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--- language:expressions [2023/09/07 10:03]
rajit [Expressions]
+++ language:expressions [2024/03/20 07:19] (current)
rajit [External circuit functions]
@@ Line 19: / Line 19: @@
    * ''bool(x)'': ''x'' must be an integer expression. This returns false if the integer is zero, and true otherwise.
+Syntactic replication is also supported for the operators ''&'', ''|'', ''^'', ''+'', and ''*''. This means the following expression is valid
+<code act>
+  (+ i : 3 : p[i] + 2*i)
+</code>
+and is equivalent to
+<code act>
+  p[0] + 2*0 + p[1] + 2*1 + p[2] + 2*2
+</code>
 ===== Parameters and constant expressions =====
@@ Line 65: / Line 73: @@
         * 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.
+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:
+<code act>
+int<2> a;
+int<3> b;
+int<4> c;
+...
+chp {
+   ...
+   x := (a + b) + c;
+   y := a + (b + c);
+   ...
+}
+</code>
+Applying the bit-width rules, the expression ''(a+b) + c'' has bit-width 5, whereas ''a + (b + c)'' has bit-width 6. While this does not have any consequences in this particular example, it could become problematic if the bitwise complement operator is used, or in the case where this expression needs to be negated (since subtraction is essentially taking the two's complement and then adding, which includes the bitwise complement operator).
+Another strange example is:
+<code act>
+...
+chp {
+   ...
+   x := x - 1;
+   y := y + (-1);
+   ...
+}
+</code>
+Now the right hand side of the first assignment takes ''x'' and ''1'' and does the subtraction that was expected. The second assignment, on the other hand, takes the bit-pattern for ''-1'' (this turns out to be ''1''), and adds it to ''y''! In other words, ''x-1'' and ''x+(-1)'' are not the same because of the way the bit-width rules operate. If you have any doubts, the ''int(...)'' operator can be used to specify the bit-width.
 ===== Functions =====
@@ Line 208: / Line 248: @@
 An example of file I/O implemented with external functions can be found in the ''actsim'' [[https://github.com/asyncvlsi/actsim|git repository]] in the ''simlib/'' directory.
+===== Operator Precedence =====
+The operators have the following precedence, from the highest to lowest:
+   - ''~'', ''!''
+   - ''*'', ''/'', ''%''
+   - ''+'', ''-''
+   - ''<<'', ''>>'', ''>>>'', ''<'', ''>'', ''<='', ''>='', ''='', ''!=''
+   - ''&''
+   - ''^''
+   - ''|''
+   - ''?''