[Tarantool-patches] [PATCH luajit] Simplify handling of instable types in TNEW/TDUP load forwarding.

[Sergey Kaplun]

From: Mike Pall <mike>

Thanks to Peter Cawley.

(cherry picked from commit c42c62e71a45a677b8b1cbf749bd33cf4d5918ff)

This patch is a follow-up for the commit
b89186cb03e79359847cdbeb000a830cc464db35 ("Fix handling of instable
types in TNEW/TDUP load forwarding."). The problem still occurs when
TDUP load forwarding is performed for the boolean value. The
`fwd_ahload()` always takes the type of IR without checking the type of
the TValue for this iteration.

This patch solves the issue by returning the expected type of the TValue
from the constant table (or `nil` for TNEW IR) instead. In the case of
type instability, the error in `loop_unroll()` will be raised, and the
loop will be unrolled for one more iteration to prevent inconsistencies.

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

Part of tarantool/tarantool#9595
---

Branch: https://github.com/tarantool/luajit/tree/skaplun/lj-994-instable-pri-types
Tarantool PR: https://github.com/tarantool/tarantool/pull/9608
Related issues:
* https://github.com/tarantool/tarantool/issues/9595
* https://github.com/LuaJIT/LuaJIT/issues/994

 src/lj_opt_mem.c                              | 20 +++++++++----------
 ...instable-types-during-loop-unroll.test.lua | 15 +++++++++++++-
 2 files changed, 23 insertions(+), 12 deletions(-)

diff --git a/src/lj_opt_mem.c b/src/lj_opt_mem.c
index 24a490d5..9044f09a 100644
--- a/src/lj_opt_mem.c
+++ b/src/lj_opt_mem.c
@@ -217,25 +217,23 @@ static TRef fwd_ahload(jit_State *J, IRRef xref)
 	}
 	ref = store->prev;
       }
-      if (ir->o == IR_TNEW && !irt_isnil(fins->t))
-	return 0;  /* Type instability in loop-carried dependency. */
-      if (irt_ispri(fins->t)) {
-	return TREF_PRI(irt_type(fins->t));
-      } else if (irt_isnum(fins->t) || (LJ_DUALNUM && irt_isint(fins->t)) ||
-		 irt_isstr(fins->t)) {
+      /* Simplified here: let loop_unroll() figure out any type instability. */
+      if (ir->o == IR_TNEW) {
+	return TREF_NIL;
+      } else {
 	TValue keyv;
 	cTValue *tv;
 	IRIns *key = IR(xr->op2);
 	if (key->o == IR_KSLOT) key = IR(key->op1);
 	lj_ir_kvalue(J->L, &keyv, key);
 	tv = lj_tab_get(J->L, ir_ktab(IR(ir->op1)), &keyv);
-	if (itype2irt(tv) != irt_type(fins->t))
-	  return 0;  /* Type instability in loop-carried dependency. */
-	if (irt_isnum(fins->t))
+	if (tvispri(tv))
+	  return TREF_PRI(itype2irt(tv));
+	else if (tvisnum(tv))
 	  return lj_ir_knum_u64(J, tv->u64);
-	else if (LJ_DUALNUM && irt_isint(fins->t))
+	else if (tvisint(tv))
 	  return lj_ir_kint(J, intV(tv));
-	else
+	else if (tvisgcv(tv))
 	  return lj_ir_kstr(J, strV(tv));
       }
       /* Othwerwise: don't intern as a constant. */
diff --git a/test/tarantool-tests/lj-994-instable-types-during-loop-unroll.test.lua b/test/tarantool-tests/lj-994-instable-types-during-loop-unroll.test.lua
index f240bdd2..a0cc8487 100644
--- a/test/tarantool-tests/lj-994-instable-types-during-loop-unroll.test.lua
+++ b/test/tarantool-tests/lj-994-instable-types-during-loop-unroll.test.lua
@@ -10,7 +10,7 @@ local test = tap.test('lj-994-instable-types-during-loop-unroll'):skipcond({
 
 -- TODO: test that compiled traces don't always exit by the type
 -- guard. See also the comment for the TDUP test chunk.
-test:plan(2)
+test:plan(3)
 
 -- TNEW.
 local result
@@ -46,4 +46,17 @@ for _ = 1, 5 do
 end
 test:is(result, true, 'TDUP load forwarding was successful')
 
+-- TDUP, primitive types.
+for i = 1, 5 do
+  local t = slot
+  -- Now use constant key slot to get necessary branch.
+  -- LJ_TRERR_GFAIL isn't triggered here.
+  -- See `fwd_ahload()` in <src/lj_opt_mem.c> for details.
+  result = t[1]
+  -- The constant tables should contain different booleans
+  -- (primitive types).
+  slot = i % 2 ~= 0 and {false} or {true}
+end
+test:is(result, true, 'TDUP load forwarding (primitive types) was successful')
+
 test:done(true)
-- 
2.43.0