adds saturating addition and subtraction operators #609
Conversation
This PR introduces saturating arithmetic operators for signed and unsigned integers of fixed width (per our discussion in the WG).
p4-16/spec/P4-16-spec.mdk
Outdated
| when the correct answer is 258, it is less surprising to get an answer of 255 | ||
| from saturating arithmetic than to get an answer of 2 from modular arithmetic. | ||
| At this time, P4 defines saturating operations only for multiplication and | ||
| addition. For an unsigned integer with bit-width of `W`, the minimum value is `0` |
There was a problem hiding this comment.
Should "multiplication and addition" be replaced with "addition and subtraction"? Those are the operators defined later.
There was a problem hiding this comment.
+1 on replacing multiplication with subtraction.
| @@ -2815,6 +2831,8 @@ to bit-strings of the same width: | |||
| by `|`. | |||
| - Bitwise "complement" of a single bit-string, denoted by `~`. | |||
| - Bitwise "xor" of two bit-strings of the same width, denoted by `^`. | |||
| - Saturating addition, denoted by `|+|`. | |||
There was a problem hiding this comment.
Another possibility is [+], which suggests confinement to a range.
There was a problem hiding this comment.
I recommend using |+|. [] are too overloaded.
p4-16/spec/P4-16-spec.mdk
Outdated
| @@ -2852,6 +2870,10 @@ operations simply "wrap around", similar to C operations on unsigned values. | |||
| Hence, attempting to represent large values using `W` bits will only keep | |||
| the least-significant `W` bits of the value. | |||
|
|
|||
| P4 also supports saturating arithmetic (addition and subtraction) for signed | |||
There was a problem hiding this comment.
For signed integers I suspect we want this to work even when adding negative values.
There was a problem hiding this comment.
There is nothing in the text that suggests it should not. The minimum value is -2^(W-1). Do you think we need to add additional clarification?
p4-16/spec/P4-16-spec.mdk
Outdated
| Saturating arithmetic has advantages, in particular when used as counters. The | ||
| the result of a saturating counter max-ing out is much closer to the real | ||
| result than a counter that overflows and wraps around. According to Wikipedia | ||
| (https://en.wikipedia.org/wiki/Saturation_arithmetic) saturating arithmetic is |
There was a problem hiding this comment.
Convert to Markdown URL?
p4-16/spec/P4-16-spec.mdk
Outdated
| @@ -2777,6 +2777,22 @@ keep the least-significant W bits of the value). In particular, P4 | |||
| does not have arithmetic exceptions---the result of an arithmetic | |||
| operation is defined for all possible inputs. | |||
|
|
|||
| P4 target architectures may also support saturating arithmetic. All saturating | |||
There was a problem hiding this comment.
"may also" -> "may optionally"?
This PR introduces saturating arithmetic operators for signed and
unsigned integers of fixed width (per our discussion in the WG).