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Avoid Overly Long Expressions in Specman e Code

Comments(0)Filed under: Functional Verification, Specman, e language, AF, parsing, long expressions, e code

When you write your e code, a good practice is to avoid expressions that are "overly long" even though they are completely legal. While there is no hard definition of what constitutes an overly long expression, such long expressions can lead to human errors and parser processing errors.

Very long expressions are hard to read and understand. This also makes them error prone, as an accidental syntax error in the middle of such an expression is hard to notice.

Furthermore, such an expression can lead to undesirable results from the Specman parser. It can take it long time to parse, and in some cases (especially if you use a 32-bit platform) it can eventually run out of memory resources and crash. This is all the more likely to happen if the expression contains a real syntax error (which in a shorter expression would normally lead to a syntax error message). Thus, by avoiding expressions that are too long, you benefit twice:

1.  You better avoid introducing accidental syntax errors in the first place.

2.  You help the Specman parser to detect such errors faster in the event that they do occur.

On top of that, it is a good habit to use parentheses in long expressions where appropriate. This not only makes the code more readable, it can actually, in certain cases, make parsing faster.

One last recommendation is to break a long expression into several smaller ones. To illustrate the above recommendations, let's take a look at the following code:

print x == 0 or x == 1 or x == 2 or x == 3 or x == 4 or x == 5 or x == 6 or x == 7 or x == 8 or x == 9 or x == 10 or x == 11 or x == 12 or x == 13 or x == 14 or x == 15 or x == 16 or x == 17 or x == 18 or x == 19 or;

This expression contains a syntax error -- it has an extra "or" at the end. Because the expression is very long, the parser starts to perform a very long calculation, instead of immediately reporting the error. (In this case, the long calculation is avoided when the error is fixed.) Also, the long calculation might eventually lead to a crash.

If you add parentheses as follows, the expression becomes more readable, and you will likely not make the above syntax error in the first place:

print (x == 0) or (x == 1) or (x == 2) or (x == 3) or (x == 4) or (x == 5) or (x == 6) or (x == 7) or (x == 8) or (x == 9) or (x == 10) or (x == 11) or (x == 12) or (x == 13) or (x == 14) or (x == 15) or (x == 16) or (x == 17) or (x == 18) or (x == 19);

You can also break the expression into two smaller ones:

var tmp1 := (x == 0) or (x == 1) or (x == 2) or (x == 3) or (x == 4) or (x == 5) or (x == 6) or (x == 7) or (x == 8) or (x == 9);

var tmp2 := (x == 10) or (x == 11) or (x == 12) or (x == 13) or (x == 14) or (x == 15) or (x == 16) or (x == 17) or (x == 18) or (x == 19);

print tmp1 or tmp2;

In order to improve usability, and ease the pain of this limitation, Specman will automatically issue a warning message, where possible, when parsing an expression actually starts taking too long. This warning will not refer to an exact syntax error, even if there is one; it will, however, refer to the source line in which the problematic expression resides. You will then be able to stop parsing by pressing Control/C, and then examine your code and correct it if needed. This warning will be added in the coming HF releases of Specman, starting from 10.2.

Yuri Tsoglin

e Language team, Specman R&D


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