87 lines
2.6 KiB
Text
87 lines
2.6 KiB
Text
.sp 2
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.NH
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Improvements to the compiler
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.nh
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.sp
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.LP
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In consideration of portability, a restricted option could be implemented.
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Under this option, the extensions and warnings should be considered as errors.
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.LP
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The restrictions imposed by the standard on the control variable of a
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for-statment should be implemented (\fBISO 6.8.3.9\fR).
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.LP
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To check whether a function returns a valid result, the following algorithm
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could be used. When a function is entered a hidden temporary variable of
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type boolean is created. This variable is initialized with the value false.
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The variable is set to true, when an assignment to the function name occurs.
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On exit of the function a test is performed on the variable. If the value
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of the variable is false, a run-time error occurs.
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.br
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Note: The check has to be done run-time.
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.LP
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The \fIundefined value\fR should be implemented. A problem arises with
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local variables, for which space on the stack is allocated. A possible
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solution would be to generate code for the initialization of the local
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variables with the undefined value at the beginning of a procedure or
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function.
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.br
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The implementation for the global variables is easy, because \fBbss\fR
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blocks are used.
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.LP
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Closely related to the last point is the generation of warnings when
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variables are never used or assigned. This is not yet implemented.
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.LP
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The error messages could specify more details about the errors occurred,
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if some additional testing is done.
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.bp
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.LP
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Every time the compiler detects sets with different base-types, a warning
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is given. Sometimes this is superfluous.
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.nf
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\fBprogram\fR sets(output);
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\fBtype\fR
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week = (sunday, monday, tuesday, wednesday, thursday, friday, saturday);
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workweek = monday..friday;
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\fBvar\fR
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s : \fBset of\fR workweek;
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day : week;
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\fBbegin\fR
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day := monday;
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s := [day]; (* warning *)
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day := saturday;
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s := [day]; (* warning *)
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\fBend\fR.
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.fi
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The new compiler gives two warnings, the first one is redundant.
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.LP
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A nasty point in the compiler is the way the procedures \fIread, readln,
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write\fR and \fIwriteln\fR are handled (see also section 2.2). They have
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been added to the grammar. This implies that they can not be redefined as
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opposed to the other required procedures and functions. They should be
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removed from the grammar altogether. This could imply that more semantic
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checks have to be performed.
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.LP
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No effort is made to detect possible run-time errors during compilation.
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.br
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E.g. a : \fBarray\fR[1..10] \fBof\fI something\fR, and the array selection
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a[11] would occur.
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.LP
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Some assistance to implement the improvements mentioned above, can be
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obtained from [PCV].
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