Index mapping: Difference between revisions

Revision as of 16:55, 26 November 2015

Revision as of 17:15, 26 November 2015

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===C example 1===

ThisRoger Sayle gives an example<ref>{{cite journal|last1=Sayle|first1=Roger Anthony|title=A Superoptimizer Analysis of Multiway Branch Code Generation|journal=Proceedings of the GCC Developers’ Summit|date=June 17, 2008|pages=103-116|url=https://www.nextmovesoftware.com/technology/SwitchOptimization.pdf|accessdate=26 November 2015}}</ref> of eliminating a Cmultiway functionbranch –caused returning TRUE ifby a monthswitch (x) contains 30 days (otherwise FALSE), illustrates the concept succinctlystatement:

<source lang="c++">

if (((unsigned)x > 12) || ((unsigned)x <= 0) return 0; /*x>12 or x<=0?*/

bool Has30Days(int m) {

static const int T[12] ={0,0,0,1,0,1,0,0,1,0,1,0}; /* 0-based table 'if 30 days =1,else 0' */

switch (m) {

return T[x - 1]; /* return with boolean 1 = true, 0=false */

case 4: // April

case 6: // June

case 9: // September

case 11: // November

return true;

default:

return false;

}

}

</source>

Which can be replaced with a table lookup:

<source lang="c++">

bool Has30Days(int m) {

static const bool T[] = { 0, 0, 0, 0, 1, 0, 1, 0, 0, 1, 0, 1, 0 };

return T[m];

}

</source>