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pc: add $ operator

--- a/sys/man/1/pc

+++ b/sys/man/1/pc

@@ -33,6 +33,9 @@

 .TP

 .B < >= < <= == !=

 .PP

+The \fB$\fR operator performs sign extension. \fIn\fB$\fIx\fR truncates \fIx\fR to \fIn\fR bits and sign extends.

+If \fIn\fR is omitted, it is inferred from the highest set bit (the result is always ≤ 0 in this case).

+.PP

 Variables can be defined using

 .BR = .

 The builtin variable

--- a/sys/src/cmd/pc.y

+++ b/sys/src/cmd/pc.y

@@ -195,6 +195,10 @@

 		else

 			mpassign(b, a);

 		break;

+	case '$':

+		a->b = b->b;

+		mpxtend(b, mptoi(a), a);

+		break;

 	}

 	numdecref(b);

 	return a;

@@ -389,6 +393,7 @@

 %left unary

 %left '*' '/' '%'

 %right LOEXP

+%right '$'

 %{

 	int save;

@@ -501,6 +506,7 @@

 	| '-' expr %prec unary { $$ = nummod($2); if($$ != nil) mpsub(mpzero, $$, $$); }

 	| '~' expr %prec unary { $$ = nummod($2); if($$ != nil) mpnot($$, $$); }

 	| '!' expr %prec unary { $$ = nummod($2); if($$ != nil) {itomp(mpcmp($$, mpzero) == 0, $$); $$->b = 0; } }

+	| '$' expr { $$ = nummod($2); if($$ != nil) if($2->sign > 0) mpxtend($2, mpsignif($2), $$); else mpassign($2, $$); }

 	| expr '?' expr ':' expr %prec '?' {

 		if($1 == nil || mpcmp($1, mpzero) != 0){

 			$$ = $3;

@@ -541,6 +547,7 @@

 		if($$ == nil) error("no last result");

 		else numincref($$);

 	}

+	| expr '$' expr { $$ = numbin('$', $1, $3); }

 elist: { $$.n = 0; } | elist1

 elist1: expr { $$.x[0] = $1; $$.n = 1; }