[Tarantool-patches] [PATCH 1/1] tuple: don't truncate float in :update()
Nikita Pettik
korablev at tarantool.org
Sun Jan 26 21:45:54 MSK 2020
On 26 Jan 17:44, Vladislav Shpilevoy wrote:
> Before the patch there were the rules:
> * float +/- double = double
> * double +/- double = double
> * float +/- float = float
>
> The rules were applied regardless of values. That led to a problem
> when float + float exceeding maximal float value could fit into
> double, but was stored as an infinity.
>
> The patch makes so that if a floating point arithmetic operation
> result fits into float, it is stored as float. Otherwise as
> double. Regardless of initial types.
>
> This alongside saves some memory for cases when doubles can be
> stored as floats, and therefore takes 4 less bytes. Although
> these cases are rare, because any not integer value stored in a
> double may have a long garbage tail in its fraction.
>
> Closes #4701
> ---
> Branch: https://github.com/tarantool/tarantool/tree/gerold103/gh-4701-update-float-truncate
> Issue: https://github.com/tarantool/tarantool/issues/4701
>
> I am not sure about the patch correctness. Perhaps we should not
> save double + double as float even when it fits. It would break
> DOUBLE data type, which we are going to introduce, because from
> what I understood, it is going to store MP_DOUBLE only.
>
> On the other hand, DOUBLE is not implemented yet
I guess double is already on board. See d8193eb1c
> and when it will
> be implemented, we may decide to allow to store MP_FLOAT there.
>
> src/box/xrow_update_field.c | 18 ++++++++--
> test/box/update.result | 66 +++++++++++++++++++++++++++++++++++++
> test/box/update.test.lua | 46 ++++++++++++++++++++++++++
> 3 files changed, 127 insertions(+), 3 deletions(-)
>
> diff --git a/src/box/xrow_update_field.c b/src/box/xrow_update_field.c
> index 7c0f5fb5e..31429ee37 100644
> --- a/src/box/xrow_update_field.c
> +++ b/src/box/xrow_update_field.c
> @@ -400,13 +400,25 @@ xrow_update_arith_make(struct xrow_update_op *op,
> unreachable();
> break;
> }
> - if (lowest_type == XUPDATE_TYPE_DOUBLE) {
> + float fc = (float) c;
> + /*
> + * A value may be saved as double even if it looks
> + * like fitting a float. For example, 0.01 + 0.01
> + * may be stored as double. This is because
> + * 0.01 may be stored as 0.009999999999999, what
> + * looks like double precision. And there is no
> + * way how to check if this is actually 0.01.
> + * By the same reason FLT_MAX can't be used to
> + * detect whether a value fits float, because it
> + * may be <= FLT_MAX, but may have a double
> + * precision in its fraction part.
> + */
> + if (c != (double) fc) {
> ret->type = XUPDATE_TYPE_DOUBLE;
> ret->dbl = c;
> } else {
> - assert(lowest_type == XUPDATE_TYPE_FLOAT);
> ret->type = XUPDATE_TYPE_FLOAT;
> - ret->flt = (float)c;
> + ret->flt = fc;
> }
> } else {
> decimal_t a, b, c;
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