The Tor Browser: comparison js/src/jit-test/tests/jaeger/loops/multiply-by-int32min.js

comparison: js/src/jit-test/tests/jaeger/loops/multiply-by-int32min.js

js/src/jit-test/tests/jaeger/loops/multiply-by-int32min.js

branch: TOR_BUG_9701
changeset 15: b8a032363ba2

equal deleted inserted replaced

--1:000000000000
+:e9e9f400628c
+function foo()
+{
+// Range analysis incorrectly computes a range for the multiplication.  Once
+// that incorrect range is computed, the goal is to compute a new value whose
+// range analysis *thinks* is in int32_t range, but which goes past it using
+// JS semantics.
+//
+// On the final iteration, in JS semantics, the multiplication produces 0, and
+// the next addition 0x7fffffff.  Adding any positive integer to that goes
+// past int32_t range: here, (0x7fffffff + 5) or 2147483652.
+//
+// Range analysis instead thinks the multiplication produces a value in the
+// range [INT32_MIN, INT32_MIN], and the next addition a value in the range
+// [-1, -1].  Adding any positive value to that doesn't overflow int32_t range
+// but *does* overflow the actual range in JS semantics.  Thus omitting
+// overflow checks produces the value 0x80000004, which interpreting as signed
+// is (INT32_MIN + 4) or -2147483644.
+//
+// For this test to trigger the bug it was supposed to trigger:
+//
+//   * 0x7fffffff must be the LHS, not RHS, of the addition in the loop, and
+//   * i must not be incremented using ++
+//
+// The first is required because JM LoopState doesn't treat *both* V + mul and
+// mul + V as not overflowing, when V is known to be int32_t -- only V + mul.
+// (JM pessimally assumes V's type might change before it's evaluated.  This
+// obviously can't happen if V is a constant, but JM's puny little mind
+// doesn't detect this possibility now.)
+//
+// The second is required because JM LoopState only ignores integer overflow
+// on multiplications if the enclosing loop is a "constrainedLoop" (the name
+// of the relevant field).  Loops become unconstrained when unhandled ops are
+// found in the loop.  Increment operators generate a DUP op, which is not
+// presently a handled op, causing the loop to become unconstrained.
+for (var i = 0; i < 15; i = i + 1) {
+var y = (0x7fffffff + ((i & 1) * -2147483648)) + 5;
+}
+return y;
+}
+assertEq(foo(), (0x7fffffff + ((14 & 1) * -2147483648)) + 5);
+function bar()
+{
+// Variation on the theme of the above test with -1 as the other half of the
+// INT32_MIN multiplication, which *should* result in -INT32_MIN on multiply
+// (exceeding int32_t range).
+//
+// Here, range analysis again thinks the range of the multiplication is
+// INT32_MIN.  We'd overflow-check except that adding zero (on the LHS, see
+// above) prevents overflow checking, so range analysis thinks the range is
+// [INT32_MIN, INT32_MIN] when -INT32_MIN is actually possible.  This direct
+// result of the multiplication is already out of int32_t range, so no need to
+// add anything to bias it outside int32_t range to get a wrong result.
+for (var i = 0; i < 17; i = i + 1) {
+var y = (0 + ((-1 + (i & 1)) * -2147483648));
+}
+return y;
+}
+assertEq(bar(), (0 + ((-1 + (16 & 1)) * -2147483648)));