interpret lines that look like

	//line 10 units.y
which is equiv to c
	#line 10 units.y
the purpose is to generate diagnostics
that correctly point to preprocessed source.

R=rsc
CC=golang-dev
https://golang.org/cl/1863042

src/cmd/gc/lex.c

@@ -25,6 +25,7 @@ static void	ungetc(int);
 static int32	getr(void);
 static int	escchar(int, int*, vlong*);
 static void	addidir(char*);
+static int	getlinepragma(void);
 
 static char *goos, *goarch, *goroot;
 
@@ -658,8 +659,8 @@ l0:
 			}
 		}
 		if(c1 == '/') {
+			c = getlinepragma();
 			for(;;) {
-				c = getr();
 				if(c == '\n') {
 					ungetc(c);
 					goto l0;
@@ -668,6 +669,7 @@ l0:
 					yyerror("eof in comment");
 					errorexit();
 				}
+				c = getr();
 			}
 		}
 		if(c1 == '=') {
@@ -1106,6 +1108,63 @@ caseout:
 	return LLITERAL;
 }
 
+/*
+ * read and interpret syntax that looks like
+ * //line 15 parse.y
+ * as a discontenuity in sequential line numbers.
+ * the next line of input comes from parse.y:15
+ */
+static int
+getlinepragma(void)
+{
+	int i, c, n;
+	char *cp, *ep;
+	Hist *h;
+
+	for(i=0; i<5; i++) {
+		c = getr();
+		if(c != "line "[i])
+			return c;
+	}
+
+	n = 0;
+	for(;;) {
+		c = getr();
+		if(!isdigit(c))
+			break;
+		n = n*10 + (c-'0');
+	}
+
+	if(c != ' ' || n == 0)
+		return c;
+
+	cp = lexbuf;
+	ep = lexbuf+sizeof(lexbuf)-5;
+	for(;;) {
+		c = getr();
+		if(c == ' ')
+			continue;
+		if(c == '\n')
+			break;
+		*cp++ = c;
+		if(cp >= ep)
+			break;
+	}
+	*cp = 0;
+//	n--;	// weve already seen the newline
+	if(n > 0) {
+		// try to avoid allocating file name over and over
+		for(h=hist; h!=H; h=h->link) {
+			if(h->name != nil && strcmp(h->name, lexbuf) == 0) {
+				linehist(h->name, n, 0);
+				return c;
+			}
+		}
+		linehist(strdup(lexbuf), n, 0);
+	}
+	return c;
+}
+
 int32
 yylex(void)
 {

src/cmd/gc/subr.c

@@ -228,11 +228,14 @@ linehist(char *file, int32 off, int relative)
 	if(debug['i']) {
 		if(file != nil) {
 			if(off < 0)
-				print("pragma %s at line %L\n", file, lineno);
+				print("pragma %s at line %L\n", file, lexlineno);
 			else
-				print("import %s at line %L\n", file, lineno);
+			if(off > 0)
+				print("line %s at line %L\n", file, lexlineno);
+			else
+				print("import %s at line %L\n", file, lexlineno);
 		} else
-			print("end of import at line %L\n", lineno);
+			print("end of import at line %L\n", lexlineno);
 	}
 
 	if(off < 0 && file[0] != '/' && !relative) {
@@ -894,12 +897,21 @@ Lconv(Fmt *fp)
 		if(lno < h->line)
 			break;
 		if(h->name) {
-			if(n < HISTSZ) {	/* beginning of file */
+			if(h->offset > 0) {
+				// #line directive
+				if(n > 0 && n < HISTSZ) {
+					a[n-1].line = h;
+					a[n-1].ldel = h->line - h->offset + 1;
+				}
+			} else {
+				// beginning of file
+				if(n < HISTSZ) {
 					a[n].incl = h;
 					a[n].idel = h->line;
 					a[n].line = 0;
 				}
 				n++;
+			}
 			continue;
 		}
 		n--;
@@ -921,12 +933,12 @@ Lconv(Fmt *fp)
 		}
 		if(a[i].line)
 			fmtprint(fp, "%s:%ld[%s:%ld]",
-				a[i].line->name, lno-a[i].ldel+1,
-				a[i].incl->name, lno-a[i].idel+1);
+				a[i].line->name, lno-a[i].ldel,
+				a[i].incl->name, lno-a[i].idel);
 		else
 			fmtprint(fp, "%s:%ld",
 				a[i].incl->name, lno-a[i].idel+1);
-		lno = a[i].incl->line - 1;	/* now print out start of this file */
+		lno = a[i].incl->line - 1;	// now print out start of this file
 	}
 	if(n == 0)
 		fmtprint(fp, "<epoch>");

src/cmd/goyacc/goyacc.go

@@ -675,7 +675,7 @@ outer:
 	//
 	if t == MARK {
 		if !lflag {
-			fmt.Fprintf(ftable, "\n//line %v:%v\n", infile, lineno)
+			fmt.Fprintf(ftable, "\n//line %v %v\n", lineno, infile)
 		}
 		for {
 			c := getrune(finput)
@@ -2066,6 +2066,7 @@ nextk:
 func output() {
 	var c, u, v int
 
+	fmt.Fprintf(ftable, "\n//line 1 yacctab\n")
 	fmt.Fprintf(ftable, "var\tyyExca = []int {\n")
 
 	noset := mkset()
@@ -2825,8 +2826,10 @@ func others() {
 		c = getrune(finput)
 	}
 
-	parts := strings.Split(yaccpar, "yyrun()", 2)
 	// copy yaccpar
+	fmt.Fprintf(ftable, "\n//line 1 yaccpar\n")
+
+	parts := strings.Split(yaccpar, "yyrun()", 2)
 	fmt.Fprintf(ftable, "%v", parts[0])
 	ftable.Write(fcode.Bytes())
 	fmt.Fprintf(ftable, "%v", parts[1])