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Fix right shift on negative BigInts

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Closes #17662.
This commit is contained in:
Tau 2024-07-11 13:57:37 +02:00 • committed by Andrew Kelley
parent 3ad81c40c0
commit 2b99b04285
2 changed files with 40 additions and 6 deletions
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29
lib/std/math/big/int.zig
View file

@ -1173,7 +1173,9 @@ pub const Mutable = struct {
/// Asserts there is enough memory to fit the result. The upper bound Limb count is
/// `a.limbs.len - (shift / (@sizeOf(Limb) * 8))`.
pub fn shiftRight(r: *Mutable, a: Const, shift: usize) void {
if (a.limbs.len <= shift / limb_bits) {
const full_limbs_shifted_out = shift / limb_bits;
const remaining_bits_shifted_out = shift % limb_bits;
if (a.limbs.len <= full_limbs_shifted_out) {
// Shifting negative numbers converges to -1 instead of 0
if (a.positive) {
r.len = 1;
@ -1186,14 +1188,29 @@ pub const Mutable = struct {
}
return;
}
const nonzero_negative_shiftout = if (a.positive) false else nonzero: {
for (a.limbs[0..full_limbs_shifted_out]) |x| {
if (x != 0)
break :nonzero true;
}
if (remaining_bits_shifted_out == 0)
break :nonzero false;
const not_covered: Log2Limb = @intCast(limb_bits - remaining_bits_shifted_out);
break :nonzero a.limbs[full_limbs_shifted_out] << not_covered != 0;
};
llshr(r.limbs[0..], a.limbs[0..a.limbs.len], shift);
r.normalize(a.limbs.len - (shift / limb_bits));
r.positive = a.positive;
// Shifting negative numbers converges to -1 instead of 0
if (!r.positive and r.len == 1 and r.limbs[0] == 0) {
r.limbs[0] = 1;
r.len = a.limbs.len - full_limbs_shifted_out;
if (nonzero_negative_shiftout) {
if (full_limbs_shifted_out > 0) {
r.limbs[a.limbs.len - full_limbs_shifted_out] = 0;
r.len += 1;
}
r.addScalar(r.toConst(), -1);
}
r.normalize(r.len);
r.positive = a.positive;
}
/// r = ~a under 2s complement wrapping semantics.

17
lib/std/math/big/int_test.zig
View file

@ -2066,6 +2066,23 @@ test "shift-right negative" {
defer arg3.deinit();
try a.shiftRight(&arg3, 1232);
try testing.expect((try a.to(i32)) == -1); // -10 >> 1232 == -1
var arg4 = try Managed.initSet(testing.allocator, -5);
defer arg4.deinit();
try a.shiftRight(&arg4, 2);
try testing.expect(try a.to(i32) == -2); // -5 >> 2 == -2
var arg5 = try Managed.initSet(testing.allocator, -0xffff0000eeee1111dddd2222cccc3333);
defer arg5.deinit();
try a.shiftRight(&arg5, 67);
try testing.expect(try a.to(i64) == -0x1fffe0001dddc223);
var arg6 = try Managed.initSet(testing.allocator, -0x1ffffffffffffffff);
defer arg6.deinit();
try a.shiftRight(&arg6, 1);
try a.shiftRight(&a, 1);
a.setSign(true);
try testing.expect(try a.to(u64) == 0x8000000000000000);
}
test "sat shift-left simple unsigned" {