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Do not invoke UB in BigInt shr operations

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Shifting a value of type T by an amount that's greater or equal to the
size of the type itself is UB.

Spotted by @tgschultz
This commit is contained in:
LemonBoy
2019-04-26 15:57:47 +02:00
committed by Andrew Kelley
parent 4df2f3d74f
commit 9ec4ccc68f
2 changed files with 5 additions and 3 deletions
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2
src/bigint.cpp
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@@ -1395,7 +1395,7 @@ void bigint_shr(BigInt *dest, const BigInt *op1, const BigInt *op2) {
uint64_t shift_amt = bigint_as_unsigned(op2);
if (op1->digit_count == 1) {
dest->data.digit = op1_digits[0] >> shift_amt;
dest->data.digit = (shift_amt < 64) ? op1_digits[0] >> shift_amt : 0;
dest->digit_count = 1;
dest->is_negative = op1->is_negative;
bigint_normalize(dest);

6
test/stage1/behavior/bit_shifting.zig
View File

@@ -90,7 +90,9 @@ fn testShardedTable(comptime Key: type, comptime mask_bit_count: comptime_int, c
// #2225
test "comptime shr of BigInt" {
comptime {
var n = 0xdeadbeef0000000000000000;
std.debug.assert(n >> 64 == 0xdeadbeef);
var n0 = 0xdeadbeef0000000000000000;
std.debug.assert(n0 >> 64 == 0xdeadbeef);
var n1 = 17908056155735594659;
std.debug.assert(n1 >> 64 == 0);
}
}