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fix dec multiplication with overflow and saturation

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fixes #7276
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Brendan Hansknecht 2024-11-30 10:47:43 -08:00
parent a4fd57e54f
commit 562032e2d8
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GPG key ID: 0EA784685083E75B
1 changed files with 21 additions and 13 deletions
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34
crates/compiler/builtins/bitcode/src/dec.zig
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@ -338,12 +338,11 @@ pub const RocDec = extern struct {
}
}));
const unsigned_answer: i128 = mul_and_decimalize(self_u128, other_u128);
const unsigned_answer = mul_and_decimalize(self_u128, other_u128);
if (is_answer_negative) {
return .{ .value = RocDec{ .num = -unsigned_answer }, .has_overflowed = false };
return .{ .value = RocDec{ .num = -unsigned_answer.value }, .has_overflowed = unsigned_answer.has_overflowed };
} else {
return .{ .value = RocDec{ .num = unsigned_answer }, .has_overflowed = false };
return .{ .value = RocDec{ .num = unsigned_answer.value }, .has_overflowed = unsigned_answer.has_overflowed };
}
}
@ -430,7 +429,15 @@ pub const RocDec = extern struct {
pub fn mulSaturated(self: RocDec, other: RocDec) RocDec {
const answer = RocDec.mulWithOverflow(self, other);
return answer.value;
if (answer.has_overflowed) {
if (answer.value.num < 0) {
return RocDec.max;
} else {
return RocDec.min;
}
} else {
return answer.value;
}
}
pub fn div(self: RocDec, other: RocDec) RocDec {
@ -592,7 +599,7 @@ inline fn count_trailing_zeros_base10(input: i128) u6 {
return count;
}
fn mul_and_decimalize(a: u128, b: u128) i128 {
fn mul_and_decimalize(a: u128, b: u128) WithOverflow(i128) {
const answer_u256 = mul_u128(a, b);
var lhs_hi = answer_u256.hi;
@ -710,14 +717,15 @@ fn mul_and_decimalize(a: u128, b: u128) i128 {
overflowed = overflowed | answer[1];
const d = answer[0];
if (overflowed == 1) {
roc_panic("Decimal multiplication overflow!", 0);
}
// Final 512bit value is d, c, b, a
// need to left shift 321 times
// 315 - 256 is 59. So left shift d, c 59 times.
return @as(i128, @intCast(c >> 59 | (d << (128 - 59))));
// need to right shift 315 times
// 315 - 256 is 59. So shift c right 59 times.
// d takes up the higher space above c, so shift it left by (128 - 59 = 69).
// Since d is being shift left 69 times, all of those 69 bits (+1 for the sign bit)
// must be zero. Otherwise, we have an overflow.
const d_high_bits = d >> 58;
return .{ .value = @as(i128, @intCast(c >> 59 | (d << (128 - 59)))), .has_overflowed = overflowed | d_high_bits != 0 };
}
// Multiply two 128-bit ints and divide the result by 10^DECIMAL_PLACES