linrrzqqq opened a new pull request, #63954:
URL: https://github.com/apache/doris/pull/63954
Problem Summary:
`signbit` in Nereids FE constant folding used `value < 0` to determine the
sign, which treats `-0.0` as non-negative and folds it to `false`. This is
inconsistent with:
- the runtime BE implementation, which uses `std::signbit`
- the documented `signbit` behavior, which distinguishes `+0.0` and `-0.0`
`+0.0` and `-0.0` compare equal numerically, so `value < 0` cannot
distinguish them. Their difference is only recorded in the floating-point sign
bit. Using raw bits makes FE constant folding consistent with BE runtime
semantics.
before:
```text
Doris> set debug_skip_fold_constant=true;
Query OK, 0 rows affected (0.024 sec)
Doris> select signbit(cast('+0.0' as double)) , signbit(cast('-0.0' as
double));
+---------------------------------+---------------------------------+
| signbit(cast('+0.0' as double)) | signbit(cast('-0.0' as double)) |
+---------------------------------+---------------------------------+
| 0 | 1 |
+---------------------------------+---------------------------------+
1 row in set (0.108 sec)
Doris> set debug_skip_fold_constant=false;
Query OK, 0 rows affected (0.002 sec)
Doris> select signbit(cast('+0.0' as double)) , signbit(cast('-0.0' as
double));
+---------------------------------+---------------------------------+
| signbit(cast('+0.0' as double)) | signbit(cast('-0.0' as double)) |
+---------------------------------+---------------------------------+
| 0 | 0 |
+---------------------------------+---------------------------------+
```
now:
```text
Doris> set debug_skip_fold_constant=true;
Query OK, 0 rows affected (0.012 sec)
Doris> select signbit(cast('+0.0' as double)) , signbit(cast('-0.0' as
double));
+---------------------------------+---------------------------------+
| signbit(cast('+0.0' as double)) | signbit(cast('-0.0' as double)) |
+---------------------------------+---------------------------------+
| 0 | 1 |
+---------------------------------+---------------------------------+
1 row in set (0.070 sec)
Doris> set debug_skip_fold_constant=false;
Query OK, 0 rows affected (0.002 sec)
Doris> select signbit(cast('+0.0' as double)) , signbit(cast('-0.0' as
double));
+---------------------------------+---------------------------------+
| signbit(cast('+0.0' as double)) | signbit(cast('-0.0' as double)) |
+---------------------------------+---------------------------------+
| 0 | 1 |
+---------------------------------+---------------------------------+
1 row in set (0.010 sec)
```
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