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std.math.big.int: fix shiftRight sign handling

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Closes #21311

The sign of the result `r` needs to be initialized before the correction
`r.addScalar(r.toConst(), -1)`, or the intended end result could be off
by 2 (depending on the original sign of `r`).
This commit is contained in:
Ian Johnson 2024-09-05 23:05:21 -04:00 committed by Andrew Kelley
parent 3929cac154
commit 3543f28320
2 changed files with 10 additions and 1 deletions
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2
lib/std/math/big/int.zig
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@ -1202,6 +1202,7 @@ pub const Mutable = struct {
llshr(r.limbs[0..], a.limbs[0..a.limbs.len], shift);
r.len = a.limbs.len - full_limbs_shifted_out;
r.positive = a.positive;
if (nonzero_negative_shiftout) {
if (full_limbs_shifted_out > 0) {
r.limbs[a.limbs.len - full_limbs_shifted_out] = 0;
@ -1210,7 +1211,6 @@ pub const Mutable = struct {
r.addScalar(r.toConst(), -1);
}
r.normalize(r.len);
r.positive = a.positive;
}
/// r = ~a under 2s complement wrapping semantics.

9
lib/std/math/big/int_test.zig
View File

@ -2083,6 +2083,15 @@ test "shift-right negative" {
try a.shiftRight(&a, 1);
a.setSign(true);
try testing.expect(try a.to(u64) == 0x8000000000000000);
var arg7 = try Managed.initSet(testing.allocator, -32767);
defer arg7.deinit();
a.setSign(false);
try a.shiftRight(&arg7, 4);
try testing.expect(try a.to(i16) == -2048);
a.setSign(true);
try a.shiftRight(&arg7, 4);
try testing.expect(try a.to(i16) == -2048);
}
test "sat shift-left simple unsigned" {