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core: Fix shift behavior on non-x86_64 #1316

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core: Fix shift behavior on non-x86_64 #1316

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42 changes: 33 additions & 9 deletions core/vm.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -16,6 +16,7 @@ limitations under the License.

#include <cassert>
#include <cmath>
#include <cstdint>
#include <cstdlib>

#include <memory>
Expand Down Expand Up @@ -2670,16 +2671,25 @@ class Interpreter {
int64_t long_r = safeDoubleToInt64(rhs.v.d, ast.location);
long_r = long_r % 64;

// Additional safety check for left shifts to prevent undefined behavior.
// Left-shift that would move the highest set bit into the sign bit position is undefined.
if (long_r >= 1 && abs(long_l) >= (1LL << (63 - long_r))) {
throw makeError(ast.location,
"numeric value outside safe integer range for bitwise operation.");
if (long_r > 0) {
// Compute bounds without UB
uint64_t abs_l = long_l < 0
? static_cast<uint64_t>(-static_cast<uint64_t>(long_l))
: static_cast<uint64_t>(long_l);

// Avoid (1LL << 63) UB by staying in unsigned
uint64_t limit = UINT64_C(1) << (63 - long_r);

if (abs_l >= limit) {
throw makeError(
ast.location,
"numeric value outside safe integer range for bitwise operation.");
}
}

// Left-shift of a negative number is undefined until C++20.
// Perform the shift on unsigned int to avoid that.
scratch = makeNumber(static_cast<int64_t>(static_cast<uint64_t>(long_l) << long_r));
// Shift in unsigned domain to avoid UB
uint64_t result = static_cast<uint64_t>(long_l) << long_r;
scratch = makeNumber(static_cast<int64_t>(result));
} break;

case BOP_SHIFT_R: {
Expand All @@ -2689,7 +2699,21 @@ class Interpreter {
int64_t long_l = safeDoubleToInt64(lhs.v.d, ast.location);
int64_t long_r = safeDoubleToInt64(rhs.v.d, ast.location);
long_r = long_r % 64;
scratch = makeNumber(long_l >> long_r);

// Avoid implementation-defined behavior for negative values
if (long_l >= 0) {
scratch = makeNumber(static_cast<int64_t>(
static_cast<uint64_t>(long_l) >> long_r));
} else {
if (long_r == 0) {
scratch = makeNumber(long_l);
} else {
uint64_t u = static_cast<uint64_t>(long_l);
uint64_t shifted = u >> long_r;
uint64_t sign_mask = (~UINT64_C(0)) << (64 - long_r);
scratch = makeNumber(static_cast<int64_t>(shifted | sign_mask));
}
}
} break;

case BOP_BITWISE_AND: {
Expand Down
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