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)));