Differences

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--- language:expressions [2024/07/19 14:20] – [Conversion from/to integer to/from pure structures] rajit
+++ language:expressions [2025/03/06 22:27] (current) – [Bit-width rules] rajit
@@ Line 48: / Line 48: @@
 ===== Expressions in CHP =====
-The same expression syntax is also used in the ''chp'' and ''dataflow'' sub-languages. Once again, constant expressions are simplified as above. However, expressions can also include variables that are determined at run-time rather than expansion time (i.e. when the circuit is executing the specified computation.) In this case, we need rules to determine the bit-width of an expression. The rules are as follows:
+The same expression syntax is also used in the ''chp'' and ''dataflow'' sub-languages. Once again, constant expressions are simplified as above. However, expressions can also include variables that are determined at run-time rather than expansion time (i.e. when the circuit is executing the specified computation.) In this case, we need rules to determine the bit-width of an expression.
+==== Bit-width rules ====
+The rules that are used to determine the bit-width of an expression involving circuit variables are as follows:
    * Each variable has the bit-width specified by its type.
-   * A constant uses the minimum number of bits needed to represent it. Note that a negative constant is assumed to be a two's complement value, an its bit-width is determined in the same way.
+   * A constant uses the minimum number of bits needed to represent it. Note that a negative constant is assumed to be a two's complement value, an its bit-width is determined in the same way. If you are unsure about what is going to happen and want to control the bit-width of a constant, ''int(const, width)'' can be used. Be careful about is negative constants, since the ''int(.)'' operation will zero-extend a value and not sign-extend it.
    * For unary operators, the bit-width of the result is the same as the bit-width of the argument
    * For binary operators and ternary operators where the result is an integer, let //left// be the bit-width of the left-hand side of the operator, and //right// be the bit-width of the right hand side. There are six categories of result bit-widths:
@@ Line 72: / Line 76: @@
      - //left// + 2^//right// - 1
         * left shift ''<<''
-  * For concatenation, the bit-width is the sum of all the components. For the bitfield extraction, the bitwidth is determined by the number of bits extracted.
+  * For concatenation, the bit-width is the sum of all the components.
+  * For bitfield extraction, the bitwidth is determined by the number of bits extracted.
+==== Idiosyncrasies ====
 While these bit-width rules are nice because you never lose bits, they can have some unexpected consequences. One of the not-so-nice effects of these rules is that, technically, addition is no longer associative in general! For example, consider the following two different assignment statements: