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Add definition for compound assignment operators #1144

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Add definition for compound assignment operators #1144

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34 changes: 34 additions & 0 deletions p4-16/spec/P4-16-spec.mdk
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Original file line number Diff line number Diff line change
Expand Up @@ -3177,6 +3177,8 @@ side-effects. P4 expressions are evaluated as follows:
the second operand is only evaluated if necessary.
- The conditional operator `e1 ? e2 : e3` evaluates `e1`, and then
either evaluates `e2` or `e3`.
- Compound assignment expressions are evaluated as described in Section
[#sec-compound-assignment].
- All other expressions are evaluated left-to-right as they appear in
the source program.
- Method and function calls are evaluated as described in Section
Expand Down Expand Up @@ -5275,6 +5277,38 @@ types (e.g. `structs`) are copied recursively, and all components
of `header`s are copied, including "validity" bits. Assignment is
not defined for `extern` values.

### Compound assignment { #sec-compound-assignment }

Compound assignment operators provide a shorter syntax for assigning
the result of an arithmetic or bitwise operator. The compound assignment
operator expressions have the form `LHS op RHS`, where `op` is one of those
in the table below, and `LHS` and `RHS` are expressions with types appropriate
for the operator.

|----------|-----------------------------------|------------|---------------|
| Operator | Operator name | Example | Equivalent of |
+----------+:---------------------------------:+:----------:+:-------------:+
| `=` | basic assignment | `a = b` | NA |
| `+=` | addition assignment | `a += b` | `a = a + b` |
| `-=` | subtraction assignment | `a -= b` | `a = a - b` |
| `*=` | multiplication assignment | `a *= b` | `a = a * b` |
| `/=` | division assignment | `a /= b` | `a = a / b` |
| `%=` | modulo assignment | `a %= b` | `a = a % b` |
| `&=` | bitwise AND assignment | `a &= b` | `a = a & b` |
| `|=` | bitwise OR assignment | `a |= b` | `a = a | b` |
| `^=` | bitwise XOR assignment | `a ^= b` | `a = a ^ b` |
| `<<=` | bitwise left shift assignment | `a <<= b` | `a = a << b` |
| `>>=` | bitwise right shift assignment | `a >>= b` | `a = a >> b` |
| `|+|=` | saturating addition assignment | `a |+|= b` | `a = a |+| b` |
| `|-|=` | saturating subtraction assignment | `a |-|= b` | `a = a |-| b` |
|----------|-----------------------------------|----------- |---------------|

The behavior of every builtin compound assignment expression with the form `E1 op= E2`,
where `E1` is a modifiable l-value expression and `E2` is an r-value expression, is exactly
the same as the behavior of the expression `E1 = E1 op E2`, except that `E1` is evaluated
only once. Like other P4 expressions where subexpressions are evaluated left to right,
`E1` is evaluated before `E2`.

## Empty statement { #sec-empty-stmt }

The empty statement, written `;` is a no-op.
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14 changes: 14 additions & 0 deletions p4-16/spec/grammar.mdk
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Expand Up @@ -726,6 +726,20 @@ annotationToken
| "?"
| "."
| "="
| "+="
| "-="
| "*="
| "/="
| "%="
| "&="
| "|="
| "&&="
| "||="
| "^="
| "<<="
| ">>="
| "|+|="
| "|-|="
| ";"
| "@"
| UNKNOWN_TOKEN
Expand Down