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<title>12.2.6 Pragma ST_nosideeffects </title>
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<a name="1212399"> </a><font color="#003366" face="Verdana, Arial, Helvetica, sans-serif">12.2.6 Pragma ST_nosideeffects</font>
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<p>
<a name="1204656"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">This pragma enables <strong>st20cc</strong> to be provided with more information about functions, whose behavior could otherwise be ambiguous, and therefore helps to maximize the degree of optimization performed.</font>
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<a name="1204657"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">Syntax:</font>
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<a name="1206326"> </a><font size=2 face=Courier><strong>#pragma ST_nosideeffects (<font size=2 face=Courier><strong><em>function-name</em></strong></font>)<br></strong></font>
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<a name="1204658"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">where <font size=2 face=Courier><strong><em>function-name</em></strong></font> must have been declared but not defined or instanced before the pragma is encountered. A warning is generated and the pragma ignored if:</font>
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<p> <font size=2 face="Verdana, Arial, Helvetica, sans-serif"><li ><a name="1204659"> </a>the named function has been defined, part defined or instanced before the pragma is encountered,</font></p>
<p> <font size=2 face="Verdana, Arial, Helvetica, sans-serif"><li ><a name="1204660"> </a>its parameter is not a function identifier,</font></p>
<p> <font size=2 face="Verdana, Arial, Helvetica, sans-serif"><li ><a name="1204661"> </a>it is given more than one parameter or none at all.</font></p>
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<p>
<a name="1204662"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">Functions marked as side-effect free enable <strong>st20cc</strong> to avoid making worst-case assumptions when compiling variables which are modified when the function is called. A function is side-effect free when, within the body of a function:</font>
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<p> <font size=2 face="Verdana, Arial, Helvetica, sans-serif"><li ><a name="1204663"> </a>there are no assignments to variables which are defined outside the function, including writing through a pointer where the pointer points to a variable defined outside the function, for example I/O or "passing by reference" are not allowed,</font></p>
<p> <font size=2 face="Verdana, Arial, Helvetica, sans-serif"><li ><a name="1204664"> </a>there are no assignments to static variables (although initialization of static variables within definitions are allowed),</font></p>
<p> <font size=2 face="Verdana, Arial, Helvetica, sans-serif"><li ><a name="1204665"> </a>there are no reads from or assignments to volatile variables,</font></p>
<p> <font size=2 face="Verdana, Arial, Helvetica, sans-serif"><li ><a name="1204666"> </a>there are no calls to functions which may have side-effects.</font></p>
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<a name="1204667"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">Ensure that a function meets the above criteria before marking it with the pragma <font size=2 face=Courier><strong>ST_nosideeffects</strong></font>.<font face="Verdana, Arial, Helvetica, sans-serif"><b> </b></font>If a function is marked as side-effect free and it does not meet the above criteria, <strong>st20cc</strong> reports an error. Assembler inserts, that is, <font size=2 face=Courier><strong>_ _asm</strong></font> statements and pointers to functions, are a particular risk area because unconsciously, a function may be introduced which breaks the above criteria. For this reason, <strong>st20cc</strong> generates a warning each time assembler code or a pointer to a function is encountered.</font>
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<a name="1205474"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif"><em>Note: These warning messages may be disabled by using the <font face="Verdana, Arial, Helvetica, sans-serif"><b>ws</b></font> command line option.</em></font>
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<p>
<a name="1204669"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif"><strong>st20cc</strong> may mark a function as side-effect free, even though the pragma <font size=2 face=Courier><strong>ST_nosideeffects</strong></font><font face="Verdana, Arial, Helvetica, sans-serif"><b> </b></font>has not been applied. Use of the <font face="Verdana, Arial, Helvetica, sans-serif"><b>fcheck</b></font><font face="Verdana, Arial, Helvetica, sans-serif"><b>referenced</b></font> command line option causes an information message to be generated if this occurs.</font>
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<a name="1204670"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif"><strong>st20cc</strong> only marks those functions that it detects as unambiguously side-effect free and which are not already marked. It is possible for there to be functions which are side-effect free that <strong>st20cc</strong> does not detect; for such functions, only the application of the pragma <font size=2 face=Courier><strong>ST_nosideeffects</strong></font><font face="Verdana, Arial, Helvetica, sans-serif"><b> </b></font>causes these functions to be marked as side-effect free.</font>
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<p>
<a name="1204671"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">If a function that is side-effect free is externally visible, then it may be useful to put the pragma <font size=2 face=Courier><strong>ST_nosideeffects</strong></font> into the header file following the prototype of the function. This enables <strong>st20cc</strong> to generate better code for calls to that function from other files.</font>
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