Fix FOLD rule for BUFHDR append.

Reported by XmiliaH.

(cherry-picked from commit bc1bdbf)

`bufput_append()` may fold `BUFHDR RESET` + `BUFPUT` IRs to `BUFHDR
APPEND` even if the right operand (`BUFSTR`) is the PHI. If it's not the
last IR in the `BUFSTR` chain, this may lead to an incorrect resulting
value in the buffer, which contains a longer string since `APPEND` is
used instead of `RESET`.

This patch adds the corresponding check inside the fold rule.

Sergey Kaplun:
* added the description and the test for the problem

Part of tarantool/tarantool#9145

Reviewed-by: Maxim Kokryashkin <[email protected]>
Reviewed-by: Sergey Bronnikov <[email protected]>
Signed-off-by: Igor Munkin <[email protected]>
(cherry picked from commit e895818)

src/lj_opt_fold.c

@@ -584,7 +584,8 @@ LJFOLDF(bufput_append)
   if ((J->flags & JIT_F_OPT_FWD) &&
       !(fleft->op2 & IRBUFHDR_APPEND) &&
       fleft->prev == fright->op2 &&
-      fleft->op1 == IR(fright->op2)->op1) {
+      fleft->op1 == IR(fright->op2)->op1 &&
+      !(irt_isphi(fright->t) && IR(fright->op2)->prev)) {
     IRRef ref = fins->op1;
     IR(ref)->op2 = (fleft->op2 | IRBUFHDR_APPEND);  /* Modify BUFHDR. */
     IR(ref)->op1 = fright->op1;

test/tarantool-tests/lj-791-fold-bufhdr-append.test.lua

@@ -0,0 +1,54 @@
+local tap = require('tap')
+
+-- Test file to demonstrate the incorrect LuaJIT's optimization
+-- `bufput_append()` for BUFPUT IR.
+-- See also https://github.com/LuaJIT/LuaJIT/issues/791.
+
+local test = tap.test('lj-791-fold-bufhdr-append'):skipcond({
+  ['Test requires JIT enabled'] = not jit.status(),
+})
+
+test:plan(1)
+
+local EMPTY_STR = ''
+local prefix = 'Lu'
+local result
+
+jit.opt.start('hotloop=1')
+
+-- The interesting part of IRs is the following (non-GC64 mode):
+-- 0006     str BUFSTR 0005 0003
+-- 0007  >  str SLOAD  #2   T
+-- 0008     p32 BUFHDR [0x400004a0] RESET
+-- 0009     p32 BUFPUT 0008 "Lu"
+-- 0010     p32 BUFPUT 0009 0007
+-- 0011   + str BUFSTR 0010 0008
+-- 0012   + int ADD    0001 +1
+-- 0013  >  int LE     0012 +5
+-- 0014  >  --- LOOP ------------
+-- 0015     p32 BUFHDR [0x400004a0] RESET
+
+-- The instruction to be folded is the following:
+-- 0016     p32 BUFPUT 0015 0011
+--
+-- The 0011 operand is PHI, which is not the last IR in the BUFSTR
+-- chain (`ir->prev = REF_BIAS + 0006`). Folding this IR leads to
+-- this resulting IR:
+-- p32 BUFHDR 0010  APPEND
+-- Which appends to buffer instead of resetting, so the resulting
+-- string contains one more symbol.
+
+-- XXX: Use 5 iterations to run the variant part of the loop.
+for _ = 1, 5 do
+  result = prefix .. 'a'
+  -- We need a non-constant string to be appended to prevent more
+  -- aggressive optimizations. Use an empty string for
+  -- convenience. Also, use a constant string in the first operand
+  -- in the concatenation operator for more readable `jit.dump`
+  -- output.
+  prefix = 'Lu' .. EMPTY_STR
+end
+
+test:is(result, 'Lua', 'BUFPUT APPEND optimization is not applied for PHIs')
+
+test:done(true)