Redundant code: Difference between revisions

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In [[computer programming]], '''redundant code''' is [[source code]] or compiled code inthat ais [[computerunnecessary. program]]Code that iscan unnecessary,be suchremoved without affecting its desired behavior is as:redundant.

* recomputing a value that has previously been calculated<ref>[http://doi.acm.org/10.1145/349214.349233 Debray, S. K., Evans, W., Muth, R., and De Sutter, B. 2000. Compiler techniques for code compaction. ACM Trans. Program. Lang. Syst. 22, 2 (Mar. 2000), 378–415.]</ref> and is still available,

* code that is never executed (known as [[unreachable code]]),

* code which is executed but has no external effect (e.g., does not change the output produced by a program; known as [[dead code]]).

==Categories==

A [[NOP (code)|NOP]] instruction might be considered to be redundant code that has been explicitly inserted to pad out the [[instruction (computer science)|instruction]] stream or introduce a time delay, for example to create a timing loop by "wasting time". [[Identifier]]s that are declared, but never referenced, are termed '''redundant declarations'''.

Noteble categoreis of redundant code include:

*; recomputingRecomputing: Recomputing a value that has previously been calculated<ref>[http://doi.acm.org/10.1145/349214.349233 Debray, S. K., Evans, W., Muth, R., and De Sutter, B. 2000. Compiler techniques for code compaction. ACM Trans. Program. Lang. Syst. 22, 2 (Mar. 2000), 378–415.]</ref> and is still available, .

==Examples==

<source lang="C">

int foo(int iX)

{

int iY = iX*2;

*; [[Dead code]]: Code whichthat is executed but has no external effect; (e.g., does not change the output produced by a program;. known as [[dead code]]).

return iX*2;

}

</source>

; [[Unreachable code]]: Code that is never executed. Also, called dead code.

The second <tt>iX*2</tt> expression is redundant code and can be replaced by a reference to the variable <tt>iY</tt>. Alternatively, the definition <tt>int iY = iX*2</tt> can instead be removed.

; [[NOP (code)|NOP]] padding: A NOP instruction might be considered redundant if it's for padding. But if the NOP is required for proper functionality then it's not redundant.

Consider:

; Unused [[identifier]]: Declared, but never referenced is a redundant declaration.

<source lang="C">

#define min(A,B) ((A)<(B)?(A):(B))

==Examples==

int shorter_magnitude(int u1, int v1, int u2, int v2)

In the following [[C (computer language)|C]] code, the second <code>x * 2</code> expression is redundant code. Line 2 can be removed, or alternatively, line 3 can be changed to {{code| return y;}}.

{

/* Returns the shorter magnitude of (u1,v1) and (u2,v2) */

<syntaxhighlight lang="C" line>

return sqrt(min(u1*u1 + v1*v1, u2*u2 + v2*v2));

int foo(int iXx) {

int iYy = iXx * 2;

return iXx * 2;

}

</syntaxhighlight>

</source>

AsA amore consequencesubtle ofexample usinginvolves the [[C preprocessor]], thethat compilerinserts willcode only witness thebefore expandedcompilation. formConsider:

<sourcesyntaxhighlight lang="C">

#define min(A,B) ((A)<(B)?(A):(B))

int shorter_magnitude(int u1, int v1, int u2, int v2)

int shorter_magnitude(int u1a, int v1b, int u2c, int v2d) {

{

return sqrt(min(u1a*u1a + v1b*v1b, u2c*u2c + v2d*v2d));

int temp;

if (u1*u1 + v1*v1 < u2*u2 + v2*v2)

temp = u1*u1 + v1*v1; /* Redundant */

else

temp = u2*u2 + v2*v2; /* Redundant */

return sqrt(temp);

}

</syntaxhighlight>

</source>

After preprocessing, the code expands to code that evaluates both {{code|a*a + b*b}} and {{code|c*c + d*d}} twice. To eliminate the duplicate code, the macro {{code|min}} could ge converted to a function.

Because the use of min/max macros is very common, modern compilers are trained to recognize and eliminate redundancy caused by their use.

<sourcesyntaxhighlight lang="C">

There is ''no'' redundancy, however, in the following code:

int shorter_magnitude(int u1a, int v1b, int u2c, int v2d) {

<source lang="C">

return sqrt(((a*a + b*b)<(c*c + d*d)?(a*a + b*b):(c*c + d*d)));

#define max(A,B) ((A)>(B)?(A):(B))

int random(int cutoff, int range)

{

/* Returns (cutoff, cutoff, cutoff, ...,

cutoff+1, cutoff+2, ... range) */

return max(cutoff, rand()%range);

}

</syntaxhighlight>

</source>

The reason is that its implementation is incorrect. If the initial call to rand(), modulo range, is greater than or equal to cutoff, rand() will be called a second time for a second computation of rand()%range, which may result in a value that is actually lower than the cutoff. The max macro will thus not work for this function.

==See also==

* {{Annotated link |Code bloat}}

* [[{{Annotated link |Code bloat]]reuse}}

* {{Annotated link |Common subexpression elimination}}

* [[Duplicate code]]

* {{Annotated link |Don't repeat yourself}}

* [[{{Annotated link |Duplicate code]]}}

* {{Annotated link |Redundancy (information theory)}}

* {{Annotated link |Code refactoring}}

* {{Annotated link |Code smell}}

==References==