if prof_states then
     if prof_states == "v" then
-      key_state = map_vmmode[vmmode] or "TRACE "..vmmode
+      if map_vmmode[vmmode] then
+        key_state = map_vmmode[vmmode]
+      else
+         -- Sampling a trace: make an understandable one-line description.
+         local tr = tonumber(vmmode)
+         local info = jutil.traceinfo(tr)
+         local extra = dump.info[tr]
+         -- Show the parent of this trace (if this is a side trace)
+         local parent = ""
+         if extra and extra.otr and extra.oex then
+            parent = "("..extra.otr.."/"..extra.oex..")"
+         end
+         -- Show what the end of the trace links to (e.g. loop or other trace)
+         local lnk = ""
+         local link, ltype = info.link, info.linktype
+         if     link == tr or link == 0 then lnk = "->"..ltype
+         elseif ltype == "root"         then lnk = "->"..link
+         else                                lnk = "->"..link.." "..ltype end
+         -- Show the current zone (if zone profiling is enabled)
+         local z = ""
+         if zone and zone:get() then
+            z = (" %-16s"):format(zone:get())
+         end
+         -- Show the source location where the trace starts
+         local loc = ""
+         if extra and extra.func then
+            local fi = jutil.funcinfo(extra.func, extra.pc)
+            if fi.loc then loc = fi.loc end
+         end
+         local s = ("TRACE %3d %-8s %-10s%s %s"):format(vmmode, parent, lnk, z, loc)
+         key_state = map_vmmode[vmmode] or s
+      end
     else
       key_state = zone:get() or "(none)"
     end
@@ -250,8 +283,8 @@ local function prof_start(mode)
   mode = mode:gsub("%-?%d+", function(s) prof_depth = tonumber(s); return "" end)
   local m = {}
   for c in mode:gmatch(".") do m[c] = c end
-  prof_states = m.z or m.v
-  if prof_states == "z" then zone = require("jit.zone") end
+  prof_states = m.v or m.z
+  if m.z == "z" then zone = require("jit.zone") end
   local scope = m.l or m.f or m.F or (prof_states and "" or "f")
   local flags = (m.p or "")
   prof_raw = m.r