407 lines
8.3 KiB
Text
407 lines
8.3 KiB
Text
.sp 1.5i
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.de CL
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.ft R
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c\\$1
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.ft 5
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\fIcode statement-\\$1
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.ft 5
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\fBbra *\fRexit_label
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.ft 5
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..
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.NH
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Translation of Pascal to EM code
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.nh
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.LP
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.sp
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A short description of the translation of Pascal constructs to EM code is
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given in the following paragraphs. The EM instructions and Pascal terminal
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symbols are printed in \fBboldface\fR. A sentence in \fIitalics\fR is a
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description of a group of EM (pseudo)instructions.
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.sp
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.NH 2
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Global Variables
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.LP
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.sp
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For every global variable, a \fBbss\fR block is reserved. To enhance the
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readability of the EM-code generated, the variable-identifier is used as
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a data label to address the block.
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.sp
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.NH 2
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Expressions
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.LP
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.sp
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Operands are always evaluated, so the execution of
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.br
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.ti +3m
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\fBif\fR ( p <> nil ) \fBand\fR ( p^.value <> 0 ) \fBthen\fR .....
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.br
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might cause a run-time error, if p is equal to nil.
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.LP
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The left-hand operand of a dyadic operator is almost always evaluated before
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the right-hand side. Peculiar evaluations exist for the following cases:
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.sp
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the expression: set1 <= set2, is evaluated as follows :
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.nf
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- evaluate set2
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- evaluate set1
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- compute set2+set1
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- test set2 and set2+set1 for equality
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.fi
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.sp
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the expression: set1 >= set2, is evaluated as follows :
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.nf
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- evaluate set1
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- evaluate set2
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- compute set1+set2
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- test set1 and set1+set2 for equality
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.fi
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.sp
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Where allowed, according to the standard, constant integral expressions are
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compile-time evaluated while an effort is made to report overflow on target
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machine basis. The integral expressions are evaluated in the type \fIarith\fR.
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The size of an arith is assumed to be at least the size of the integer type
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on the target machine. If the target machine's integer size is less than the
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size of an arith, overflow can be detected at compile-time. However, the
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following call to the standard procedure new, \fInew(p, 3+5)\fR, is illegal,
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because the second parameter is not a constant according to the grammar.
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.sp
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Constant floating expressions are not compile-time evaluated, because the
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precision on the target machine and the precision on the machine on which the
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compiler runs could be different. The boolean expression \fI(1.0 + 1.0) = 2.0\fR
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could evaluate to false.
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.sp
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.NH 2
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Statements
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.NH 3
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Assignment Statement
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\fRPASCAL :
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.ti +3m
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\f5(variable-access | function-identifier) \fB:=\f5 expression
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\fREM :
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.nf
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.in +3m
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.ft I
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evaluate expression
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store in variable-access or function-identifier
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.ft R
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.in -3m
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.fi
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In case of a function-identifier, a hidden temporary variable is used to
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keep the function result.
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.bp
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.NH 3
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Goto Statement
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\fRPASCAL :
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.ti +3m
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\fBGOTO\f5 label
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\fREM :
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.in +3m
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Two cases can be distinguished :
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.br
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- local goto,
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.ti +2m
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in which a \fBbra\fR is generated.
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- non-local goto,
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.in +2m
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.ll -1i
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a goto_descriptor is build, containing the ProgramCounter of the instruction
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jumped to and an offset in the target procedure frame which contains the
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value of the StackPointer after the jump. The code for the jump itself is to
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load the address of the goto_descriptor, followed by a push of the LocalBase
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of the target procedure and a \fBcal\fR $_gto. A message is generated to
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indicate that a procedure or function contains a statement which is the
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target of a non-local goto.
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.ll +1i
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.in -2m
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.in -3m
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.sp 2
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.NH 3
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If Statement
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\fRPASCAL :
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.in +3m
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.ft 5
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\fBIF\f5 boolean-expression \fBTHEN\f5 statement
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.in -3m
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\fREM :
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.nf
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.in +3m
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\fIevaluation boolean-expression
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\fBzeq \fR*exit_label
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\fIcode statement
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\fRexit_label
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.in -3m
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.fi
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.sp 2
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\fRPASCAL :
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.in +3m
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.ft 5
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\fBIF\f5 boolean-expression \fBTHEN\f5 statement-1 \fBELSE\f5 statement-2
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.in -3m
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\fREM :
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.nf
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.in +3m
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\fIevaluation boolean-expression
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\fBzeq \fR*else_label
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\fIcode statement-1
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\fBbra \fR*exit_label
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\fRelse_label
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\fIcode statement-2
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\fRexit_label
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.in -3m
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.fi
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.sp 2
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.NH 3
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Repeat Statement
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\fRPASCAL :
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.in +3m
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.ft 5
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\fBREPEAT\f5 statement-sequence \fBUNTIL\f5 boolean-expression
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.in -3m
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\fREM :
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.nf
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.in +3m
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\fRrepeat_label
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\fIcode statement-sequence
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\fIevaluation boolean-expression
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\fBzeq\fR *repeat_label
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.in -3m
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.fi
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.bp
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.NH 3
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While Statement
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\fRPASCAL :
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.in +3m
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.ft 5
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\fBWHILE\f5 boolean-expression \fBDO\f5 statement
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.in -3m
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\fREM :
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.nf
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.in +3m
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\fRwhile_label
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\fIevaluation boolean-expression
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\fBzeq\fR *exit_label
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\fIcode statement
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\fBbra\fR *while_label
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\fRexit_label
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.in -3m
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.fi
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.sp 2
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.NH 3
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Case Statement
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.LP
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.sp
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The case-statement is implemented using the \fBcsa\fR and \fBcsb\fR
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instructions.
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\fRPASCAL :
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.in +3m
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\fBCASE\f5 case-expression \fBOF\f5
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.in +5m
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case-constant-list-1 \fB:\f5 statement-1 \fB;\f5
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.br
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case-constant-list-2 \fB:\f5 statement-2 \fB;\f5
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.br
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\&.
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.br
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\&.
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.br
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case-constant-list-n \fB:\f5 statement-n [\fB;\f5]
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.in -5m
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\fBEND\fR
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.in -3m
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.sp 2
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.LP
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.ll -1i
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The \fBcsa\fR instruction is used if the range of the case-expression
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value is dense, i.e.
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.br
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.ti +3m
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\f5( upperbound \- lowerbound ) / number_of_cases\fR
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.br
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is less than the constant DENSITY, defined in the file \fIdensity.h\fR.
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If the range is sparse, a \fBcsb\fR instruction is used.
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.ll +1i
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\fREM :
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.nf
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.in +3m
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\fIevaluation case-expression
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\fBbra\fR *l1
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.CL 1
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.CL 2
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.
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.
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.CL n
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.ft R
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\&.case_descriptor
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.ft 5
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\fIgeneration case_descriptor
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\fRl1
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.ft 5
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\fBlae\fR .case_descriptor
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.ft 5
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\fBcsa\fR size of (case-expression)
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\fRexit_label
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.in -3m
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.fi
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.bp
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.NH 3
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For Statement
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\fRPASCAL :
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.in +3m
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.ft 5
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\fBFOR\f5 control-variable \fB:=\f5 initial-value (\fBTO\f5 | \fBDOWNTO\f5) final-value \fBDO\f5 statement
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.ft R
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.in -3m
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The initial-value and final-value are evaluated at the beginning of the loop.
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If the values are not constant, they are evaluated once and stored in a
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temporary.
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EM :
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.nf
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.in +3m
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\fIload initial-value
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\fIload final-value
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\fBbgt\fR exit-label (* DOWNTO : \fBblt\fI exit-label\fR *)
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\fIload initial-value
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\fRl1
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\fIstore in control-variable
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\fIcode statement
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\fIload control-variable
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\fBdup\fI control-variable
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\fIload final-value
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\fBbeq\fR exit_label
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\fBinc\fI control-variable\fR (* DOWNTO : \fBdec\fI control-variable\fR *)
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\fBbra *\fRl1
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\fRexit_label
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.in -3m
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.fi
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Note: testing must be done before incrementing(decrementing) the
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control-variable,
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.br
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\h'\w'Note: 'u'because wraparound could occur, which could lead to an infinite
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loop.
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.sp 2
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.NH 3
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With Statement
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\fRPASCAL :
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.ti +3m
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\fBWITH\f5 record-variable-list \fBDO\f5 statement
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.ft R
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The statement
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.ti +3m
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\fBWITH\fR r\s-3\d1\u\s0, r\s-3\d2\u\s0, ..., r\s-3\dn\u\s0 \fBDO\f5 statement
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.ft R
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is equivalent to
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.in +3m
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\fBWITH\fR r\s-3\d1\u\s0 \fBDO\fR
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\fBWITH\fR r\s-3\d2\u\s0 \fBDO\fR
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...
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\fBWITH\fR r\s-3\dn\u\s0 \fBDO\f5 statement
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.ft R
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.in -3m
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The translation of
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.ti +3m
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\fBWITH\fR r\s-3\d1\u\s0 \fBDO\f5 statement
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.br
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.ft R
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is
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.nf
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.in +3m
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\fIpush address of r\s-3\d1\u\s0
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\fIstore address in temporary
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\fIcode statement
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.in -3m
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.fi
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.ft R
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An occurrence of a field is translated into:
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.in +3m
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\fIload temporary
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.br
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\fIadd field-offset
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.in -3m
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.bp
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.NH 2
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Procedure and Function Calls
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.ft R
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In general, the call
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.ti +5m
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p(a\s-3\d1\u\s0, a\s-3\d2\u\s0, ...., a\s-3\dn\u\s0)
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.br
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is translated into the sequence:
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.in +5m
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.nf
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\fIevaluate a\s-3\dn\u\s0
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\&.
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\&.
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\fIevaluate a\s-3\d2\u\s0
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\fIevaluate a\s-3\d1\u\s0
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\fIpush localbase
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\fBcal\fR $p
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\fIpop parameters
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.ft R
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.fi
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.in -5m
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i.e. the order of evaluation and binding of the actual-parameters is from
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right to left. In general, a copy of the actual-parameter is made when the
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formal-parameter is a value-parameter. If the formal-parameter is a
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variable-parameter, a pointer to the actual-parameter is pushed.
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In case of a function call, a \fBlfr\fR is generated, which pushes the
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function result on top of the stack.
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.sp 2
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.NH 2
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Register Messages
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.ft R
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A register message can be generated to indicate that a local variable is never
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referenced indirectly. This implies that a register can be used for a variable.
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We distinguish the following classes, given in decreasing priority:
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\(bu control-variable and final-value of a for-statement
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.br
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.ti +5m
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to speed up testing, and execution of the body of the for-statement
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.sp
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\(bu record-variable of a with-statement
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.br
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.ti +5m
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to improve the field selection of a record
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.sp
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\(bu remaining local variables and parameters
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.sp 2
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.NH 2
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Compile-time optimizations
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.ft R
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The only optimization that is performed is the evaluation of constant
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integral expressions. The optimization of constructs like
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.ti +5m
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\fBif\f5 false \fBthen\f5 statement\fR,
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.br
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is left to either the peephole optimizer, or a global optimizer.
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