x86/x64: Change ipairs_aux to match JIT backend behavior.

Thanks to Sergey Kaplun.

(cherry picked from commit 64b1f10)

In the single-number VM, `ipairs_aux()` first adds the 1 to the given
number and only after casts it to an integer. This leads to results
different from PUC Rio Lua 5.1 and inconsistent with JIT engine
recording.

This patch fixes it by casting the value to an integer before addition.

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

Part of tarantool/tarantool#12480

Reviewed-by: Evgeniy Temirgaleev <e.temirgaleev@tarantool.org>
Reviewed-by: Sergey Bronnikov <sergeyb@tarantool.org>
Signed-off-by: Sergey Kaplun <skaplun@tarantool.org>

Author: Mike Pall

src/vm_x64.dasc

@@ -1476,17 +1476,15 @@ static void build_subroutines(BuildCtx *ctx)
   |  checkint RA, ->fff_fallback
   |.else
   |  checknumtp [BASE+8], ->fff_fallback
-  |  movsd xmm0, qword [BASE+8]
+  |  cvttsd2si RAd, qword [BASE+8]
   |.endif
   |  mov PC, [BASE-8]
-  |.if DUALNUM
   |  add RAd, 1
+  |.if DUALNUM
   |  setint ITYPE, RA
   |  mov [BASE-16], ITYPE
   |.else
-  |  sseconst_1 xmm1, TMPR
-  |  addsd xmm0, xmm1
-  |  cvttsd2si RAd, xmm0
+  |  cvtsi2sd xmm0, RAd
   |  movsd qword [BASE-16], xmm0
   |.endif
   |  cmp RAd, TAB:RB->asize;  jae >2	// Not in array part?

src/vm_x86.dasc

@@ -1853,10 +1853,9 @@ static void build_subroutines(BuildCtx *ctx)
   |  mov dword [BASE-4], LJ_TISNUM
   |  mov dword [BASE-8], RD
   |.else
-  |  movsd xmm0, qword [BASE+8]
-  |  sseconst_1 xmm1, RBa
-  |  addsd xmm0, xmm1
-  |  cvttsd2si RD, xmm0
+  |  cvttsd2si RD, qword [BASE+8]
+  |  add RD, 1
+  |  cvtsi2sd xmm0, RD
   |  movsd qword [BASE-8], xmm0
   |.endif
   |  mov TAB:RB, [BASE]

test/tarantool-tests/lj-1463-ipairs-aux-consistency.test.lua

@@ -0,0 +1,32 @@
+local tap = require('tap')
+
+-- The test file to demonstrate the inconsistent behaviour between
+-- the JIT compiler and the VM for the `ipairs_aux()` function on
+-- x86 and x86_64 arches.
+-- See also: https://github.com/LuaJIT/LuaJIT/issues/1463.
+
+local test = tap.test('lj-1463-ipairs-aux-consistency'):skipcond({
+  ['Test requires JIT enabled'] = not jit.status(),
+})
+
+test:plan(4)
+
+jit.opt.start('hotloop=1')
+
+local ipairs_aux = ipairs({})
+
+local rkeys = {}
+local rvals = {}
+
+for i = 1, 4 do
+  local key, val = ipairs_aux({[0] = 0, [1] = 1}, -0.1)
+  rkeys[i] = key
+  rvals[i] = val
+end
+
+test:is(rkeys[1], 1, 'correct key result')
+test:is(rvals[1], 1, 'correct value result')
+test:samevalues(rkeys, 'consistent JIT and VM behaviour for keys')
+test:samevalues(rvals, 'consistent JIT and VM behaviour for values')
+
+test:done(true)